Chapter 8: Introduction to Structured Query Language (SQL) PDF

Introduction to Chapter Structured Query 8 Language (SQL) In this chapter...

Introduction to Chapter Structured Query 8 Language (SQL) In this chapter »» Introduction »» Structured Query “The most important motivation for the Language (SQL) research work that resulted in the relational »» Data Types and model was the objective of providing a sharp Constraints in MySQL and clear boundary between the logical and »» SQL for Data Definition physical aspects of database management.” »» SQL for Data Manipulation – E. F. Codd »» SQL for Data Query »» Data Updation and Deletion 8.1 Introduction We have learnt about Relational Database Management System (RDBMS) and purpose in the previous chapter. There are many RDBMS such as MySQL, Microsoft SQL Server, PostgreSQL, Oracle, etc. that allow us to create a database consisting of relations and to link one or more relations for efficient querying to store, retrieve and manipulate data on that database. In this chapter, we will learn how to create, populate and query database using MySQL. 2024-25 Chap 8.indd 143 19-Jul-19 3:45:57 PM  144 Informatics Practices – Class XI 8.2 Structured Query Language (SQL) One has to write application programs to access data in case of a file system. However, for database management systems there are special kind of programming languages called query language that can be used to access data from the database. The Structured Query Language (SQL) is the most popular query language used by major relational database management systems such as MySQL, ORACLE, SQL Server, etc. SQL is easy to learn as the statements comprise of descriptive English words and are not case sensitive. We can create and interact with a database using SQL in an efficient and easy way. The benefit with SQL is that we don’t have to specify how to get the data from the database. Rather, we simply specify what is to be retrieved, and SQL does the rest. Although called a query language, SQL can do much more besides querying. SQL provides statements for defining the structure of the data, manipulating data in the database, declare constraints and retrieve data from the database in various ways, depending on our requirements. In this chapter, we will learn how to create a database using MySQL as the RDBMS software. We will create a database called StudentAttendance (Figure 7.5) that we had identified in the previous chapter. We will also learn how to populate database with data, manipulate data in that and retrieve data from the database through SQL queries. 8.2.1 Installing MySQL MySQL is an open source RDBMS software which can be easily downloaded from the official website https:// dev.mysql.com/downloads. After installing MySQL, start MySQL service. The appearance of mysql> prompt (Figure 8.1) means that MySQL is ready for us to enter SQL statements. Few rules to follow while writing SQL statements in MySQL: Activity 8.1 SQL is case insensitive. That means name and NAME are same for SQL. Explore LibreOffice Base and compare it Always end SQL statements with a semicolon (;). with MySQL To enter multiline SQL statements, we don’t write ‘;’ after the first line. We put enter to continue on next line. The prompt mysql> then changes to ‘->’, 2024-25 Chap 8.indd 144 19-Jul-19 3:45:57 PM  Introduction to Structured Query Language (SQL) 145 indicating that statement is continued to the next line. After the last line, put ‘;’ and press enter. 8.3 Data Types and Constraints in MySQL Figure 8.1: MySQL Shell We know that a database consists of one or more relations and each relation (table) is made up of attributes Activity 8.2 (column). Each attribute has a data type. We can also What are the other specify constraints for each attribute of a relation. data types supported in MySQL? Are there other 8.3.1 Data type of Attribute variants of integer and float data type? Data type indicates the type of data value that an attribute can have. The data type of an attribute decides the operations that can be performed on the data of that attribute. For example, arithmetic operations can Think and Reflect Can you think of an be performed on numeric data but not on character attribute for which data. Commonly used data types in MySQL are numeric fixed length string is types, date and time types, and string (character and suitable? byte) types as shown in Table 8.1. Table 8.1 Commonly used data types in MySQL Data type Description CHAR(n) Specifies character type data of length n where n could be any value from 0 to 255. CHAR is of fixed length, means, declaring CHAR (10) implies to reserve spaces for 10 characters. If data does not have 10 characters (for example, ‘city’ has four characters), MySQL fills the remaining 6 characters with spaces padded on the right. VARCHAR(n) Specifies character type data of length ‘n’ where n could be any value from 0 to 65535. But unlike CHAR, VARCHAR is a variable-length data type. That is, declaring VARCHAR (30) means a maximum of 30 characters can be stored but the actual allocated bytes will depend on the length of entered string. So ‘city’ in VARCHAR (30) will occupy the space needed to store 4 characters only. 2024-25 Chap 8.indd 145 19-Jul-19 3:45:57 PM  146 Informatics Practices – Class XI INT INT specifies an integer value. Each INT value occupies 4 bytes of storage. The range of values allowed in integer type are -2147483648 to 2147483647. For values larger than that, we have to use BIGINT, which occupies 8 bytes. FLOAT Holds numbers with decimal points. Each FLOAT value occupies 4 bytes. DATE The DATE type is used for dates in 'YYYY-MM-DD' format. YYYY is the 4 digit year, MM is the 2 digit month and DD is the 2 digit date. The supported range is '1000-01-01' to '9999-12-31'. 8.3.2 Constraints Think and Reflect Which two constraints Constraints are certain types of restrictions on the data when applied together values that an attribute can have. They are used to will produce a Primary ensure the accuracy and reliability of data. However, it Key constraint? is not mandatory to define constraint for each attribute of a table. Table 8.2 lists various SQL constraints. Table 8.2 Commonly used SQL Constraints Constraint Description NOT NULL Ensures that a column cannot have NULL values where NULL means missing/ unknown/not applicable value. UNIQUE Ensures that all the values in a column are distinct/unique. DEFAULT A default value specified for the column if no value is provided. PRIMARY KEY The column which can uniquely identify each row or record in a table. FOREIGN KEY The column which refers to value of an attribute defined as primary key in another table. 8.4 SQL for Data Definition SQL provides commands for defining the relation schemas, modifying relation schemas and deleting relations. These are called Data Definition Language (DDL) through which the set of relations are specified, including their schema, data type for each attribute, the constraints as well as the security and access related authorisations. Data definition starts with the create statement. This statement is used to create a database and its tables (relations). Before creating a database, we should be clear about the number of tables in the database, the columns (attributes) in each table along with the data type of each column. This is how we decide the relation schema. 8.4.1 CREATE Database To create a database, we use the CREATE DATABASE statement as shown in the following syntax: CREATE DATABASE databasename; 2024-25 Chap 8.indd 146 19-Jul-19 3:45:57 PM  Introduction to Structured Query Language (SQL) 147 To create a database called StudentAttendance, we will type following command at mysql prompt. mysql> CREATE DATABASE StudentAttendance; Query OK, 1 row affected (0.02 sec) Note: In LINUX environment, names for database and tables are case-sensitive whereas in WINDOWS, there is no such Show differentiation. However, as a good practice, it is suggested to write database or table name in the same letter cases that were used at the time of their creation. A DBMS can manage multiple databases on one computer. Therefore, we need to select the database that we want to use. Once the database is selected, we can proceed with creating tables or querying data. Write the following SQL statement for using the database: mysql> USE StudentAttendance; Database changed Activity 8.3 Initially, the created database is empty. It can be Type the statement checked by using the Show tables command that lists show database;. Does names of all the tables within a database. it show the name of StudentAttendance mysql> SHOW TABLES; database? Empty set (0.06 sec) 8.4.2 CREATE Table After creating database StudentAttendance, we need to define relations (create tables) in this database and specify attributes for each relation along with data types for each attribute. This is done using the CREATE TABLE statement. Syntax: CREATE TABLE tablename( attributename1 datatype constraint, attributename2 datatype constraint, : attributenameN datatype constraint); It is important to observe the following points with respect to the Create Table statement: N is the degree of the relation, means there are N columns in the table. Attribute name specifies the name of the column in the table. Datatype specifies the type of data that an attribute can hold. Constraint indicates the restrictions imposed on the values of an attribute. By default, each attribute can take NULL values except for the primary key. 2024-25 Chap 8.indd 147 19-Jul-19 3:45:57 PM  148 Informatics Practices – Class XI Let us identify data types of the attributes of table STUDENT along with their constraint, if any. Assuming maximum students in a class to be 100 and values of roll number in a sequence from 1 to 100, we know that 3 digits are sufficient to store values for the attribute RollNumber. Hence, data type INT is appropriate for this attribute. Total number of characters in student names (SName) can differ. Assuming maximum characters in a name as 20, we use VARCHAR(20) for SName column. Data type for the attribute SDateofBirth is DATE and supposing the school uses guardian’s 12 digit Aadhaar number as GUID, we can declare GUID as CHAR (12) since Aadhaar number is of fixed length and we are not going to perform any mathematical operation on GUID. Table 8.3, 8.4 and 8.5 show the chosen data type and constraint for each attribute of the relations STUDENT, GUARDIAN and ATTENDANCE, respectively. Table 8.3 Data types and constraints for the attributes of relation STUDENT Attribute Name Data expected to be stored Data type Constraint RollNumber Numeric value consisting of maximum 3 digits INT PRIMARY KEY SName Variant length string of maximum 20 characters VARCHAR(20) NOT NULL SDateofBirth Date value DATE NOT NULL GUID Numeric value consisting of 12 digits CHAR (12) FOREIGN KEY Table 8.4 Data types and constraints for the attributes of relation GUARDIAN Attribute Name Data expected to be stored Data type Constraint GUID Numeric value consisting of 12 digit Aadhaar CHAR (12) PRIMARY KEY number GName Variant length string of maximum 20 VARCHAR(20) NOT NULL characters GPhone Numeric value consisting of 10 digits CHAR(10) NULL UNIQUE GAddress Variant length string of size 30 characters VARCHAR(30) NOT NULL Table 8.5 Data types and constraints for the attributes of relation ATTENDANCE. Attribute Name Data expected to be stored Data type Constraint AttendanceDate Date value DATE PRIMARY KEY* RollNumber Numeric value consisting of maximum 3 INT PRIMARY KEY* digits FOREIGN KEY AttendanceStatus ‘P’ for present and ‘A’ for absent CHAR(1) NOT NULL *means part of composite primary key Once data types and constraints are identified, let us create tables without specifying constraint along with the attribute name for simplification. We will learn to incorporate constraints on attributes in Section 8.4.4. 2024-25 Chap 8.indd 148 19-Jul-19 3:45:57 PM  Introduction to Structured Query Language (SQL) 149 Example 8.1 Create table STUDENT. mysql> CREATE TABLE STUDENT( Think and Reflect -> RollNumber INT, Can we have a CHAR -> SName VARCHAR(20), or VARCHAR data type -> SDateofBirth DATE, for contact number -> GUID CHAR(12), (mobile, landline)? -> PRIMARY KEY (RollNumber)); Query OK, 0 rows affected (0.91 sec) Note: ‘,’ is used to separate two attributes and each statement terminates with a semi-colon (;). The symbol ‘->’ indicates line continuation as SQL statement may not complete in a single line. Activity 8.4 8.4.3 DESCRIBE Table We can view the structure of an already created table Create the other two relations GUARDIAN using the describe statement. and ATTENDANCE Syntax: as per data types DESCRIBE tablename; given in Table 8.4 and 8.5, and view their MySQL also supports the short form DESC of DESCRIBE structures. Don't add to get description of table. To retrieve details about the any constraint in the structure of relation STUDENT, we can write DESC or two tables. DESCRIBE followed by table name: mysql> DESC STUDENT; +--------------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +--------------+-------------+------+-----+---------+-------+ | RollNumber | int | NO | PRI | NULL | | | SName | varchar(20) | YES | | NULL | | | SDateofBirth | date | YES | | NULL | | | GUID | char(12) | YES | | NULL | | +--------------+-------------+------+-----+---------+-------+ 4 rows in set (0.06 sec) The show table command will now return the table STUDENT: mysql> SHOW TABLES; +------------------------------+ | Tables_in_studentattendance | +------------------------------+ | student | +------------------------------+ 1 row in set (0.00 sec) 8.4.4 ALTER Table After creating a table we may realize that we need to add/remove an attribute or to modify the datatype of an existing attribute or to add constraint in attribute. In all such cases, we need to change or alter the structure of the table by using the alter statement. Syntax: ALTER TABLE tablename ADD/Modify/DROP attribute1, attribute2,.. 2024-25 Chap 8.indd 149 19-Jul-19 3:45:57 PM  150 Informatics Practices – Class XI (A) Add primary key to a relation Let us now alter the tables created in Activity 8.4. The below MySQL statement adds a primary key to the GUARDIAN relation: mysql> ALTER TABLE GUARDIAN ADD PRIMARY KEY (GUID); Query OK, 0 rows affected (1.14 sec) Records: 0 Duplicates: 0 Warnings: 0 Now let us add primary key to the ATTENDANCE relation. The primary key of this relation is a composite key made up of two attributes — AttendanceDate and RollNumber. mysql> ALTER TABLE ATTENDANCE -> ADD PRIMARY KEY(AttendanceDate, -> RollNumber); Query OK, 0 rows affected (0.52 sec) Records: 0 Duplicates: 0 Warnings: 0 (B) Add foreign key to a relation Once primary keys are added the next step is to add foreign keys to the relation (if any). A relation may have multiple foreign keys and each foreign key is defined on a single attribute. Following points need to be observed while adding foreign key to a relation: The referenced relation must be already created. The referenced attribute must be a part of primary key of the referenced relation. Data types and size of referenced and referencing attributes must be same. Syntax: ALTER TABLE table_name ADD FOREIGN KEY(attribute name) REFERENCES referenced_table_name (attribute name); Let us now add foreign key to the table STUDENT. Table 8.3 shows that attribute GUID (the referencing Think and Reflect attribute) is a foreign key and it refers to attribute GUID Name foreign keys in (the referenced attribute) of table GUARDIAN (Table 8.4). table ATTENDANCE Hence, STUDENT is the referencing table and GUARDIAN and STUDENT. Is there is the referenced table. any foreign key in table GUARDIAN. mysql> ALTER TABLE STUDENT -> ADD FOREIGN KEY(GUID) REFERENCES -> GUARDIAN(GUID); Query OK, 0 rows affected (0.75 sec) Records: 0 Duplicates: 0 Warnings: 0 (C) Add constraint UNIQUE to an existing attribute In GUARDIAN table, attribute GPhone has a constraint UNIQUE which means no two values in that column should be same. Syntax: 2024-25 Chap 8.indd 150 19-Jul-19 3:45:57 PM  Introduction to Structured Query Language (SQL) 151 ALTER TABLE table_name ADD UNIQUE (attribute name); Let us now add the constraint UNIQUE with attribute GPhone of the table GUARDIAN as shown at table 8.4. Activity 8.5 mysql> ALTER TABLE GUARDIAN Add foreign key in -> ADD UNIQUE(GPhone); the ATTENDANCE Query OK, 0 rows affected (0.44 sec) table (use fig. 8.1 to Records: 0 Duplicates: 0 Warnings: 0 identify referencing and (D) Add an attribute to an existing table referenced tables). Sometimes, we may need to add an additional attribute in a table. It can be done using the syntax given below: ALTER TABLE table_name ADD attribute_name DATATYPE; Suppose the principal of the school has decided to award scholarship to some needy students for which income of the guardian must be known. But school has not maintained income attribute with table GUARDIAN so far. Therefore, the database designer now needs to add a new attribute income of data type INT in the table GUARDIAN. mysql> ALTER TABLE GUARDIAN -> ADD income INT; Query OK, 0 rows affected (0.47 sec) Records: 0 Duplicates: 0 Warnings: 0 Think and Reflect What are the minimum (E) Modify datatype of an attribute and maximum income We can modify data types of the existing attributes of a values that can be table using the following ALTER statement. entered in the income attribute given the data Syntax: type is INT? ALTER TABLE table_name MODIFY attribute DATATYPE; Suppose we need to change the size of attribute GAddress from VARCHAR(30) to VARCHAR(40) of the GUARDIAN table. The MySQL statement will be: mysql> ALTER TABLE GUARDIAN -> MODIFY GAddress VARCHAR(40); Query OK, 0 rows affected (0.11 sec) Records: 0 Duplicates: 0 Warnings: 0 (F) Modify constraint of an attribute When we create a table, by default each attribute takes NULL value except for the attribute defined as primary key. We can change an attribute’s constraint from NULL to NOT NULL using alter statement. Syntax: ALTER TABLE table_name MODIFY attribute DATATYPE NOT NULL; Note: We have to specify the data type of the attribute along with constraint NOT NULL while using MODIFY. 2024-25 Chap 8.indd 151 19-Jul-19 3:45:57 PM  152 Informatics Practices – Class XI Notes To associate NOT NULL constraint with attribute SName of table STUDENT (table 8.3), we write the following MySQL statement: mysql> ALTER TABLE STUDENT -> MODIFY SName VARCHAR(20) NOT NULL; Query OK, 0 rows affected (0.47 sec) Records: 0 Duplicates: 0 Warnings: 0 (G) Add default value to an attribute If we want to specify default value for an attribute, then use the following syntax: ALTER TABLE table_name MODIFY attribute DATATYPE DEFAULT default_value; To set default value of SDateofBirth of STUDENT to 15th May 2000, we write the following statement: mysql> ALTER TABLE STUDENT -> MODIFY SDateofBirth DATE DEFAULT -> 2000-05-15; Query OK, 0 rows affected (0.08 sec) Records: 0 Duplicates: 0 Warnings: 0 Note: We have to specify the data type of the attribute along with DEFAULT while using MODIFY. (H) Remove an attribute Using ALTER, we can remove attributes from a table, as shown in the below syntax: ALTER TABLE table_name DROP attribute; To remove the attribute income from the table GUARDIAN (8.4), we can write the following MySQL statement: mysql> ALTER TABLE GUARDIAN DROP income; Query OK, 0 rows affected (0.42 sec) Records: 0 Duplicates: 0 Warnings: 0 (I) Remove primary key from the table While creating a table, we may have specified incorrect primary key. In such case, we need to drop the existing primary key of the table and add a new primary key. Syntax: ALTER TABLE table_name DROP PRIMARY KEY; To remove primary key of table GUARDIAN (Table 8.4), we write the following MySQL statement: mysql> ALTER TABLE GUARDIAN DROP PRIMARY KEY; Query OK, 0 rows affected (0.72 sec) Records: 0 Duplicates: 0 Warnings: 0 Note: We have dropped primary key from GUARDIAN table, but each table should have a primary key to maintain uniqueness. Hence, we have to use ADD command to specify primary key for the GUARDIAN table as shown in earlier examples. 2024-25 Chap 8.indd 152 19-Jul-19 3:45:57 PM  Introduction to Structured Query Language (SQL) 153 8.4.5 DROP Statement Notes Sometimes a table in a database or the database itself needs to be removed. We can use DROP statement to remove a database or a table permanently from the system. However, one should be very cautious while using this statement as it cannot be undone. Syntax to drop a table: DROP TABLE table_name; Syntax to drop a database: DROP DATABASE database_name; Cautions: 1) Using the Drop statement to remove a database will ultimately remove all the tables within it. 2) DROP statement will remove the tables or database created by you. Hence you may apply DROP statement at the end of the chapter. 8.5 SQL for Data Manipulation In the previous section, we created the database StudentAttendance having three relations STUDENT, GUARDIAN and ATTENDANCE. When we create a table, only its structure is created but the table has no data. To populate records in the table, INSERT statement is used. Similarly, table records can be deleted or updated using SQL data manipulation statements. Data Manipulation using a database means either retrieval (access) of existing data, insertion of new data, removal of existing data or modification of existing data in the database. 8.5.1 INSERTION of Records INSERT INTO statement is used to insert new records in a table. Its syntax is: INSERT INTO tablename VALUES(value 1, value 2,....); Here, value 1 corresponds to attribute 1, value 2 corresponds to attribute 2 and so on. Note that we need not to specify attribute names in insert statement if there are exactly same number of values in the INSERT statement as the total number of attributes in the table. Caution: While populating records in a table with foreign key, ensure that records in referenced tables are already populated. 2024-25 Chap 8.indd 153 19-Jul-19 3:45:58 PM  154 Informatics Practices – Class XI Let us insert some records in the StudentAttendance database. We shall insert records in the GUARDIAN table first as it does not have any foreign key. We are going to insert the records given in Table 8.6. Table 8.6 Records to be inserted into the GUARDIAN table GUID GName GPhone GAddress 444444444444 Amit Ahuja 5711492685 G-35, Ashok Vihar, Delhi 111111111111 Baichung Bhutia 3612967082 Flat no. 5, Darjeeling Appt., Shimla 101010101010 Himanshu Shah 4726309212 26/77, West Patel Nagar, Ahmedabad 333333333333 Danny Dsouza S -13, Ashok Village, Daman 466444444666 Sujata P. 3801923168 HNO-13, B- block, Preet Vihar, Madurai The below statement inserts the first record in the table. mysql> INSERT INTO GUARDIAN -> VALUES (444444444444, 'Amit Ahuja', -> 5711492685, 'G-35,Ashok vihar, Delhi' ); Query OK, 1 row affected (0.01 sec) We can use the SQL statement SELECT * from table_ name to view the inserted records. The SELECT statement will be explained in next section. mysql> SELECT * from GUARDIAN; +--------------+-----------------+------------+-------------------------------+ | GUID | GName | Gphone | GAddress | +--------------+-----------------+------------+-------------------------------+ | 444444444444 | Amit Ahuja | 5711492685 | G-35, Ashok vihar, Delhi | +--------------+-----------------+------------+-------------------------------+ 1 row in set (0.00 sec) If we want to provide values only for some of the attributes in a table (supposing other attributes having NULL or any other default value), then we shall specify the attribute name alongside each data value as shown in the following syntax of INSERT INTO statement. Syntax: INSERT INTO tablename (column1, column2,...) VALUES (value1, value2,...); To insert the fourth record of Table 8.6 where GPhone is not given, we need to insert values in the other three fields (GPhone was set to NULL by default at the time of table creation). In this case, we have to specify the Activity 8.6 names of attributes in which we want to insert values. The values must be given in the same order in which Write SQL statements to insert the remaining attributes are written in INSERT command. 3 rows of table 8.6 in mysql> INSERT INTO GUARDIAN(GUID, GName, GAddress) table GUARDIAN. -> VALUES (333333333333, 'Danny Dsouza', 2024-25 Chap 8.indd 154 3/31/2023 3:57:16 PM  Introduction to Structured Query Language (SQL) 155 -> 'S -13, Ashok Village, Daman' ); Query OK, 1 row affected (0.03 sec) Note: Text and date values must be enclosed in ‘ ’ (single quotes). mysql> SELECT * from GUARDIAN; +--------------+--------------+------------+----------------------------------+ | GUID | GName | Gphone | GAddress | +--------------+--------------+------------+----------------------------------+ | 333333333333 | Danny Dsouza | NULL | S -13, Ashok Village, Daman | | 444444444444 | Amit Ahuja | 5711492685 | G-35, Ashok vihar, Delhi | +--------------+--------------+------------+----------------------------------+ 2 rows in set (0.00 sec) Let us now insert the records given in Table 8.7 into the STUDENT table. Table 8.7 Records to be inserted into the STUDENT table RollNumber SName SDateofBirth GUID 1 Atharv Ahuja 2003-05-15 444444444444 2 Daizy Bhutia 2002-02-28 111111111111 3 Taleem Shah 2002-02-28 4 John Dsouza 2003-08-18 333333333333 5 Ali Shah 2003-07-05 101010101010 6 Manika P. 2002-03-10 466444444666 To insert the first record of Table 8.7, we write the following MySQL statement mysql> INSERT INTO STUDENT -> VALUES(1,'Atharv Ahuja','2003-05-15', -> 444444444444); Query OK, 1 row affected (0.11 sec) OR mysql> INSERT INTO STUDENT (RollNumber, SName, -> SDateofBirth, GUID) -> VALUES (1,'Atharv Ahuja','2003-05-15', -> 444444444444); Query OK, 1 row affected (0.02 sec) mysql> SELECT * from STUDENT; +------------+--------------+--------------+--------------+ | RollNumber | SName | SDateofBirth | GUID | +------------+--------------+--------------+--------------+ | 1 | Atharv Ahuja | 2003-05-15 | 444444444444 | +------------+--------------+--------------+--------------+ 1 row in set (0.00 sec) Let us now insert the third record of Table 8.7 where GUID is NULL. Recall that GUID is foreign key of this Recall that Date is table and thus can take NULL value. Hence, we can put stored in “YYYY-MM- DD” format. NULL value for GUID and insert the record by using the following statement: 2024-25 Chap 8.indd 155 19-Jul-19 3:45:58 PM  156 Informatics Practices – Class XI mysql> INSERT INTO STUDENT -> VALUES(3, 'Taleem Shah','2002-02-28', -> NULL); Query OK, 1 row affected (0.05 sec) mysql> SELECT * from STUDENT; +------------+--------------+--------------+--------------+ | RollNumber | SName | SDateofBirth | GUID | +------------+--------------+--------------+--------------+ | 1 | Atharv Ahuja | 2003-05-15 | 444444444444 | | 3 | Taleem Shah | 2002-02-28 | NULL | +------------+--------------+--------------+--------------+ 2 rows in set (0.00 sec) We had to write NULL in the above MySQL statement because when not giving the column names, we need to give values for all the columns. Otherwise, we have to give names of attributes along with the values if we need to insert data only for certain attributes, as shown in the next query: Activity 8.7 mysql> INSERT INTO STUDENT (RollNumber, SName, Write SQL statements -> SDateofBirth) VALUES (3, 'Taleem Shah',' to insert the remaining -> 2002-02-28'); 4 rows of table 8.7 in Query OK, 1 row affected (0.05 sec) table STUDENT. In the above statement we are informing DBMS to insert the corresponding values for the mentioned columns and GUID would be assigned NULL value. mysql> SELECT * from STUDENT; +------------+--------------+--------------+--------------+ | RollNumber | SName | SDateofBirth | GUID | +------------+--------------+--------------+--------------+ | 1 | Atharv Ahuja | 2003-05-15 | 444444444444 | | 3 | Taleem Shah | 2002-02-28 | NULL | +------------+--------------+--------------+--------------+ 2 rows in set (0.00 sec) 8.6 SQL for Data Query Think and Reflect Which of the above So far we have learnt how to create database as well syntax should be as to store and manipulate data. We are interested to used when we are store data in a database as it is easier to retrieve data not sure of the order in future from databases in whatever way we want. (with respect to the The Structured Query Language (SQL) has efficient column) in which the values are to be mechanisms to retrieve data stored in multiple tables inserted in the table? in a MySQL database (or any other RDBMS). The Can we insert two user enters the SQL commands called queries where records with the the specific requirements for data to be retrieved are same roll number? provided. The SQL statement SELECT is used to retrieve data from the tables in a database and is also called query statement. 2024-25 Chap 8.indd 156 19-Jul-19 3:45:58 PM  Introduction to Structured Query Language (SQL) 157 8.6.1 SELECT Statement The SQL statement SELECT is used to retrieve data from the tables in a database and the output is also displayed in tabular form. Syntax: SELECT attribute1, attribute2,... FROM table_name WHERE condition Here, attribute1, attribute2,... are the column names of the table table_name from which we want to retrieve data. The FROM clause is always written with SELECT clause as it specifies the name of the table from which data is to be retrieved. The WHERE clause is optional and is used to retrieve data that meet specified condition(s). Example 8.2 To display the name and date of birth of student with roll number 2, we write the following query: mysql> SELECT SName, SDateofBirth -> FROM STUDENT -> WHERE RollNumber = 1; +--------------+--------------+ | SName | SDateofBirth | +--------------+--------------+ Think and Reflect | Atharv Ahuja | 2003-05-15 | Can you think of +--------------+--------------+ examples from daily 1 row in set (0.03 sec) life where storing 8.6.2 QUERYING using Database OFFICE and querying data in a database can be Different organisations maintain databases to helpful? store data in the form of tables. Let us consider the database OFFICE of an organisation that has many related tables like EMPLOYEE, DEPARTMENT and so on. Every EMPLOYEE in the database is assigned to a DEPARTMENT and his/her Department number (DeptId) is stored as a foreign key in the table EMPLOYEE. Let us consider some data for the table ‘EMPLOYEE’ as shown in Table 8.8 and apply the SELECT statement to retrieve data: Table 8.8 EMPLOYEE EmpNo Ename Salary Bonus Deptld 101 Aaliya 10000 234 D02 102 Kritika 60000 123 D01 103 Shabbir 45000 566 D01 104 Gurpreet 19000 565 D04 105 Joseph 34000 875 D03 2024-25 Chap 8.indd 157 19-Jul-19 3:45:58 PM  158 Informatics Practices – Class XI Notes 106 Sanya 48000 695 D02 107 Vergese 15000 D01 108 Nachaobi 29000 D05 109 Daribha 42000 D04 110 Tanya 50000 467 D05 (A) Retrieve selected columns The following query displays employee numbers of all the employees: mysql> SELECT EmpNo -> FROM EMPLOYEE; +-------+ | EmpNo | +-------+ | 101 | | 102 | | 103 | | 104 | | 105 | | 106 | | 107 | | 108 | | 109 | | 110 | +-------+ 10 rows in set (0.41 sec) To display the employee number and employee name of all the employees, we write the following query: mysql> SELECT EmpNo, Ename -> FROM EMPLOYEE; +-------+----------+ | EmpNo | Ename | +-------+----------+ | 101 | Aaliya | | 102 | Kritika | | 103 | Shabbir | | 104 | Gurpreet | | 105 | Joseph | | 106 | Sanya | | 107 | Vergese | | 108 | Nachaobi | | 109 | Daribha | | 110 | Tanya | +-------+----------+ 10 rows in set (0.00 sec) (B) Renaming of columns In case we want to rename any column while displaying the output, we can do so by using alias 'AS' in the query as: Display Employee name as Name in the output for all the employees. mysql> SELECT EName AS Name 2024-25 Chap 8.indd 158 19-Jul-19 3:45:58 PM  Introduction to Structured Query Language (SQL) 159 -> FROM EMPLOYEE; Notes +----------+ | Name | +----------+ | Aaliya | | Kritika | | Shabbir | | Gurpreet | | Joseph | | Sanya | | Vergese | | Nachaobi | | Daribha | | Tanya | +----------+ 10 rows in set (0.00 sec) Example 8.3 Display names of all employees along with their annual salary (Salary*12). While displaying query result, rename EName as Name. mysql> SELECT EName AS Name, Salary*12 -> FROM EMPLOYEE; +----------+-----------+ | Name | Salary*12 | +----------+-----------+ | Aaliya | 120000 | | Kritika | 720000 | | Shabbir | 540000 | | Gurpreet | 228000 | | Joseph | 408000 | | Sanya | 576000 | | Vergese | 180000 | | Nachaobi | 348000 | | Daribha | 504000 | | Tanya | 600000 | +----------+-----------+ 10 rows in set (0.02 sec) Observe that in the output, Salary*12 is displayed as the column name for the annual salary column. In the output table, we can use alias to rename that column as Annual Salary as shown below: mysql> SELECT Ename AS Name, Salary*12 AS -> 'Annual Salary' -> FROM EMPLOYEE; +----------+---------------+ | Name | Annual Salary | +----------+---------------+ | Aaliya | 120000 | | Kritika | 720000 | | Shabbir | 540000 | | Gurpreet | 228000 | | Joseph | 408000 | | Sanya | 576000 | | Vergese | 180000 | | Nachaobi | 348000 | | Daribha | 504000 | | Tanya | 600000 | +----------+---------------+ 10 rows in set (0.00 sec) 2024-25 Chap 8.indd 159 19-Jul-19 3:45:58 PM  160 Informatics Practices – Class XI Notes Note: i) Annual Salary will not be added as a new column in the database table. It is just for displaying the output of the query. ii) If an aliased column name has space as in the case of Annual Salary, it should be enclosed in quotes as 'Annual Salary'. (C) DISTINCT Clause By default, SQL shows all the data retrieved through query as output. However, there can be duplicate values. The SELECT statement when combined with DISTINCT clause, returns records without repetition (distinct records). For example, while retrieving employee’s department number, there can be duplicate values as many employees are assigned to same department. To display unique department number for all the employees, we use DISTINCT as shown below: mysql> SELECT DISTINCT DeptId -> FROM EMPLOYEE; +--------+ | DeptId | +--------+ | D02 | | D01 | | D04 | | D03 | | D05 | +--------+ 5 rows in set (0.03 sec) (D) WHERE Clause The WHERE clause is used to retrieve data that meet some specified conditions. In the OFFICE database, more than one employee can have the same salary. To display distinct salaries of the employees working in the department number D01, we write the following query in which the condition to select the employee whose department number is D01 is specified using the WHERE clause: mysql> SELECT DISTINCT Salary -> FROM EMPLOYEE -> WHERE Deptid='D01'; As the column DeptId is of string type, its values are enclosed in quotes ('D01'). +--------+ | Salary | +--------+ | 60000 | | 45000 | | 15000 | +--------+ 3 rows in set (0.02 sec) 2024-25 Chap 8.indd 160 19-Jul-19 3:45:58 PM  Introduction to Structured Query Language (SQL) 161 In the above example, we have used = operator in WHERE clause. We can also use other relational operators (=, !=) to specify conditions. The logical operators AND, OR, and NOT are used with WHERE clause to combine multiple conditions. Example 8.4 Display all the employees who are earning more than 5000 and work in department with DeptId D04. mysql> SELECT * -> FROM EMPLOYEE -> WHERE Salary > 5000 AND DeptId = 'D04'; +-------+----------+--------+-------+--------+ | EmpNo | Ename | Salary | Bonus | DeptId | +-------+----------+--------+-------+--------+ | 104 | Gurpreet | 19000 | 565 | D04 | | 109 | Daribha | 42000 | NULL | D04 | +-------+----------+--------+-------+--------+ 2 rows in set (0.00 sec) Example 8.5 The following query displays records of all the Think and Reflect employees except Aaliya. What will happen if mysql> SELECT * in the above query -> FROM EMPLOYEE we write “Aaliya” as -> WHERE NOT Ename = 'Aaliya'; “AALIYA” or “aaliya” +-------+----------+--------+-------+--------+ or “AaLIYA”? Will the | EmpNo | Ename | Salary | Bonus | DeptId | query generate the same +-------+----------+--------+-------+--------+ output or an error? | 102 | Kritika | 60000 | 123 | D01 | | 103 | Shabbir | 45000 | 566 | D01 | | 104 | Gurpreet | 19000 | 565 | D04 | | 105 | Joseph | 34000 | 875 | D03 | | 106 | Sanya | 48000 | 695 | D02 | | 107 | Vergese | 15000 | NULL | D01 | | 108 | Nachaobi | 29000 | NULL | D05 | | 109 | Daribha | 42000 | NULL | D04 | | 110 | Tanya | 50000 | 467 | D05 | +-------+----------+--------+-------+--------+ 9 rows in set (0.00 sec) Example 8.6 The following query displays name and department number of all those employees who are earning Activity 8.8 salary between 20000 and 50000 (both values inclusive). Compare the output mysql> SELECT Ename, DeptId produced by the query -> FROM EMPLOYEE in example 8.6 and -> WHERE Salary>=20000 AND Salary 5000 OR | Nachaobi | D05 | DeptId= 20; | Daribha | D04 | | Tanya | D05 | +----------+--------+ 6 rows in set (0.00 sec) 2024-25 Chap 8.indd 161 19-Jul-19 3:45:58 PM  162 Informatics Practices – Class XI Notes The above query defines a range that can also be checked using a comparison operator BETWEEN. mysql> SELECT Ename, DeptId -> FROM EMPLOYEE -> WHERE Salary BETWEEN 20000 AND 50000; +----------+--------+ | Ename | DeptId | +----------+--------+ | Shabbir | D01 | | Joseph | D03 | | Sanya | D02 | | Nachaobi | D05 | | Daribha | D04 | | Tanya | D05 | +----------+--------+ 6 rows in set (0.03 sec) Note: The BETWEEN operator defines the range of values in which the column value must fall into, to make the condition true. Example 8.7 The following query displays details of all the employees who are working either in DeptId D01, D02 or D04. mysql> SELECT * -> FROM EMPLOYEE -> WHERE DeptId = 'D01' OR DeptId = 'D02' OR -> DeptId = 'D04'; +-------+----------+--------+-------+--------+ | EmpNo | Ename | Salary | Bonus | DeptId | +-------+----------+--------+-------+--------+ | 101 | Aaliya | 10000 | 234 | D02 | | 102 | Kritika | 60000 | 123 | D01 | | 103 | Shabbir | 45000 | 566 | D01 | | 104 | Gurpreet | 19000 | 565 | D04 | | 106 | Sanya | 48000 | 695 | D02 | | 107 | Vergese | 15000 | NULL | D01 | | 109 | Daribha | 42000 | NULL | D04 | +-------+----------+--------+-------+--------+ 7 rows in set (0.00 sec) (E) MEMBERSHIP OPERATOR IN The IN operator compares a value with a set of values and returns true if the value belongs to that set. The above query can be rewritten using IN operator as shown below: mysql> SELECT * -> FROM EMPLOYEE -> WHERE DeptId IN ('D01', 'D02' , 'D04'); +-------+----------+--------+-------+--------+ | EmpNo | Ename | Salary | Bonus | DeptId | +-------+----------+--------+-------+--------+ | 101 | Aaliya | 10000 | 234 | D02 | | 102 | Kritika | 60000 | 123 | D01 | | 103 | Shabbir | 45000 | 566 | D01 | | 104 | Gurpreet | 19000 | 565 | D04 | | 106 | Sanya | 48000 | 695 | D02 | | 107 | Vergese | 15000 | NULL | D01 | | 109 | Daribha | 42000 | NULL | D04 | +-------+----------+--------+-------+--------+ 7 rows in set (0.00 sec) 2024-25 Chap 8.indd 162 19-Jul-19 3:45:58 PM  Introduction to Structured Query Language (SQL) 163 Example 8.8 The following query displays details of all the Notes employees except those working in department number D01 or D02. mysql> SELECT * -> FROM EMPLOYEE -> WHERE DeptId NOT IN('D01', 'D02'); +-------+----------+--------+-------+--------+ | EmpNo | Ename | Salary | Bonus | DeptId | +-------+----------+--------+-------+--------+ | 104 | Gurpreet | 19000 | 565 | D04 | | 105 | Joseph | 34000 | 875 | D03 | | 108 | Nachaobi | 29000 | NULL | D05 | | 109 | Daribha | 42000 | NULL | D04 | | 110 | Tanya | 50000 | 467 | D05 | +-------+----------+--------+-------+--------+ 5 rows in set (0.00 sec) Note: Here we need to combine NOT with IN as we want to retrieve all records except with DeptId D01 and D02. (F) ORDER BY Clause ORDER BY clause is used to display data in an ordered (arranged) form with respect to a specified column. By default, ORDER BY displays records in ascending order of the specified column’s values. To display the records in descending order, the DESC (means descending) keyword needs to be written with that column. Example 8.9 The following query displays details of all the employees in ascending order of their salaries. mysql> SELECT * -> FROM EMPLOYEE -> ORDER BY Salary; +-------+----------+--------+-------+--------+ | EmpNo | Ename | Salary | Bonus | DeptId | +-------+----------+--------+-------+--------+ | 101 | Aaliya | 10000 | 234 | D02 | | 107 | Vergese | 15000 | NULL | D01 | | 104 | Gurpreet | 19000 | 565 | D04 | | 108 | Nachaobi | 29000 | NULL | D05 | | 105 | Joseph | 34000 | 875 | D03 | | 109 | Daribha | 42000 | NULL | D04 | | 103 | Shabbir | 45000 | 566 | D01 | | 106 | Sanya | 48000 | 695 | D02 | | 110 | Tanya | 50000 | 467 | D05 | | 102 | Kritika | 60000 | 123 | D01 | +-------+----------+--------+-------+--------+ 10 rows in set (0.05 sec) Example 8.10 The following query displays details of all the employees in descending order of their salaries. mysql> SELECT * -> FROM EMPLOYEE -> ORDER BY Salary DESC; 2024-25 Chap 8.indd 163 19-Jul-19 3:45:58 PM  164 Informatics Practices – Class XI +-------+----------+--------+-------+--------+ | EmpNo | Ename | Salary | Bonus | DeptId | +-------+----------+--------+-------+--------+ | 102 | Kritika | 60000 | 123 | D01 | | 110 | Tanya | 50000 | 467 | D05 | | 106 | Sanya | 48000 | 695 | D02 | | 103 | Shabbir | 45000 | 566 | D01 | | 109 | Daribha | 42000 | NULL | D04 | | 105 | Joseph | 34000 | 875 | D03 | | 108 | Nachaobi | 29000 | NULL | D05 | | 104 | Gurpreet | 19000 | 565 | D04 | | 107 | Vergese | 15000 | NULL | D01 | | 101 | Aaliya | 10000 | 234 | D02 | +-------+----------+--------+-------+--------+ 10 rows in set (0.00 sec) (G) Handling NULL Values SQL supports a special value called NULL to represent a missing or unknown value. For example, the village column in a table called address will have no value for cities. Hence, NULL is used to represent such unknown values. It is important to note that NULL is different from 0 (zero). Also, any arithmetic operation performed with NULL value gives NULL. For example: 5 + NULL = NULL because NULL is unknown hence the result is also unknown. In order to check for NULL value in a column, Activity 8.9 we use IS NULL. Execute the following two queries and find Example 8.11 The following query displays details of all out what will happen if those employees who have not been given a bonus. This we specify two columns implies that the bonus column will be blank. in the ORDER BY clause: mysql> SELECT * -> FROM EMPLOYEE SELECT * -> WHERE Bonus IS NULL; FROM EMPLOYEE ORDER BY Salary, +-------+----------+--------+-------+--------+ Bonus; | EmpNo | Ename | Salary | Bonus | DeptId | +-------+----------+--------+-------+--------+ SELECT * | 107 | Vergese | 15000 | NULL | D01 | | 108 | Nachaobi | 29000 | NULL | D05 | FROM EMPLOYEE | 109 | Daribha | 42000 | NULL | D04 | ORDER BY Salary,Bonus +-------+----------+--------+-------+--------+ desc; 3 rows in set (0.00 sec) Example 8.12 The following query displays names of all the employees who have been given a bonus. This implies that the bonus column will not be blank. mysql> SELECT EName -> FROM EMPLOYEE -> WHERE Bonus IS NOT NULL; +----------+ | EName | +----------+ | Aaliya | | Kritika | | Shabbir | | Gurpreet | | Joseph | | Sanya | | Tanya | +----------+ 7 rows in set (0.00 sec) 2024-25 Chap 8.indd 164 19-Jul-19 3:45:58 PM  Introduction to Structured Query Language (SQL) 165 (H) Substring pattern matching Notes Many a times we come across situations where we don’t want to query by matching exact text or value. Rather, we are interested to find matching of only a few characters or values in column values. For example, to find out names starting with ‘T’ or to find out pin codes starting with ‘60’. This is called substring pattern matching. We cannot match such patterns using = operator as we are not looking for exact match. SQL provides LIKE operator that can be used with WHERE clause to search for a specified pattern in a column. The LIKE operator makes use of the following two wild card characters: % (percent)— used to represent zero, one, or multiple characters _ (underscore)— used to represent a single character Example 8.13 The following query displays details of all those employees whose name starts with 'K'. mysql> SELECT * -> FROM EMPLOYEE -> WHERE Ename LIKE 'K%'; +-------+---------+--------+-------+--------+ | EmpNo | Ename | Salary | Bonus | DeptId | +-------+---------+--------+-------+--------+ | 102 | Kritika | 60000 | 123 | D01 | +-------+---------+--------+-------+--------+ 1 row in set (0.00 sec) Example 8.14 The following query displays details of all those employees whose name ends with 'a'. mysql> SELECT * -> FROM EMPLOYEE -> WHERE Ename LIKE '%a'; +-------+---------+--------+-------+--------+ | EmpNo | Ename | Salary | Bonus | DeptId | +-------+---------+--------+-------+--------+ | 101 | Aaliya | 10000 | 234 | D02 | | 102 | Kritika | 60000 | 123 | D01 | | 106 | Sanya | 48000 | 695 | D02 | | 109 | Daribha | 42000 | NULL | D04 | | 110 | Tanya | 50000 | 467 | D05 | +-------+---------+--------+-------+--------+ 5 rows in set (0.00 sec) Example 8.15 The following query displays details of all those employees whose name consists of exactly 5 letters and starts with any letter but has ‘ANYA’ after that. mysql> SELECT * 2024-25 Chap 8.indd 165 19-Jul-19 3:45:58 PM  166 Informatics Practices – Class XI -> FROM EMPLOYEE -> WHERE Ename LIKE '_ANYA'; Think and Reflect +-------+-------+--------+-------+--------+ | EmpNo | Ename | Salary | Bonus | DeptId | When we type first letter +-------+-------+--------+-------+--------+ of a contact name in | 106 | Sanya | 48000 | 695 | D02 | our contact list in our | 110 | Tanya | 50000 | 467 | D05 | mobile phones all the +-------+-------+--------+-------+--------+ names containing that 2 rows in set (0.00 sec) character are displayed. Can you relate SQL Example 8.16 The following query displays names of all the statement with the employees containing 'se' as a substring in name. process? List other real mysql> SELECT Ename life situations where you -> FROM EMPLOYEE can visualize an SQL -> WHERE Ename LIKE '%se%'; statement in operation. +---------+ | Ename | +---------+ | Joseph | | Vergese | +---------+ 2 rows in set (0.00 sec) Example 8.17 The following query displays names of all employees containing 'a' as the second character. mysql> SELECT EName -> FROM EMPLOYEE -> WHERE Ename LIKE '_a%'; +----------+ | EName | +----------+ | Aaliya | | Sanya | | Nachaobi | | Daribha | | Tanya | +----------+ 5 rows in set (0.00 sec) 8.7 Data Updation and Deletion Updation and deletion of data are also the parts of SQL data manipulation. In this section, we are going to apply these two data manipulation methods. 8.7.1 Data Updation We may need to make changes in the value(s) of one or more columns of existing records in a table. For example, we may require some changes in address, phone number or spelling of name, etc. The UPDATE statement is used to make such modifications in the existing data. Syntax: UPDATE table_name SET attribute1 = value1, attribute2 = value2,... 2024-25 Chap 8.indd 166 19-Jul-19 3:45:58 PM  Introduction to Structured Query Language (SQL) 167 WHERE condition; The STUDENT Table 8.7 has NULL value for GUID for student with roll number 3. Also, suppose students with roll numbers 3 and 5 are siblings. So, in STUDENT table, we need to fill the GUID value for student with roll number 3 as 101010101010. In order to update or change GUID of a particular row (record), we need to specify that record using WHERE clause, as shown below: mysql> UPDATE STUDENT -> SET GUID = 101010101010 -> WHERE RollNumber = 3; Query OK, 1 row affected (0.06 sec) Rows matched: 1 Changed: 1 Warnings: 0 We can then verify the updated data using the statement SELECT * FROM STUDENT. Caution : If we miss the where clause in the UPDATE statement then the GUID of all the records will be changed to 101010101010. We can also update values for more than one column using the UPDATE statement. Suppose, the guardian (Table 8.6) with GUID 466444444666 has requested to change the Address to 'WZ - 68, Azad Avenue, Bijnour, MP' and Phone number to '4817362092'. mysql> UPDATE GUARDIAN -> SET GAddress = 'WZ - 68, Azad Avenue, -> Bijnour, MP', GPhone = 9010810547 -> WHERE GUID = 466444444666; Query OK, 1 row affected (0.06 sec) Rows matched: 1 Changed: 1 Warnings: 0 mysql> SELECT * FROM GUARDIAN ; +------------+---------------+----------+------------------------------------+ |GUID |GName |Gphone |GAddress | +------------+---------------+----------+------------------------------------+ |444444444444|Amit Ahuja |5711492685|G-35, Ashok vihar, Delhi | |111111111111|Baichung Bhutia|3612967082|Flat no. 5, Darjeeling Appt., Shimla| |101010101010|Himanshu Shah |4726309212|26/77, West Patel Nagar, Ahmedabad | |333333333333|Danny Dsouza |NULL |S -13, Ashok Village, Daman | |466444444666|Sujata P. |3801923168|WZ - 68, Azad Avenue, Bijnour, MP | +------------+---------------+----------+------------------------------------+ 5 rows in set (0.00 sec) 8.7.2 Data Deletion The DELETE statement is used to delete one or more record(s) from a table. Syntax: DELETE FROM table_name