Introduction to SQL.pdf

Chapter 3: Introduction to SQL Database System Concepts, 7th Ed. ©Silberschatz, Korth and Sudarshan See www.db-book.com for conditions on re-use  Outline  Overview of The SQL Query Language  SQL Data D...

Chapter 3: Introduction to SQL Database System Concepts, 7th Ed. ©Silberschatz, Korth and Sudarshan See www.db-book.com for conditions on re-use  Outline  Overview of The SQL Query Language  SQL Data Definition  Basic Query Structure of SQL Queries  Additional Basic Operations  Set Operations  Null Values  Aggregate Functions  Nested Subqueries  Modification of the Database Database System Concepts - 7th Edition 3.2 ©Silberschatz, Korth and Sudarshan  History  IBM Sequel language developed as part of System R project at the IBM San Jose Research Laboratory  Renamed Structured Query Language (SQL)  ANSI and ISO standard SQL: SQL-86 SQL-89 SQL-92 SQL:1999 (language name became Y2K compliant!) SQL:2003  Commercial systems offer most, if not all, SQL-92 features, plus varying feature sets from later standards and special proprietary features. Not all examples here may work on your particular system. Database System Concepts - 7th Edition 3.3 ©Silberschatz, Korth and Sudarshan  SQL Parts  DML -- provides the ability to query information from the database and to insert tuples into, delete tuples from, and modify tuples in the database.  integrity – the DDL includes commands for specifying integrity constraints.  View definition -- The DDL includes commands for defining views.  Transaction control –includes commands for specifying the beginning and ending of transactions.  Embedded SQL and dynamic SQL -- define how SQL statements can be embedded within general-purpose programming languages.  Authorization – includes commands for specifying access rights to relations and views. Database System Concepts - 7th Edition 3.4 ©Silberschatz, Korth and Sudarshan  Data Definition Language The SQL data-definition language (DDL) allows the specification of information about relations, including:  The schema for each relation.  The type of values associated with each attribute.  The Integrity constraints  The set of indices to be maintained for each relation.  Security and authorization information for each relation.  The physical storage structure of each relation on disk. Database System Concepts - 7th Edition 3.5 ©Silberschatz, Korth and Sudarshan  Domain Types in SQL  char(n). Fixed length character string, with user-specified length n.  varchar(n). Variable length character strings, with user-specified maximum length n.  int. Integer (a finite subset of the integers that is machine-dependent).  smallint. Small integer (a machine-dependent subset of the integer domain type).  numeric(p,d). Fixed point number, with user-specified precision of p digits, with d digits to the right of decimal point. (ex., numeric(3,1), allows 44.5 to be stores exactly, but not 444.5 or 0.32)  real, double precision. Floating point and double-precision floating point numbers, with machine-dependent precision.  float(n). Floating point number, with user-specified precision of at least n digits.  More are covered in Chapter 4. Database System Concepts - 7th Edition 3.6 ©Silberschatz, Korth and Sudarshan  Create Table Construct  An SQL relation is defined using the create table command: create table r (A1 D1, A2 D2,..., An Dn, (integrity-constraint1),..., (integrity-constraintk)) r is the name of the relation each Ai is an attribute name in the schema of relation r Di is the data type of values in the domain of attribute Ai  Example: create table instructor ( ID char(5), name varchar(20), dept_name varchar(20), salary numeric(8,2)) Database System Concepts - 7th Edition 3.7 ©Silberschatz, Korth and Sudarshan  Integrity Constraints in Create Table  Types of integrity constraints primary key (A1,..., An ) foreign key (Am,..., An ) references r not null  SQL prevents any update to the database that violates an integrity constraint.  Example: create table instructor ( ID char(5), name varchar(20) not null, dept_name varchar(20), salary numeric(8,2), primary key (ID), foreign key (dept_name) references department); Database System Concepts - 7th Edition 3.8 ©Silberschatz, Korth and Sudarshan  And a Few More Relation Definitions  create table student ( ID varchar(5), name varchar(20) not null, dept_name varchar(20), tot_cred numeric(3,0), primary key (ID), foreign key (dept_name) references department);  create table takes ( ID varchar(5), course_id varchar(8), sec_id varchar(8), semester varchar(6), year numeric(4,0), grade varchar(2), primary key (ID, course_id, sec_id, semester, year) , foreign key (ID) references student, foreign key (course_id, sec_id, semester, year) references section); Database System Concepts - 7th Edition 3.9 ©Silberschatz, Korth and Sudarshan  And more still  create table course ( course_id varchar(8), title varchar(50), dept_name varchar(20), credits numeric(2,0), primary key (course_id), foreign key (dept_name) references department); Database System Concepts - 7th Edition 3.10 ©Silberschatz, Korth and Sudarshan  Updates to tables  Insert insert into instructor values ('10211', 'Smith', 'Biology', 66000);  Delete Remove all tuples from the student relation  delete from student  Drop Table drop table r  Alter alter table r add A D  where A is the name of the attribute to be added to relation r and D is the domain of A.  All exiting tuples in the relation are assigned null as the value for the new attribute. alter table r drop A  where A is the name of an attribute of relation r  Dropping of attributes not supported by many databases. Database System Concepts - 7th Edition 3.11 ©Silberschatz, Korth and Sudarshan  Basic Query Structure  A typical SQL query has the form: select A1, A2,..., An from r1, r2,..., rm where P Ai represents an attribute Ri represents a relation P is a predicate.  The result of an SQL query is a relation. Database System Concepts - 7th Edition 3.12 ©Silberschatz, Korth and Sudarshan  The select Clause  The select clause lists the attributes desired in the result of a query corresponds to the projection operation of the relational algebra  Example: find the names of all instructors: select name from instructor  NOTE: SQL names are case insensitive (i.e., you may use upper- or lower-case letters.) E.g., Name ≡ NAME ≡ name Some people use upper case wherever we use bold font. Database System Concepts - 7th Edition 3.13 ©Silberschatz, Korth and Sudarshan  The select Clause (Cont.)  SQL allows duplicates in relations as well as in query results.  To force the elimination of duplicates, insert the keyword distinct after select.  Find the department names of all instructors, and remove duplicates select distinct dept_name from instructor  The keyword all specifies that duplicates should not be removed. select all dept_name from instructor Database System Concepts - 7th Edition 3.14 ©Silberschatz, Korth and Sudarshan  The select Clause (Cont.)  An asterisk in the select clause denotes “all attributes” select * from instructor  An attribute can be a literal with no from clause select '437' Results is a table with one column and a single row with value “437” Can give the column a name using: select '437' as FOO  An attribute can be a literal with from clause select 'A' from instructor Result is a table with one column and N rows (number of tuples in the instructors table), each row with value “A” Database System Concepts - 7th Edition 3.15 ©Silberschatz, Korth and Sudarshan  The select Clause (Cont.)  The select clause can contain arithmetic expressions involving the operation, +, –, ∗, and /, and operating on constants or attributes of tuples. The query: select ID, name, salary/12 from instructor would return a relation that is the same as the instructor relation, except that the value of the attribute salary is divided by 12. Can rename “salary/12” using the as clause: select ID, name, salary/12 as monthly_salary Database System Concepts - 7th Edition 3.16 ©Silberschatz, Korth and Sudarshan  The where Clause  The where clause specifies conditions that the result must satisfy Corresponds to the selection predicate of the relational algebra.  To find all instructors in Comp. Sci. dept select name from instructor where dept_name = 'Comp. Sci.'  SQL allows the use of the logical connectives and, or, and not  The operands of the logical connectives can be expressions involving the comparison operators =, =, and.  Comparisons can be applied to results of arithmetic expressions  To find all instructors in Comp. Sci. dept with salary > 80000 select name from instructor where dept_name = 'Comp. Sci.' and salary > 80000 Database System Concepts - 7th Edition 3.17 ©Silberschatz, Korth and Sudarshan  The from Clause  The from clause lists the relations involved in the query Corresponds to the Cartesian product operation of the relational algebra.  Find the Cartesian product instructor X teaches select ∗ from instructor, teaches generates every possible instructor – teaches pair, with all attributes from both relations. For common attributes (e.g., ID), the attributes in the resulting table are renamed using the relation name (e.g., instructor.ID)  Cartesian product not very useful directly, but useful combined with where-clause condition (selection operation in relational algebra). Database System Concepts - 7th Edition 3.18 ©Silberschatz, Korth and Sudarshan  Examples  Find the names of all instructors who have taught some course and the course_id select name, course_id from instructor , teaches where instructor.ID = teaches.ID  Find the names of all instructors in the Art department who have taught some course and the course_id select name, course_id from instructor , teaches where instructor.ID = teaches.ID and instructor. dept_name = 'Art' Database System Concepts - 7th Edition 3.19 ©Silberschatz, Korth and Sudarshan  The Rename Operation  The SQL allows renaming relations and attributes using the as clause: old-name as new-name  Find the names of all instructors who have a higher salary than some instructor in 'Comp. Sci'. select distinct T.name from instructor as T, instructor as S where T.salary > S.salary and S.dept_name = 'Comp. Sci.’  Keyword as is optional and may be omitted instructor as T ≡ instructor T Database System Concepts - 7th Edition 3.20 ©Silberschatz, Korth and Sudarshan  Self Join Example  Relation emp-super  Find the supervisor of “Bob”  Find the supervisor of the supervisor of “Bob”  Can you find ALL the supervisors (direct and indirect) of “Bob”? Database System Concepts - 7th Edition 3.21 ©Silberschatz, Korth and Sudarshan  String Operations  SQL includes a string-matching operator for comparisons on character strings. The operator like uses patterns that are described using two special characters: percent ( % ). The % character matches any substring. underscore ( _ ). The _ character matches any character.  Find the names of all instructors whose name includes the substring “dar”. select name from instructor where name like '%dar%'  Match the string “100%” like '100 \%' escape '\' in that above we use backslash (\) as the escape character. Database System Concepts - 7th Edition 3.22 ©Silberschatz, Korth and Sudarshan  String Operations (Cont.)  Patterns are case sensitive.  Pattern matching examples: 'Intro%' matches any string beginning with “Intro”. '%Comp%' matches any string containing “Comp” as a substring. '_ _ _' matches any string of exactly three characters. '_ _ _ %' matches any string of at least three characters.  SQL supports a variety of string operations such as concatenation (using “||”) converting from upper to lower case (and vice versa) finding string length, extracting substrings, etc. Database System Concepts - 7th Edition 3.23 ©Silberschatz, Korth and Sudarshan  Ordering the Display of Tuples  List in alphabetic order the names of all instructors select distinct name from instructor order by name  We may specify desc for descending order or asc for ascending order, for each attribute; ascending order is the default. Example: order by name desc  Can sort on multiple attributes Example: order by dept_name, name Database System Concepts - 7th Edition 3.24 ©Silberschatz, Korth and Sudarshan  Where Clause Predicates  SQL includes a between comparison operator  Example: Find the names of all instructors with salary between $90,000 and $100,000 (that is, ≥ $90,000 and ≤ $100,000) select name from instructor where salary between 90000 and 100000  Tuple comparison select name, course_id from instructor, teaches where (instructor.ID, dept_name) = (teaches.ID, 'Biology'); Database System Concepts - 7th Edition 3.25 ©Silberschatz, Korth and Sudarshan  Set Operations  Find courses that ran in Fall 2017 or in Spring 2018 (select course_id from section where sem = 'Fall' and year = 2017) union (select course_id from section where sem = 'Spring' and year = 2018)  Find courses that ran in Fall 2017 and in Spring 2018 (select course_id from section where sem = 'Fall' and year = 2017) intersect (select course_id from section where sem = 'Spring' and year = 2018)  Find courses that ran in Fall 2017 but not in Spring 2018 (select course_id from section where sem = 'Fall' and year = 2017) except (select course_id from section where sem = 'Spring' and year = 2018) Database System Concepts - 7th Edition 3.26 ©Silberschatz, Korth and Sudarshan  Set Operations (Cont.)  Set operations union, intersect, and except Each of the above operations automatically eliminates duplicates  To retain all duplicates use the union all, intersect all except all. Database System Concepts - 7th Edition 3.27 ©Silberschatz, Korth and Sudarshan  Null Values  It is possible for tuples to have a null value, denoted by null, for some of their attributes  null signifies an unknown value or that a value does not exist.  The result of any arithmetic expression involving null is null Example: 5 + null returns null  The predicate is null can be used to check for null values. Example: Find all instructors whose salary is null. select name from instructor where salary is null  The predicate is not null succeeds if the value on which it is applied is not null. Database System Concepts - 7th Edition 3.28 ©Silberschatz, Korth and Sudarshan  Null Values (Cont.)  SQL treats as unknown the result of any comparison involving a null value (other than predicates is null and is not null). Example: 5 < null or null null or null = null  The predicate in a where clause can involve Boolean operations (and, or, not); thus the definitions of the Boolean operations need to be extended to deal with the value unknown. and : (true and unknown) = unknown, (false and unknown) = false, (unknown and unknown) = unknown or: (unknown or true) = true, (unknown or false) = unknown (unknown or unknown) = unknown  Result of where clause predicate is treated as false if it evaluates to unknown Database System Concepts - 7th Edition 3.29 ©Silberschatz, Korth and Sudarshan  Aggregate Functions  These functions operate on the multiset of values of a column of a relation, and return a value avg: average value min: minimum value max: maximum value sum: sum of values count: number of values Database System Concepts - 7th Edition 3.30 ©Silberschatz, Korth and Sudarshan  Aggregate Functions Examples  Find the average salary of instructors in the Computer Science department select avg (salary) from instructor where dept_name= 'Comp. Sci.';  Find the total number of instructors who teach a course in the Spring 2018 semester select count (distinct ID) from teaches where semester = 'Spring' and year = 2018;  Find the number of tuples in the course relation select count (*) from course; Database System Concepts - 7th Edition 3.31 ©Silberschatz, Korth and Sudarshan  Aggregate Functions – Group By  Find the average salary of instructors in each department select dept_name, avg (salary) as avg_salary from instructor group by dept_name; Database System Concepts - 7th Edition 3.32 ©Silberschatz, Korth and Sudarshan  Aggregation (Cont.)  Attributes in select clause outside of aggregate functions must appear in group by list select dept_name, ID, avg (salary) from instructor group by dept_name; Database System Concepts - 7th Edition 3.33 ©Silberschatz, Korth and Sudarshan  Aggregate Functions – Having Clause  Find the names and average salaries of all departments whose average salary is greater than 42000 select dept_name, avg (salary) as avg_salary from instructor group by dept_name having avg (salary) > 42000;  Note: predicates in the having clause are applied after the formation of groups whereas predicates in the where clause are applied before forming groups Database System Concepts - 7th Edition 3.34 ©Silberschatz, Korth and Sudarshan  Nested Subqueries  SQL provides a mechanism for the nesting of subqueries. A subquery is a select-from-where expression that is nested within another query.  The nesting can be done in the following SQL query select A1, A2,..., An from r1, r2,..., rm where P as follows: From clause: ri can be replaced by any valid subquery Where clause: P can be replaced with an expression of the form: B (subquery) B is an attribute and to be defined later. Select clause: Ai can be replaced be a subquery that generates a single value. Database System Concepts - 7th Edition 3.35 ©Silberschatz, Korth and Sudarshan  Set Membership Database System Concepts - 7th Edition 3.36 ©Silberschatz, Korth and Sudarshan  Set Membership  Find courses offered in Fall 2017 and in Spring 2018 select distinct course_id from section where semester = 'Fall' and year= 2017 and course_id in (select course_id from section where semester = 'Spring' and year= 2018);  Find courses offered in Fall 2017 but not in Spring 2018 select distinct course_id from section where semester = 'Fall' and year= 2017 and course_id not in (select course_id from section where semester = 'Spring' and year= 2018); Database System Concepts - 7th Edition 3.37 ©Silberschatz, Korth and Sudarshan  Set Membership (Cont.)  Name all instructors whose name is neither “Mozart” nor Einstein” select distinct name from instructor where name not in ('Mozart', 'Einstein')  Find the total number of (distinct) students who have taken course sections taught by the instructor with ID 10101 select count (distinct ID) from takes where (course_id, sec_id, semester, year) in (select course_id, sec_id, semester, year from teaches where teaches.ID= 10101);  Note: Above query can be written in a much simpler manner. The formulation above is simply to illustrate SQL features Database System Concepts - 7th Edition 3.38 ©Silberschatz, Korth and Sudarshan  Set Comparison Database System Concepts - 7th Edition 3.39 ©Silberschatz, Korth and Sudarshan  Set Comparison – “some” Clause  Find names of instructors with salary greater than that of some (at least one) instructor in the Biology department. select distinct T.name from instructor as T, instructor as S where T.salary > S.salary and S.dept name = 'Biology';  Same query using > some clause select name from instructor where salary > some (select salary from instructor where dept name = 'Biology'); Database System Concepts - 7th Edition 3.40 ©Silberschatz, Korth and Sudarshan  Definition of “some” Clause  F some r ⇔ ∃ t ∈ r such that (F t ) Where can be: , =, ≠ 0 (5 < some 5 ) = true (read: 5 < some tuple in the relation) 6 0 (5 < some 5 ) = false 0 (5 = some 5 ) = true 0 (5 ≠ some 5 ) = true (since 0 ≠ 5) (= some) ≡ in However, (≠ some) ≡ not in Database System Concepts - 7th Edition 3.41 ©Silberschatz, Korth and Sudarshan  Set Comparison – “all” Clause  Find the names of all instructors whose salary is greater than the salary of all instructors in the Biology department. select name from instructor where salary > all (select salary from instructor where dept name = 'Biology'); Database System Concepts - 7th Edition 3.42 ©Silberschatz, Korth and Sudarshan  Definition of “all” Clause  F all r ⇔ ∀ t ∈ r (F t) 0 (5 < all 5 ) = false 6 6 (5 < all 10 ) = true 4 (5 = all 5 ) = false 4 (5 ≠ all 6 ) = true (since 5 ≠ 4 and 5 ≠ 6) (≠ all) ≡ not in However, (= all) ≡ in Database System Concepts - 7th Edition 3.43 ©Silberschatz, Korth and Sudarshan  Test for Empty Relations  The exists construct returns the value true if the argument subquery is nonempty.  exists r ⇔ r ≠ Ø  not exists r ⇔ r = Ø Database System Concepts - 7th Edition 3.44 ©Silberschatz, Korth and Sudarshan  Use of “exists” Clause  Yet another way of specifying the query “Find all courses taught in both the Fall 2017 semester and in the Spring 2018 semester” select course_id from section as S where semester = 'Fall' and year = 2017 and exists (select * from section as T where semester = 'Spring' and year= 2018 and S.course_id = T.course_id);  Correlation name – variable S in the outer query  Correlated subquery – the inner query Database System Concepts - 7th Edition 3.45 ©Silberschatz, Korth and Sudarshan  Use of “not exists” Clause  Find all students who have taken all courses offered in the Biology department. select distinct S.ID, S.name from student as S where not exists ( (select course_id from course where dept_name = 'Biology') except (select T.course_id from takes as T where S.ID = T.ID)); First nested query lists all courses offered in Biology Second nested query lists all courses a particular student took  Note that X – Y = Ø ⇔ X ⊆ Y  Note: Cannot write this query using = all and its variants Database System Concepts - 7th Edition 3.46 ©Silberschatz, Korth and Sudarshan  Test for Absence of Duplicate Tuples  The unique construct tests whether a subquery has any duplicate tuples in its result.  The unique construct evaluates to “true” if a given subquery contains no duplicates.  Find all courses that were offered at most once in 2017 select T.course_id from course as T where unique ( select R.course_id from section as R where T.course_id= R.course_id and R.year = 2017); Database System Concepts - 7th Edition 3.47 ©Silberschatz, Korth and Sudarshan  Subqueries in the From Clause Database System Concepts - 7th Edition 3.48 ©Silberschatz, Korth and Sudarshan  Subqueries in the Form Clause  SQL allows a subquery expression to be used in the from clause  Find the average instructors’ salaries of those departments where the average salary is greater than $42,000.” select dept_name, avg_salary from ( select dept_name, avg (salary) as avg_salary from instructor group by dept_name) where avg_salary > 42000;  Note that we do not need to use the having clause  Another way to write above query select dept_name, avg_salary from ( select dept_name, avg (salary) from instructor group by dept_name) as dept_avg (dept_name, avg_salary) where avg_salary > 42000; Database System Concepts - 7th Edition 3.49 ©Silberschatz, Korth and Sudarshan  With Clause  The with clause provides a way of defining a temporary relation whose definition is available only to the query in which the with clause occurs.  Find all departments with the maximum budget with max_budget (value) as (select max(budget) from department) select department.name from department, max_budget where department.budget = max_budget.value; Database System Concepts - 7th Edition 3.50 ©Silberschatz, Korth and Sudarshan  Complex Queries using With Clause  Find all departments where the total salary is greater than the average of the total salary at all departments with dept _total (dept_name, value) as (select dept_name, sum(salary) from instructor group by dept_name), dept_total_avg(value) as (select avg(value) from dept_total) select dept_name from dept_total, dept_total_avg where dept_total.value > dept_total_avg.value; Database System Concepts - 7th Edition 3.51 ©Silberschatz, Korth and Sudarshan  Scalar Subquery  Scalar subquery is one which is used where a single value is expected  List all departments along with the number of instructors in each department select dept_name, ( select count(*) from instructor where department.dept_name = instructor.dept_name) as num_instructors from department;  Runtime error if subquery returns more than one result tuple Database System Concepts - 7th Edition 3.52 ©Silberschatz, Korth and Sudarshan  Modification of the Database  Deletion of tuples from a given relation.  Insertion of new tuples into a given relation  Updating of values in some tuples in a given relation Database System Concepts - 7th Edition 3.53 ©Silberschatz, Korth and Sudarshan  Deletion  Delete all instructors delete from instructor  Delete all instructors from the Finance department delete from instructor where dept_name= 'Finance’;  Delete all tuples in the instructor relation for those instructors associated with a department located in the Watson building. delete from instructor where dept name in (select dept name from department where building = 'Watson'); Database System Concepts - 7th Edition 3.54 ©Silberschatz, Korth and Sudarshan  Deletion (Cont.)  Delete all instructors whose salary is less than the average salary of instructors delete from instructor where salary < (select avg (salary) from instructor); Problem: as we delete tuples from instructor, the average salary changes Solution used in SQL: 1. First, compute avg (salary) and find all tuples to delete 2. Next, delete all tuples found above (without recomputing avg or retesting the tuples) Database System Concepts - 7th Edition 3.55 ©Silberschatz, Korth and Sudarshan  Insertion  Add a new tuple to course insert into course values ('CS-437', 'Database Systems', 'Comp. Sci.', 4);  or equivalently insert into course (course_id, title, dept_name, credits) values ('CS-437', 'Database Systems', 'Comp. Sci.', 4);  Add a new tuple to student with tot_creds set to null insert into student values ('3003', 'Green', 'Finance', null); Database System Concepts - 7th Edition 3.56 ©Silberschatz, Korth and Sudarshan  Insertion (Cont.)  Make each student in the Music department who has earned more than 144 credit hours an instructor in the Music department with a salary of $18,000. insert into instructor select ID, name, dept_name, 18000 from student where dept_name = 'Music' and total_cred > 144;  The select from where statement is evaluated fully before any of its results are inserted into the relation. Otherwise queries like insert into table1 select * from table1 would cause problem Database System Concepts - 7th Edition 3.57 ©Silberschatz, Korth and Sudarshan  Updates  Give a 5% salary raise to all instructors update instructor set salary = salary * 1.05  Give a 5% salary raise to those instructors who earn less than 70000 update instructor set salary = salary * 1.05 where salary < 70000;  Give a 5% salary raise to instructors whose salary is less than average update instructor set salary = salary * 1.05 where salary < (select avg (salary) from instructor); Database System Concepts - 7th Edition 3.58 ©Silberschatz, Korth and Sudarshan  Updates (Cont.)  Increase salaries of instructors whose salary is over $100,000 by 3%, and all others by a 5% Write two update statements: update instructor set salary = salary * 1.03 where salary > 100000; update instructor set salary = salary * 1.05 where salary