SQL Fundamentals

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Questions and Answers

What is SQL primarily used for?

Building websites

Storing data in relational databases (correct)

Creating graphics and images

Manipulating data in spreadsheets

What does SQL stand for?

Structured Query Language

SQL statements always end with a _____

semicolon

What does the 's' parameter indicate in the numeric(p,s) data type?

maximum number of digits stored to the right of the decimal point Signup and view all the answers

SQL statements are case-sensitive.

False Signup and view all the answers

Match the SQL command with its function:

CREATE = Creates new databases, tables, and views DROP = Drops commands, views, tables, and databases INSERT = Adds new rows of data to the database UPDATE = Modifies existing database data Signup and view all the answers

What does the 'p' parameter indicate in the numeric(p,s) data type?

maximum total number of digits that can be stored (both to the left and right of the decimal point) Signup and view all the answers

What is the range of values for the 'smallmoney' data type?

-214,748.3648 to 214,748.3647 Signup and view all the answers

What is the range of values for the 'datetime' data type?

From January 1, 1753 to December 31, 9999 with an accuracy of 3.33 milliseconds Signup and view all the answers

A primary key column can have NULL values.

False Signup and view all the answers

What SQL statement is used to update existing records in a table?

UPDATE Signup and view all the answers

What does the WHERE clause determine in an SQL UPDATE statement?

Which record(s) will be updated Signup and view all the answers

Which records will be updated if the WHERE clause is omitted in an SQL UPDATE statement?

All records in the table will be updated Signup and view all the answers

What SQL statement is used to delete existing records in a table?

DELETE Signup and view all the answers

What does the WHERE clause specify in an SQL DELETE statement?

Which record(s) should be deleted Signup and view all the answers

What happens if you omit the WHERE clause in an SQL DELETE statement?

All records in the table will be deleted Signup and view all the answers

What is the SQL SELECT TOP Clause used for?

Specify the number of records to return Signup and view all the answers

What SQL syntax is used to drop a PRIMARY KEY constraint from a table?

ALTER TABLE CUSTOMERS DROP PRIMARY KEY; Signup and view all the answers

How can you delete all rows in a table without deleting the table itself in SQL?

DELETE FROM table_name; Signup and view all the answers

What SQL syntax is used to add a PRIMARY KEY constraint on the 'ID' column in the CUSTOMERS table when it already exists?

ALTER TABLE CUSTOMER ADD PRIMARY KEY (ID); Signup and view all the answers

What is the purpose of a FOREIGN KEY in a database?

A foreign key is used to link two tables together. Signup and view all the answers

What does the UNIQUE Constraint prevent in a database table?

Adding duplicate records Signup and view all the answers

How can you drop a UNIQUE constraint in SQL?

ALTER TABLE customers DROP CONSTRAINT myuniqueconstraint; Signup and view all the answers

How can you specify a FOREIGN KEY constraint by altering a table that already has the child table (orders) created?

ALTER TABLE orders ADD FOREIGN KEY (customer_id) REFERENCES customers (id); Signup and view all the answers

What does the CHECK Constraint do in SQL?

Enables a condition to check the value being entered into a record. Signup and view all the answers

How can you define a CHECK constraint on existing records in a table?

ALTER TABLE CUSTOMERS MODIFY AGE INT NOT NULL CHECK (AGE >= 18); Signup and view all the answers

What does the DEFAULT Constraint provide in SQL?

Provides a default value to a column. Signup and view all the answers

How can you create a DEFAULT constraint on a column that already exists in a table?

ALTER TABLE Persons ADD CONSTRAINT df_City DEFAULT 'Sandnes' FOR City; Signup and view all the answers

Study Notes

SQL Overview

SQL is a computer language for storing, manipulating, and retrieving data stored in relational databases.
It is the standard language for Relational Database Management Systems.
All relational database management systems like MySQL, MS Access, Oracle, Sybase, Informix, and SQL Server use SQL as a standard database language.

SQL Syntax

SQL statements start with keywords like SELECT, INSERT, UPDATE, DELETE, ALTER, DROP, CREATE, USE, and SHOW, and end with a semicolon (;).
SQL is case insensitive, which means SELECT and select have the same meaning in SQL statements.

SQL Process

The SQL process involves determining the best way to carry out a request, and the SQL engine figures out how to interpret the task.
The process involves various components, including Query Dispatcher, Optimization Engines, Classic Query Engine, and SQL Query Engine.

Types of SQL Language

Data Manipulation Language (DML) retrieves or modifies data.
Data Definition Language (DDL) defines the structure of the data.
Data Control Language (DCL) defines the privileges granted to database users.

Data Manipulation Language (DML)

DML modifies the database instance by inserting, updating, and deleting data.
It includes commands like:
- SELECT/FROM/WHERE
- INSERT INTO/VALUES
- UPDATE/SET/WHERE
- DELETE FROM/WHERE

Data Definition Language (DDL)

DDL defines the database schema.
It includes commands like:
- CREATE
- DROP
- ALTER

SQL Constraints

NOT NULL Constraint: Ensures a column cannot have a NULL value.
DEFAULT Constraint: Provides a default value for a column.
UNIQUE Constraint: Ensures all values in a column are different.
PRIMARY KEY: Uniquely identifies each row in a database table.
FOREIGN KEY: Uniquely identifies a row in another database table.
CHECK Constraint: Ensures all values in a column satisfy certain conditions.
INDEX: Used to create and retrieve data from the database quickly.

SQL Basic Language Elements

Statements: Have a persistent effect on schemas and data, or control transactions, program flow, connections, sessions, or diagnostics.
Queries: Retrieve data based on specific criteria.
Expressions: Can produce scalar values or tables consisting of columns and rows of data.
Clauses: Are optional, constituent components of statements and queries.
Whitespace is generally ignored in SQL statements and queries.

SQL Statements/Commands

Data Manipulation:
- SELECT: Retrieves data from the database.
- INSERT: Adds new rows of data to the database.
- DELETE: Removes rows of data from the database.
- UPDATE: Modifies existing database data.
Data Definition:
- CREATE TABLE: Adds a new table to the database.
- DROP TABLE: Removes a table from the database.
- ALTER TABLE: Changes the structure of an existing table.
- CREATE VIEW: Adds a new view to the database.
- DROP VIEW: Removes a view from the database.

SQL Data Types

Character strings:
- CHAR: Fixed-length character string.
- VARCHAR: Variable-length character string.
- TEXT: Variable-length character string.
Unicode strings:
- NCHAR: Fixed-length Unicode data.
- NVARCHAR: Variable-length Unicode data.
- NTEXT: Variable-length Unicode data.
Binary types:
- BIT: Allows 0, 1, or NULL.
- BINARY: Fixed-length binary data.
- VARBINARY: Variable-length binary data.
- IMAGE: Variable-length binary data.

Extended Data Types

Variable-length character strings
Dates and times
Boolean data

Constants

Numeric constants: Written as ordinary decimal numbers.
String constants: Enclosed in single quotes.
Date and time constants: Written in a specific format.### SQL Basics
SQL is a language used to manage relational databases.
SQL is used to store, manipulate, and retrieve data stored in a database.

Creating Tables

The CREATE TABLE statement is used to create a new table in a database.
The syntax is: CREATE TABLE table_name (column1 datatype, column2 datatype, ...)
Example: CREATE TABLE Customers (CustomerName varchar(255), ContactName varchar(255), ...).

Constraints

Constraints are used to limit the type of data that can be inserted into a table.
There are several types of constraints:
- NOT NULL: Ensures that a column cannot have a null value.
- DEFAULT: Provides a default value for a column when none is specified.
- UNIQUE: Ensures that all values in a column are different.
- PRIMARY KEY: Uniquely identifies each row in a table.
- FOREIGN KEY: Links two tables together.
- CHECK: Ensures that the data in a column satisfies a specific condition.
- INDEX: Used to create and retrieve data quickly.

Modifying Tables

The ALTER TABLE statement is used to modify an existing table.
The syntax is: ALTER TABLE table_name ADD/MODIFY/DROP column_name datatype
Example: ALTER TABLE Customers ADD Email varchar(255).

Dropping Tables

The DROP TABLE statement is used to delete a table.
The syntax is: DROP TABLE table_name
Example: DROP TABLE Customers.

Deleting Data

The DELETE statement is used to delete data from a table.
The syntax is: DELETE FROM table_name WHERE condition
Example: DELETE FROM Customers WHERE CustomerName = 'Alfreds Futterkiste'.

Updating Data

The UPDATE statement is used to modify existing data in a table.
The syntax is: UPDATE table_name SET column1 = value1, column2 = value2, ... WHERE condition
Example: UPDATE Customers SET ContactName = 'Alfred Schmidt', City = 'Frankfurt' WHERE CustomerID = 1.

Selecting Data

The SELECT statement is used to select data from a table.
The syntax is: SELECT column1, column2, ... FROM table_name WHERE condition
Example: SELECT * FROM Customers WHERE Country = 'Germany'.

Filtering Data

The WHERE clause is used to filter data based on a specific condition.
The syntax is: SELECT column1, column2, ... FROM table_name WHERE condition
Example: SELECT * FROM Customers WHERE Country = 'Germany' AND City = 'Berlin'.

Sorting Data

The ORDER BY clause is used to sort data in ascending or descending order.
The syntax is: SELECT column1, column2, ... FROM table_name ORDER BY column1, column2, ... ASC|DESC
Example: SELECT * FROM Customers ORDER BY Country ASC.

Limiting Data

The LIMIT or TOP clause is used to limit the number of records returned.
The syntax is: SELECT column1, column2, ... FROM table_name LIMIT number or SELECT TOP number FROM table_name
Example: SELECT * FROM Customers LIMIT 3 or SELECT TOP 3 FROM Customers.

Joining Tables

The JOIN clause is used to combine data from two or more tables.
The syntax is: SELECT column1, column2, ... FROM table1 JOIN table2 ON table1.column_name = table2.column_name
Example: SELECT * FROM Customers JOIN Orders ON Customers.CustomerID = Orders.CustomerID.

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Description

Test your knowledge of basic SQL concepts, including querying, aggregation, and sub-queries. Learn to write effective SQL statements to retrieve and manipulate data.