SQL LECTURE 11

Lecture 11

The CREATE DATABASE Statement

The CREATE DATABASE statement is used to create a database.

SQL CREATE DATABASE Syntax

CREATE DATABASE database_name

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CREATE DATABASE Example

Now we want to create a database called "my_db".

We use the following CREATE DATABASE statement:

CREATE DATABASE my_db

Database tables can be added with the CREATE TABLE statement.

The CREATE TABLE Statement

The CREATE TABLE statement is used to create a table in a database.

SQL CREATE TABLE Syntax

CREATE TABLE table_name

(

column_name1 data_type,

column_name2 data_type,

column_name3 data_type,

....

)

The data type specifies what type of data the column can hold. For a complete reference of all the data types available in MS Access, MySQL, and SQL Server, go to our complete Data Types reference.

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CREATE TABLE Example

Now we want to create a table called "Persons" that contains five columns: P_Id, LastName, FirstName, Address, and City.

We use the following CREATE TABLE statement:

CREATE TABLE Persons

(

P_Id int,

LastName varchar(255),

FirstName varchar(255),

Address varchar(255),

City varchar(255)

)

The P_Id column is of type int and will hold a number. The LastName, FirstName, Address, and City columns are of type varchar with a maximum length of 255 characters.

The empty "Persons" table will now look like this:

P_Id    LastName    FirstName    Address    City

              

The empty table can be filled with data with the INSERT INTO statement.

SQL Constraints

Constraints are used to limit the type of data that can go into a table.

Constraints can be specified when a table is created (with the CREATE TABLE statement) or after the table is created (with the ALTER TABLE statement).

We will focus on the following constraints:

•    NOT NULL

•    UNIQUE

•    PRIMARY KEY

•    FOREIGN KEY

•    CHECK

•    DEFAULT

The next chapters will describe each constraint in detail.

By default, a table column can hold NULL values.

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SQL NOT NULL Constraint

The NOT NULL constraint enforces a column to NOT accept NULL values.

The NOT NULL constraint enforces a field to always contain a value. This means that you cannot insert a new record, or update a record without adding a value to this field.

The following SQL enforces the "P_Id" column and the "LastName" column to not accept NULL values:

CREATE TABLE Persons

(

P_Id int NOT NULL,

LastName varchar(255) NOT NULL,

FirstName varchar(255),

Address varchar(255),

City varchar(255)

)

SQL UNIQUE Constraint

The UNIQUE constraint uniquely identifies each record in a database table.

The UNIQUE and PRIMARY KEY constraints both provide a guarantee for uniqueness for a column or set of columns.

A PRIMARY KEY constraint automatically has a UNIQUE constraint defined on it.

Note that you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per table.

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SQL UNIQUE Constraint on CREATE TABLE

The following SQL creates a UNIQUE constraint on the "P_Id" column when the "Persons" table is created:

MySQL:

CREATE TABLE Persons

(

P_Id int NOT NULL,

LastName varchar(255) NOT NULL,

FirstName varchar(255),

Address varchar(255),

City varchar(255),

UNIQUE (P_Id)

)

SQL Server / Oracle / MS Access:

CREATE TABLE Persons

(

P_Id int NOT NULL UNIQUE,

LastName varchar(255) NOT NULL,

FirstName varchar(255),

Address varchar(255),

City varchar(255)

)

To allow naming of a UNIQUE constraint, and for defining a UNIQUE constraint on multiple columns, use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

CREATE TABLE Persons

(

P_Id int NOT NULL,

LastName varchar(255) NOT NULL,

FirstName varchar(255),

Address varchar(255),

City varchar(255),

CONSTRAINT uc_PersonID UNIQUE (P_Id,LastName)

)

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SQL UNIQUE Constraint on ALTER TABLE

To create a UNIQUE constraint on the "P_Id" column when the table is already created, use the following SQL:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Persons

ADD UNIQUE (P_Id)

To allow naming of a UNIQUE constraint, and for defining a UNIQUE constraint on multiple columns, use the following SQL syntax:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Persons

ADD CONSTRAINT uc_PersonID UNIQUE (P_Id,LastName)

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To DROP a UNIQUE Constraint

To drop a UNIQUE constraint, use the following SQL:

MySQL:

ALTER TABLE Persons

DROP INDEX uc_PersonID

SQL Server / Oracle / MS Access:

ALTER TABLE Persons

DROP CONSTRAINT uc_PersonID