SQL Alter Table

SQL ALTER TABLE Statement

The ALTER TABLE statement is used to add, delete, or modify columns in an existing table.

The ALTER TABLE statement is also used to add and drop various constraints on an existing table.

Common ALTER TABLE operations are:

• Add column - Adds a new column to a table
• Drop column - Deletes a column in a table
• Rename column - Renames a column
• Modify column - Changes the data type, size, or constraints of a column
• Add constraint - Adds a new constraint
• Rename table - Renames a table

ALTER TABLE - ADD Column

To add a column in a table, use the following syntax:

Syntax

ALTER TABLE table_name

ADD column_name datatype;

The following SQL adds an "Email" column to the "Customers" table:

Example

ALTER TABLE Customers
ADD Email varchar(255);

ALTER TABLE - DROP COLUMN

To delete a column in a table, use the following syntax (notice that some database systems don't allow deleting a column):

Syntax

ALTER TABLE table_name

DROP COLUMN column_name;

The following SQL deletes the "Email" column from the "Customers" table:

Example

ALTER TABLE Customers
DROP COLUMN Email;

ALTER TABLE - RENAME COLUMN

To rename a column in a table, use the following syntax:

Syntax

ALTER TABLE table_name

RENAME COLUMN old_name to new_name;

To rename a column in a table in SQL Server, use the following syntax:

Syntax for SQL Server:

EXEC sp_rename 'table_name.old_name', 'new_name', 'COLUMN';

ALTER TABLE - MODIFY Datatype

To modify the data type, size or constraints of a column in a table, use the following syntax:

Syntax for SQL Server / MS Access:

ALTER TABLE table_name

ALTER COLUMN column_name new_datatype constraint;

Syntax for MySQL / Oracle:

ALTER TABLE table_name

MODIFY column_name new_datatype constraint;

The following SQL modifies the size of the "Email" column to varchar(100), and we also add a NOT NULL constraint:

Example

ALTER TABLE Customers
MODIFY Email varchar(100) NOT NULL;

ALTER TABLE - ADD CONSTRAINT

To add a constraint to an existing table, use the following syntax:

Syntax

ALTER TABLE table_name

ADD CONSTRAINT constraint_name constraint_definition;

The following SQL adds a constraint named "CHK_Age" that is a CHECK constraint that ensures that the "Age" column has a value of 18 and above:

Example

ALTER TABLE Members
ADD CONSTRAINT CHK_Age CHECK (Age >= 18);

ALTER TABLE - Rename table

To rename a table, use the following syntax:

Syntax

ALTER TABLE table_name

RENAME TO new_table_name;

The following SQL renames the "Customers" table to "Clients":

Example

ALTER TABLE Customers

RENAME TO Clients;

SQL ALTER TABLE Example

Assume we have a "Persons" table, that looks like this:

Now we want to add a column named "DateOfBirth" in the "Persons" table.

We use the following SQL statement:

Example

ALTER TABLE Persons

ADD DateOfBirth date;

Notice that the new column, "DateOfBirth", is of type date and is going to hold a date. 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.

The "Persons" table will now look like this:

Change Data Type Example

Now we want to change the data type of the column named "DateOfBirth" in the "Persons" table.

We use the following SQL statement:

Example

ALTER TABLE Persons

ALTER COLUMN DateOfBirth year;

Notice that the "DateOfBirth" column is now of type year and is going to hold a year in a two- or four-digit format.

DROP COLUMN Example

Next, we want to delete the column named "DateOfBirth" in the "Persons" table.

We use the following SQL statement:

Example

ALTER TABLE Persons

DROP COLUMN DateOfBirth;

The "Persons" table will now look like this: