SQL ALTER TABLE Statement

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.

ALTER TABLE - ADD Column

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

ALTER TABLE table_name
ADD column_name datatype; 

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

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):

ALTER TABLE table_name
DROP COLUMN column_name; 

The following SQL deletes the “Email” column from the “Customers” table:

ALTER TABLE Customers
DROP COLUMN Email;

ALTER TABLE - ALTER/MODIFY COLUMN

To change the data type of a column in a table, use the following syntax:

SQL Server / MS Access:

ALTER TABLE table_name
ALTER COLUMN column_name datatype; 

My SQL / Oracle (prior version 10G):

ALTER TABLE table_name
MODIFY COLUMN column_name datatype; 

Oracle 10G and later:

ALTER TABLE table_name
MODIFY column_name datatype; 

SQL ALTER TABLE Example

Look at the “Persons” table:

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

We use the following SQL statement:

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.

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:

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:

ALTER TABLE Persons
DROP COLUMN DateOfBirth; 

The “Persons” table will now look like this: