SQL Alter

The ALTER TABLE Statement

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

SQL ALTER TABLE Syntax

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

ALTER TABLE table_name
ADD column_name datatype

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

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

ALTER TABLE table_name
ALTER COLUMN column_name datatype

 

SQL ALTER TABLE Example

Look at the "Persons" table:

P_Id LastName FirstName Address City
1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger

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. 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 like this:

P_Id LastName FirstName Address City DateOfBirth
1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger

 

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-digit 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 like this:

P_Id LastName FirstName Address City
1 Hansen Ola Timoteivn 10 Sandnes
2 Svendson Tove Borgvn 23 Sandnes
3 Pettersen Kari Storgt 20 Stavanger