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SQL ALTER TABLE Statement


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:

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:

Id LastName FirstName City
1 Ray Barnett OK
2 Tara Allen NY
3 Austin McCollum CT

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
The following SQL code adds a new column named DateOfBirth:
ALTER TABLE People ADD DateOfBirth date;


The "Persons" table will now look like this:

Id LastName FirstName City DateOfBirth
1 Ray Barnett OK  
2 Tara Allen NY  
3 Austin McCollum CT  

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 look like this:

Id LastName FirstName City
1 Ray Barnett OK
2 Tara Allen NY
3 Austin McCollum CT