106 SQL ALTER TABLE

The SQL ALTER TABLE statement is used to modify the structure of an existing table in a database. You can:

• Add, delete, or modify columns in a table.
• Add or drop constraints such as primary keys and foreign keys.

Main usages:

• Add a column:

  ALTER TABLE table_name ADD column_name datatype;
  

• Delete a column:

  ALTER TABLE table_name DROP COLUMN column_name;
  

• Rename a column: (syntax varies by SQL database system)

  -- General SQL
  ALTER TABLE table_name RENAME COLUMN old_name TO new_name;
  
  -- SQL Server
  EXEC sp_rename 'table_name.old_name', 'new_name', 'COLUMN';
  

• Change a column’s data type:

  -- SQL Server / MS Access
  ALTER TABLE table_name ALTER COLUMN column_name datatype;
  
  -- MySQL / Oracle (pre-10G)
  ALTER TABLE table_name MODIFY COLUMN column_name datatype;
  

Examples and explanations show step-by-step how a column can be added, changed, or deleted on a sample table.

• Real-life quiz and interactive exercise are included to reinforce the learning with solutions to add columns using the correct ALTER TABLE syntax.[1]

1

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 - RENAME COLUMN

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

SQL Server:

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

ALTER TABLE - ALTER/MODIFY DATATYPE

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. 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:

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: