SQL ALTER TABLE Keyword

The SQL ALTER TABLE keyword is used to add, delete or modify columns in an existing table. It is also used to add SQL constraints or drop SQL constraints on an existing table.

Syntax

The syntax for using ALTER TABLE statement to add or drop columns are given below:

/* ADD Column */
ALTER TABLE table_name
ADD column_name datatype;

/* DROP Column */
ALTER TABLE table_name
DROP COLUMN column_name;

The syntax for using ALTER TABLE statement to modify columns is given below:

/* SQL Server / MS Access */
ALTER TABLE table_name
ALTER COLUMN column_name datatype;

/* MySQL / Oracle (prior 10G version) */
ALTER TABLE table_name
MODIFY COLUMN column_name datatype;

/* Oracle 10G and later */
ALTER TABLE table_name
MODIFY column_name datatype;

Example:

Consider a database table called Employee with the following records:

• Add a column: To add a column named Salary in the Employee table, the SQL code is given below:

  ALTER TABLE Employee
  ADD Salary DECIMAL(18,2);
  

  
  

  This will produce the result as shown below:

  EmpID	Name	City	Age	Salary
  1	John	London	25
  2	Marry	New York	24
  3	Jo	Paris	27
  4	Kim	Amsterdam	30

• Drop a column: To drop a column named Salary in the original Employee table, the SQL code is mentioned below:

  ALTER TABLE Employee
  DROP COLUMN Age;
  

  
  

  This will produce the below mentioned result:

  EmpID	Name	City
  1	John	London
  2	Marry	New York
  3	Jo	Paris
  4	Kim	Amsterdam

• Modify a column: In the original Employee table, the datatype of City column is VARCHAR(100). To change the datatype of the column to VARCHAR(255), the SQL code is given below:

  /* SQL Server / MS Access */
  ALTER TABLE Employee
  ALTER COLUMN City VARCHAR(255);
  
  /* MySQL / Oracle (prior 10G version) */
  ALTER TABLE Employee
  MODIFY COLUMN City VARCHAR(255);
  
  /* Oracle 10G and later */
  ALTER TABLE Employee
  MODIFY City VARCHAR(255);
  

  
  

  This will change the datatype of City column from VARCHAR(100) to VARCHAR(255). For a complete reference of all the data types available in SQL database, see Data Types reference.