SQL - ALTER TABLE


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

/* My SQL / 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:

EmpIDNameCityAge
1JohnLondon25
2MarryNew York24
3JoParis27
4KimAmsterdam30

  • 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:

    EmpIDNameCityAgeSalary
    1JohnLondon25
    2MarryNew York24
    3JoParis27
    4KimAmsterdam30

  • 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:

    EmpIDNameCity
    1JohnLondon
    2MarryNew York
    3JoParis
    4KimAmsterdam

  • 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);
    
    /* My SQL / 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).


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