SQL ALTER TABLE Statement

The SQL ALTER TABLE command is used to modify the definition (structure) of a table by modifying the definition of its columns. The ALTER command is used to perform the following functions.

1) Add, drop, modify table columns 
2) Add and drop constraints 
3) Enable and Disable constraints 

Syntax to add a column

ALTER TABLE table_name ADD column_name datatype;

For Example: To add a column "experience" to the employee table, the query would be like

ALTER TABLE employee ADD experience number(3);

Syntax to drop a column

ALTER TABLE table_name DROP column_name;

For Example: To drop the column "location" from the employee table, the query would be like

ALTER TABLE employee DROP location;

Syntax to modify a column

ALTER TABLE table_name MODIFY column_name datatype;

For Example: To modify the column salary in the employee table, the query would be like

ALTER TABLE employee MODIFY salary number(15,2);