{"id":92,"date":"2026-03-06T07:11:56","date_gmt":"2026-03-06T07:11:56","guid":{"rendered":"https:\/\/gigz.pk\/sql\/?post_type=lesson&p=92"},"modified":"2026-03-16T18:52:16","modified_gmt":"2026-03-16T18:52:16","slug":"alter-table","status":"publish","type":"lesson","link":"https:\/\/gigz.pk\/sql\/lesson\/alter-table\/","title":{"rendered":"\u00a0ALTER TABLE"},"content":{"rendered":"\n

The ALTER TABLE<\/code> statement is used to modify an existing table<\/strong> in a database. You can use it to add, delete, or modify columns<\/strong>, and even change table constraints.<\/p>\n\n\n\n

1. Add a Column<\/strong><\/h3>\n\n\n\n

To add a new column to a table:<\/p>\n\n\n\n

ALTER TABLE table_name
ADD column_name datatype;<\/pre>\n\n\n\n
Example:<\/strong><\/p>\n\n\n\n
ALTER TABLE Employees
ADD DateOfBirth DATE;<\/pre>\n\n\n\n
This adds a new column DateOfBirth<\/code> to the Employees<\/code> table.<\/p>\n\n\n\n
2. Delete a Column<\/strong><\/h3>\n\n\n\n
To remove a column from a table:<\/p>\n\n\n\n
ALTER TABLE table_name
DROP COLUMN column_name;<\/pre>\n\n\n\n
Example:<\/strong><\/p>\n\n\n\n
ALTER TABLE Employees
DROP COLUMN DateOfBirth;<\/pre>\n\n\n\n
This deletes the DateOfBirth<\/code> column from the table.<\/p>\n\n\n\n
3. Modify a Column<\/strong><\/h3>\n\n\n\n
To change the datatype or size of an existing column:<\/p>\n\n\n\n
ALTER TABLE table_name
MODIFY COLUMN column_name new_datatype;<\/pre>\n\n\n\n
Example:<\/strong><\/p>\n\n\n\n
ALTER TABLE Employees
MODIFY COLUMN LastName VARCHAR(100);<\/pre>\n\n\n\n
This changes the LastName<\/code> column to allow up to 100 characters.<\/p>\n\n\n\n
4. Rename a Column<\/strong><\/h3>\n\n\n\n
To rename a column (syntax may vary by database):<\/p>\n\n\n\n
ALTER TABLE table_name
RENAME COLUMN old_name TO new_name;<\/pre>\n\n\n\n
Example:<\/strong><\/p>\n\n\n\n
ALTER TABLE Employees
RENAME COLUMN LastName TO Surname;<\/pre>\n\n\n\n
This renames LastName<\/code> to Surname<\/code>.<\/p>\n\n\n\n
5. Rename a Table<\/strong><\/h3>\n\n\n\n
To rename the entire table:<\/p>\n\n\n\n
ALTER TABLE old_table_name
RENAME TO new_table_name;<\/pre>\n\n\n\n
Example:<\/strong><\/p>\n\n\n\n
ALTER TABLE Employees
RENAME TO Staff;<\/pre>\n\n\n\n
This renames the Employees<\/code> table to Staff<\/code>.<\/p>\n\n\n\n
6. Add a Constraint<\/strong><\/h3>\n\n\n\n
You can add constraints like PRIMARY KEY<\/code>, FOREIGN KEY<\/code>, UNIQUE<\/code>, or CHECK<\/code>:<\/p>\n\n\n\n
ALTER TABLE table_name
ADD CONSTRAINT constraint_name constraint_definition;<\/pre>\n\n\n\n
Example:<\/strong><\/p>\n\n\n\n
ALTER TABLE Employees
ADD CONSTRAINT PK_EmployeeID PRIMARY KEY (EmployeeID);<\/pre>\n\n\n\n
This sets EmployeeID<\/code> as the primary key.<\/p>\n\n\n\n
Key Points<\/strong><\/h3>\n\n\n\n