SQL ALTER TABLE: Modifying Columns

The ALTER TABLE statement in SQL allows you to make changes to an existing table's structure. This includes adding, deleting, or modifying columns.

ALTER COLUMN: Changing Column Data Types

ALTER COLUMN Syntax

Use ALTER COLUMN to change a column's data type. Be cautious, as this can affect existing data; make sure your data is compatible with the new data type.

ALTER TABLE table_name
ALTER COLUMN column_name new_data_type;
      

Example: Changing BirthDate Data Type

This changes the 'BirthDate' column in the 'Employees' table to a 'year' data type. (The specific data types available depend on your database system.)

ALTER TABLE Employees
ALTER COLUMN BirthDate year;
      

DROP COLUMN: Deleting Columns

DROP COLUMN Syntax

Use DROP COLUMN to remove a column from a table. This is a destructive operation; the column and all its data will be permanently removed.

ALTER TABLE table_name
DROP COLUMN column_name;
      

Example: Deleting ContactName Column

This example removes the 'ContactName' column from the 'Customers' table.

ALTER TABLE Customers
DROP COLUMN ContactName;