SQL Keyword ALTER

The ALTER keyword can be used to perform the below changes on a column or a table.

  • The ALTER TABLE command can be used to add, modify, or delete columns of a table.
  • The ALTER TABLE command can be used to add or delete constraints of a table.
  • The ALTER COLUMN command can be used to change a column's data type.

ALTER Table Columns

The below SQL adds a new column to an existing table.

ALTER TABLE customers
ADD email varchar(255);

The below SQL deletes a column from a table.

ALTER TABLE customers
DROP COLUMN email;

The below SQL modifies the column data type to type "year".

ALTER TABLE employees
ALTER COLUMN birth_date year;

ALTER Table Constraints

The below SQL adds a PRIMARY KEY constraint to the table.

ALTER TABLE customers
ADD CONSTRAINT PK_customers PRIMARY KEY (customer_id, customer_name);

The below SQL deletes a PRIMARY KEY constraint from the table.

ALTER TABLE customers
DROP CONSTRAINT PK_customers;

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