ALTER TABLE

ALTER TABLE

The ALTER TABLE statement changes an existing table: adding, deleting, or modifying columns.

Add a column

ALTER TABLE Customers
ADD Email varchar(255);

Drop a column

ALTER TABLE Customers
DROP COLUMN Email;

Change a column's data type

-- MySQL / Oracle:
ALTER TABLE Customers
MODIFY COLUMN Email varchar(100);

-- SQL Server / MS Access:
ALTER TABLE Customers
ALTER COLUMN Email varchar(100);

Rename a column

ALTER TABLE Customers
RENAME COLUMN Email TO ContactEmail;   -- newer standard

Add / drop constraints

ALTER TABLE Persons
ADD UNIQUE (PersonID);

ALTER TABLE Persons
DROP CONSTRAINT UQ_PersonID;

TL;DR

• ALTER TABLE modifies an existing table.
• ADD / DROP COLUMN add or remove columns.
• MODIFY / ALTER COLUMN change a column type.
• It can also add or remove constraints.