ALTER TABLE

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

Add a column

sql

### ALTER TABLE Customers

### ADD Email varchar(255);

Drop a column

sql

### ALTER TABLE Customers

### DROP COLUMN Email;

Change a column's data type

sql
- **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

sql

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

Add / drop constraints

sql

### 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.