PRIMARY KEY

Description

The PRIMARY KEY constraint uniquely identifies each record in a table.

Primary keys must contain UNIQUE values, and cannot contain NULL values.

A table can have only ONE primary key; and in the table, this primary key can consist of single column (field).

SQL PRIMARY KEY on CREATE TABLE

The following SQL creates a PRIMARY KEY on the “ID” column when the “Persons” table is created:

> CREATE TABLE Persons (
    ID int NOT NULL,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Age int,
    PRIMARY KEY (ID)
);

Constraints

1. DROP PRIMARY KEY with ALTER TABLE is not supported yet.
2. Composite primary key(the primary key consists of multiple columns) is not supported yet.