SQL PRIMARY KEY

SQL PRIMARY KEY Constraint:

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.

SQL PRIMARY KEY on CREATE TABLE:

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

SQL PRIMARY KEY on ALTER TABLE:

ALTER TABLE Persons ADD PRIMARY KEY (ID);

DROP PRIMARY KEY:

ALTER TABLE Persons DROP PRIMARY KEY;