Primary Key Constraint

SQL Primary Key Constraint

A SQL Primary Key constraint is a column or a combination of several columns that contain values that uniquely identify each row in the table.
A table can contain only one PRIMARY KEY constraint.
A primary key cannot exceed 16 columns and a total key length of 900 bytes.
All columns defined within a PRIMARY KEY constraint must be defined as NOT NULL.
If nullability is not specified, then all columns participating in a PRIMARY KEY constraint will be automatically set to NOT NULL.

Example 1

CREATE TABLE Test1
(
ID INT PRIMARY KEY,
TITLE VARCHAR(250) NOT NULL,
);

Example 2

CREATE TABLE Test2
(
ID INT,
TITLE VARCHAR(250),
DESCRIPTION VARCHAR(250),
CONSTRAINT PK_TL2 PRIMARY KEY (ID, TITLE)
);

Example 3

CREATE TABLE Test3
(
ID INT NOT NULL,
TITLE VARCHAR(250) NOT NULL,
);

ALTER TABLE Test3 
ADD CONSTRAINT PKTest3_ID PRIMARY KEY (ID);