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PRIMARY KEY constraint in sql server

PRIMARY KEY- Primary key constraint is a combination of a NOT NULL constraint and a UNIQUE constraint. This constraint ensures that the specific column for a table has a unique identity.

Example-
Following we create a table PRODUCT in which column ID has primary key constraints.

CREATE TABLE PRODUCT
(
ID INT NOT NULL IDENTITY(1,1),
SKU VARCHAR(20) NOT NULL,
TITLE VARCHAR(200) NOT NULL,
PRICE MONEY NOT NULL,
DESCRIPTION VARCHAR(2000)  NULL,
DTCREATE DATETIME NULL
CONSTRAINT pk_ID PRIMARY KEY (ID)
)

If a table already exists then use the following query.

ALTER TABLE PRODUCT ADD CONSTRAINT pk_products_pid PRIMARY KEY(ID)