PRIMARY KEY

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PRIMARY KEY in SQL Server Database Table

In SQL server,PRIMARY KEY constraint uniquely identifies each row/record in a table. It can consists of single or multiple columns.

• A table can have only one primary key.


• A primary key must contain unique values and cannot have NULL values.


• Primary key can consist of single or multiple columns (fields),when multiple fields are used as a primary key, they are called as composite key.


• A primary key can be defined in either a CREATE TABLE statement or an ALTER TABLE statement.
  
  

  SQL PRIMARY KEY on CREATE TABLE

  Syntax to define the ID attribute as a primary key.

  CREATE TABLE CUSTOMERS(
     ID   INT              NOT NULL,
     NAME VARCHAR (20)     NOT NULL,
     AGE  INT              NOT NULL,
     ADDRESS  CHAR (25) ,
     SALARY   DECIMAL (18, 2),       
     PRIMARY KEY (ID)
  );
  

  To create a PRIMARY KEY constraint on the "ID" column when the CUSTOMERS table already exists, use the following SQL syntax:

  ALTER TABLE CUSTOMER ADD PRIMARY KEY (ID);
  

  NOTE − If you use the ALTER TABLE statement to add a primary key,we have to make sure that the primary key column(s) do not contain NULL values when the table was first created.

  For defining a PRIMARY KEY constraint on multiple columns, use the SQL syntax given below:

  CREATE TABLE CUSTOMERS(
     ID   INT              NOT NULL,
     NAME VARCHAR (20)     NOT NULL,
     AGE  INT              NOT NULL,
     ADDRESS  CHAR (25) ,
     SALARY   DECIMAL (18, 2),        
     PRIMARY KEY (ID, NAME)
  );
  

  To create a PRIMARY KEY constraint on the "ID" and "NAMES" columns when CUSTOMERS table already exists the following SQL syntax be used:

  ALTER TABLE CUSTOMERS 
     ADD CONSTRAINT PK_CUSTID PRIMARY KEY (ID, NAME);
  

  SQL PRIMARY KEY on ALTER TABLE

  Syntax

  ALTER TABLE table_name
  ADD CONSTRAINT constraint_name PRIMARY KEY (column1, column2, ... column_n);
  
  

  Drop Primary Key

  You can drop a primary key in SQL Server using the ALTER TABLE statement.

  Syntax

  ALTER TABLE table_name
  DROP CONSTRAINT constraint_name;
  

  Disable Primary Key

  You can disable a primary key using the ALTER TABLE statement in SQL Server.

  Syntax

  ALTER INDEX constraint_name ON table_name
  DISABLE;
  

  Enable Primary Key

  You can enable a primary key using the ALTER INDEX statement in SQL Server

  Syntax

  ALTER INDEX constraint_name ON table_name
  REBUILD;
  

  
  

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