SQL Primary Key - The Coding Shala

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SQL Primary Key

SQL Primary Key is a field that uniquely identifies each record in the database table. That means if a Primary Key is defined on a field then you cannot have two records having the same value of the fields. SQL Primary Key cannot contains NULL Values. A table can have only one Primary Key which may consist of one or more fields. If more than one column is used as a Primary Key then it is known as Composite Primary Key.
SQL Primary Key - The Coding Shala

Create Primary Key Syntax

The following syntax will create Table Persons having Primary Key as ID - 

 Create table Persons(  
    ID int,  
    Name varchar(30),  
    City varchar(30),  
    PRIMARY KEY (ID)  
    );

If Table is already created and we want to add primary key then we use alter command to add a primary key. The following syntax will add a primary key in the existing table - 

 ALTER TABLE PERSONS ADD PRIMARY KEY (ID) 

To create Primary Key constraints more than one columns then use the following SQL Command - 

 ALTER TABLE PERSONS   
   ADD CONSTRAINT PK_PID PRIMARY KEY (ID, NAME);  

Delete Primary Key

The following SQL Syntax is used to drop the primary key constraints from the existing table-  

 ALTER TABLE PERSONS DROP PRIMARY KEY ;  



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