SQL Primary Key
primary key of a table in database uniquely identifies each record in the table.Primary Key may consist of a single attribute or multiple attributes in combination.
A Primary key column canot cantain null values.
Each table should have primary key.
And each table can have only one primary key.
SYNTAX :
CREATE TABLE Employee ( E_Id int NOT NULL, LastName varchar(255) NOT NULL, FirstName varchar(255), Address varchar(255), City varchar(255), PRIMARY KEY (E_Id) )
Mysql/SQl Server/Oracle/MS Access:
CREATE TABLE Employee ( E_Id int NOT NULL, LastName varchar(255) NOT NULL, FirstName varchar(255), Address varchar(255), City varchar(255), CONSTRAINT ak_EmployeeID PRIMARY KEY (E_Id,LastName) )