Primary Key
PRIMARY KEY(Entity Integrity Constraints)
A PRIMARY KEY is one or more columns in a table used to uniquely identify each row in the table. A primary key column in a table has special attributes.
- It defines the column as a mandatory column i.e the column cannot be left blank. The NOT NULL attribute is active.
- The data held across then column must be unique.
A single column primary key is called simple key. A multicolumn primary key is called a composite primary key. The only function of primary key in a table is to uniquely identify a row. Only when a record cannot be uniquely identified using the value in a single column, will a composite primary key be defined.
Syn:- 1) columnname datatype(size) PRIMARY KEY à Column level
2) PRIMARY KEY(columns list) à Table Level
Ex:- 1 create table vision2(eno number(5) PRIMARY KEY,na varchar2(20),
sal number(5));
2Create table vision3(eno number(5),code number(5),sal number(5),
PRIMARY KEY (eno,code));
3 insert into vision2 values(100,’prasad’,3200);
4 insert into vision2 values(100,’vani’,4300);
5 insert into vision3 values(100,10,1000);
6 insert into vision3 values(200,10,2000);
7 insert into vision3 values(200,10,3200);
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