Primary Key

In Oracle, Primary key is the column which has unique values, and it cannot be null. In a table, there can be only one primary key.

Primary Key – Using CREATE TABLE statement

Syntax:

CREATE TABLE table_name  

(  

  column1 datatype null/not null,  

  column2 datatype null/not null,  

  ...  

  CONSTRAINT constraint_name PRIMARY KEY (column1, column2, ... column_n)  

);

Example

CREATE TABLE Test2(ID Number, NAME Varchar2 CONSTRAINT test2_pk PRIMARY KEY (ID));  

Primary Key – Using ALTER TABLE statement

Syntax:

 ALTER TABLE table_name  

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

Example

ALTER TABLE student ADD CONSTRAINT student_pk PRIMARY KEY(id);  

Drop Primary Key

Syntax:

ALTER TABLE table_name  

DROP CONSTRAINT constraint_name;

Example

ALTER TABLE student DROP CONSTRAINT student_pk ;  

Disable Primary Key

Syntax:

ALTER TABLE table_name  

DISABLE CONSTRAINT constraint_name;

Example

ALTER TABLE student DISABLE CONSTRAINT student_pk ;  

Enable Primary Key

Syntax:

ALTER TABLE table_name  

ENABLE CONSTRAINT constraint_name;

Example

ALTER TABLE student ENABLE CONSTRAINT student_pk ;