Oracle / PLSQL : create, drop, disable, and enable a primary key

In Oracle, a primary key is a single field or combination of fields that uniquely defines a record. None of the fields that are part of the primary key can contain a null value. A table can have only one primary key.

• In Oracle, a primary key can not contain more than 32 columns.
• A primary key can be defined in either a CREATE TABLE statement or an ALTER TABLE statement.

Create Primary Key - Using CREATE TABLE statement

The syntax to create a primary key using the CREATE TABLE statement in Oracle/PLSQL is:

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 emp
(
  emp_id numeric(10) NOT NULL,
  emp_name varchar2(50) NOT NULL,
  mgr_id numeric(10) NOT NULL,
  CONSTRAINT emp_pk PRIMARY KEY (emp_id)
);
 

A primary key with more than one field
CREATE TABLE emp
(
  emp_id numeric(10) NOT NULL,
  emp_name varchar2(50) NOT NULL,
  mgr_id numeric(10) NOT NULL,
  CONSTRAINT emp_pk PRIMARY KEY (emp_id,emp_name)
);

Create Primary Key - Using ALTER TABLE statement

The syntax to create a primary key using the ALTER TABLE statement in Oracle/PLSQL is:

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

Example

ALTER TABLE emp
ADD CONSTRAINT emp_pk PRIMARY KEY (emp_id);

A primary key with more than one field 
ALTER TABLE emp
ADD CONSTRAINT emp_pk PRIMARY KEY (emp_id,emp_name);

Drop Primary Key

The syntax to drop a primary key using the ALTER TABLE statement in Oracle/PLSQL is:

ALTER TABLE table_name
DROP CONSTRAINT constraint_name;

Example

ALTER TABLE emp
DROP CONSTRAINT emp_pk;

Disable Primary Key

The syntax to disable a primary key using the ALTER TABLE statement in Oracle/PLSQL is:

ALTER TABLE table_name
DISABLE CONSTRAINT constraint_name;

Example

ALTER TABLE emp
DISABLE CONSTRAINT emp_pk;

Enable Primary Key

The syntax to enable a primary key using the ALTER TABLE statement in Oracle/PLSQL is:

ALTER TABLE table_name
ENABLE CONSTRAINT constraint_name;

Example

ALTER TABLE emp
ENABLE CONSTRAINT emp_pk;