Oracle / PLSQL: Primary Keys

This Oracle tutorial explains how to create, drop, disable, and enable a primary key in Oracle with syntax and examples.

What is a primary key in Oracle?

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.

Note

• 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

You can create a primary key in Oracle with the CREATE TABLE statement.

Syntax

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

Let's look at an example of how to create a primary key using the CREATE TABLE statement in Oracle:

CREATE TABLE supplier
(
  supplier_id numeric(10) not null,
  supplier_name varchar2(50) not null,
  contact_name varchar2(50),
  CONSTRAINT supplier_pk PRIMARY KEY (supplier_id)
);

In this example, we've created a primary key on the supplier table called supplier_pk. It consists of only one field - the supplier_id field.

We could also create a primary key with more than one field as in the example below:

CREATE TABLE supplier
(
  supplier_id numeric(10) not null,
  supplier_name varchar2(50) not null,
  contact_name varchar2(50),
  CONSTRAINT supplier_pk PRIMARY KEY (supplier_id, supplier_name)
);

Create Primary Key - Using ALTER TABLE statement

You can create a primary key in Oracle with the ALTER TABLE statement.

Syntax

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

Let's look at an example of how to create a primary key using the ALTER TABLE statement in Oracle.

ALTER TABLE supplier
ADD CONSTRAINT supplier_pk PRIMARY KEY (supplier_id);

In this example, we've created a primary key on the existing supplier table called supplier_pk. It consists of the field called supplier_id.

We could also create a primary key with more than one field as in the example below:

ALTER TABLE supplier
ADD CONSTRAINT supplier_pk PRIMARY KEY (supplier_id, supplier_name);

Drop Primary Key

You can drop a primary key in Oracle using the ALTER TABLE statement.

Syntax

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

Let's look at an example of how to drop a primary key using the ALTER TABLE statement in Oracle.

ALTER TABLE supplier
DROP CONSTRAINT supplier_pk;

In this example, we're dropping a primary key on the supplier table called supplier_pk.

Disable Primary Key

You can disable a primary key in Oracle using the ALTER TABLE statement.

Syntax

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

Let's look at an example of how to disable a primary using the ALTER TABLE statement in Oracle.

ALTER TABLE supplier
DISABLE CONSTRAINT supplier_pk;

In this example, we're disabling a primary key on the supplier table called supplier_pk.

Enable Primary Key

You can enable a primary key in Oracle using the ALTER TABLE statement.

Syntax

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

Let's look at an example of how to enable a primary key using the ALTER TABLE statement in Oracle.

ALTER TABLE supplier
ENABLE CONSTRAINT supplier_pk;

In this example, we're enabling a primary key on the supplier table called supplier_pk.