Primary Keys

What is a primary key?

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.

Using a CREATE TABLE statement

The syntax for creating a primary key using a CREATE TABLE statement is:

CREATE TABLE table_name
(
  column1 datatype null/not null,
  column2 datatype null/not null,
  ...
  
  CONSTRAINT constraint_name PRIMARY KEY (column1, column2, ... column_n)
);

For Example

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)
);

Using an ALTER TABLE statement

The syntax for creating a primary key in an ALTER TABLE statement is:

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

For Example

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 a Primary Key

The syntax for dropping a primary key is:

ALTER TABLE table_name
drop CONSTRAINT constraint_name;

For Example

ALTER TABLE supplier
drop CONSTRAINT supplier_pk;

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

Disable a Primary Key

The syntax for disabling a primary key is:

ALTER TABLE table_name
disable CONSTRAINT constraint_name;

For Example

ALTER TABLE supplier
disable CONSTRAINT supplier_pk;

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

Enable a Primary Key

The syntax for enabling a primary key is:

ALTER TABLE table_name
enable CONSTRAINT constraint_name;

For Example

ALTER TABLE supplier
enable CONSTRAINT supplier_pk;

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