Oracle/PLSQL: 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);