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PostgreSQL: Unique Constraints

This PostgreSQL tutorial explains how to create, add, and drop unique constraints in PostgreSQL with syntax and examples.

What is a unique constraint in PostgreSQL?

A unique constraint is a single field or combination of fields that uniquely defines a record. Some of the fields can contain null values as long as the combination of values is unique.

What is the difference between a unique constraint and a primary key?

Primary Key Unique Constraint
None of the fields that are part of the primary key can contain a null value. Some of the fields that are part of the unique constraint can contain null values as long as the combination of values is unique.

Create unique Contraint - Using a CREATE TABLE statement

The syntax for creating a unique constraint using a CREATE TABLE statement in PostgreSQL is:

CREATE TABLE table_name
(
  column1 datatype [ NULL | NOT NULL ],
  column2 datatype [ NULL | NOT NULL ],
  ...

  CONSTRAINT constraint_name UNIQUE (uc_col1, uc_col2, ... uc_col_n)
);
table_name
The name of the table that you wish to create.
column1, column2
The columns that you wish to create in the table.
constraint_name
The name of the unique constraint.
uc_col1, uc_col2, ... uc_col_n
The columns that make up the unique constraint.

Example

Let's look at an example of how to create a unique constraint in PostgreSQL using the CREATE TABLE statement.

CREATE TABLE order_details
( order_detail_id integer CONSTRAINT order_details_pk PRIMARY KEY,
  order_id integer NOT NULL,
  order_date date,
  quantity integer,
  notes varchar(200),
  CONSTRAINT order_unique UNIQUE (order_id)
);

In this example, we've created a unique constraint on the order_details table called order_unique. It consists of only one field - the order_id field.

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

CREATE TABLE order_details
( order_detail_id integer CONSTRAINT order_details_pk PRIMARY KEY,
  order_id integer NOT NULL,
  order_date date,
  quantity integer,
  notes varchar(200),
  CONSTRAINT order_date_unique UNIQUE (order_id, order_date)
);

Create unique contraint - Using an ALTER TABLE statement

The syntax for creating a unique constraint using an ALTER TABLE statement in PostgreSQL is:

ALTER TABLE table_name
ADD CONSTRAINT constraint_name UNIQUE (column1, column2, ... column_n);
table_name
The name of the table to modify. This is the table that you wish to add a unique constraint to.
constraint_name
The name of the unique constraint.
column1, column2, ... column_n
The columns that make up the unique constraint.

Example

Let's look at an example of how to add a unique constraint to an existing table in PostgreSQL using the ALTER TABLE statement.

ALTER TABLE order_details
ADD CONSTRAINT order_unique UNIQUE (order_id);

In this example, we've created a unique constraint on the existing order_details table called order_unique. It consists of the field called order_id.

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

ALTER TABLE order_details
ADD CONSTRAINT order_date_unique UNIQUE (order_id, order_date);

Drop Unique Constraint

The syntax for dropping a unique constraint in PostgreSQL is:

ALTER TABLE table_name
DROP CONSTRAINT constraint_name;
table_name
The name of the table to modify. This is the table that you wish to remove the unique constraint from.
constraint_name
The name of the unique constraint to remove.

Example

Let's look at an example of how to remove a unique constraint from a table in PostgreSQL.

ALTER TABLE order_details
DROP CONSTRAINT order_unique;

In this example, we're dropping a unique constraint on the order_details table called order_unique.

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