PostgreSQL Tutorial: UNIQUE Constraint

August 1, 2023

Summary: in this tutorial, you will learn about PostgreSQL UNIQUE constraint to make sure that values stored in a column or a group of columns are unique across rows in a table.

Sometimes, you want to ensure that values stored in a column or a group of columns are unique across the whole table such as email addresses or usernames.

PostgreSQL provides you with the UNIQUE constraint that maintains the uniqueness of the data correctly.

When a UNIQUE constraint is in place, every time you insert a new row, it checks if the value is already in the table. It rejects the change and issues an error if the value already exists. The same process is carried out for updating existing data.

When you add a UNIQUE constraint to a column or a group of columns, PostgreSQL will automatically create a unique index on the column or the group of columns.

PostgreSQL UNIQUE constraint example

The following statement creates a new table named person with a UNIQUE constraint for the email column.

CREATE TABLE person (
	id SERIAL PRIMARY KEY,
	first_name VARCHAR (50),
	last_name VARCHAR (50),
	email VARCHAR (50) UNIQUE
);

Note that the UNIQUE constraint above can be rewritten as a table constraint as shown in the following query:

CREATE TABLE person (
	id SERIAL  PRIMARY KEY,
	first_name VARCHAR (50),
	last_name  VARCHAR (50),
	email      VARCHAR (50),
        UNIQUE(email)
);

First, insert a new row into the person table using INSERT statement:

INSERT INTO person(first_name,last_name,email)
VALUES('john','doe','j.doe@rockdata.net');

Second, insert another row with duplicate email.

INSERT INTO person(first_name,last_name,email)
VALUES('jack','doe','j.doe@rockdata.net');

PostgreSQL issued an error message.

ERROR:  duplicate key value violates unique constraint "person_email_key"
DETAIL:  Key (email)=(j.doe@rockdata.net) already exists.

Creating a UNIQUE constraint on multiple columns

PostgreSQL allows you to create a UNIQUE constraint to a group of columns using the following syntax:

CREATE TABLE table (
    c1 data_type,
    c2 data_type,
    c3 data_type,
    UNIQUE (c2, c3)
);

The combination of values in column c2 and c3 will be unique across the whole table. The value of the column c2 or c3 needs not to be unique.

Adding unique constraint using a unique index

Sometimes, you may want to add a unique constraint to an existing column or group of columns. Let’s take a look at the following example.

First, suppose you have a table named equipment:

CREATE TABLE equipment (
	id SERIAL PRIMARY KEY,
	name VARCHAR (50) NOT NULL,
	equip_id VARCHAR (16) NOT NULL
);

Second, create a unique index based on the equip_id column.

CREATE UNIQUE INDEX CONCURRENTLY equipment_equip_id 
ON equipment (equip_id);

Third, add a unique constraint to the equipment table using the equipment_equip_id index.

ALTER TABLE equipment 
ADD CONSTRAINT unique_equip_id 
UNIQUE USING INDEX equipment_equip_id;

Notice that the ALTER TABLE statement acquires an exclusive lock on the table. If you have any pending transactions, it will wait for all transactions to complete before changing the table. Therefore, you should check the pg_stat_activity table to see the current pending transactions that are on-going using the following query:

SELECT
	datid,
	datname,
    usename,
	state
FROM
	pg_stat_activity;

You should look at the result to find the state column with the value idle in transaction. Those are the transactions that are pending to complete.

In this tutorial, you have learned about UNIQUE constraints and how to use them to enforce values stored in a column or a group of columns unique across rows within the same table.

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