UNIQUE Constraint in PostgreSQL

Understanding the UNIQUE Constraint in PostgreSQL: The Definitive Guide


Data integrity is the cornerstone of any robust database system, and PostgreSQL offers various constraints to help maintain this integrity. One such constraint is the UNIQUE constraint, which ensures that all values in a column are unique. Let’s explore the ins and outs of this fundamental constraint and how to make the most of it in PostgreSQL.

What is the UNIQUE Constraint?

The UNIQUE constraint enforces the uniqueness of values in one or multiple columns. Unlike a PRIMARY KEY constraint, a table can have multiple UNIQUE constraints, and the columns involved can also contain NULL values (since NULL is considered unique).


To define a UNIQUE constraint, you can use the following syntax during table creation:

CREATE TABLE table_name (
    column1 data_type UNIQUE,
    column2 data_type,

For composite UNIQUE constraints involving multiple columns:

CREATE TABLE table_name (
    column1 data_type,
    column2 data_type,
    UNIQUE (column1, column2),

Example Using the tv_series Table

Let’s extend our tv_series table to include a code column, which should have unique values:

ALTER TABLE public.tv_series

With this addition, PostgreSQL will ensure that any value you insert into the code column will be unique across the table.

Common Errors and Solutions

Error Message:

ERROR: duplicate key value violates unique constraint "tv_series_code_key"


You encounter this error when trying to insert or update a record with a value in the code column that already exists in the table.


To avoid this error:

  • Ensure that the value you’re inserting or updating is unique.
  • If applicable, you can use database functions to generate unique values.

Error Message:

ERROR: multiple primary keys for table "tv_series" are not allowed


This error arises when you attempt to add another PRIMARY KEY constraint to a column already constrained by a UNIQUE constraint.


You can:

  • Drop the existing PRIMARY KEY or UNIQUE constraint before setting a new PRIMARY KEY.
  • Choose a different column for the PRIMARY KEY if needed.

Best Practices and Tips

  • The UNIQUE constraint also creates a unique B-tree index on the column, which can speed up data retrieval. However, remember that maintaining this index has a performance cost during data insertion and updates.
  • If you expect to perform a lot of searches based on a column, a UNIQUE constraint can be beneficial beyond enforcing data integrity.
  • Carefully consider the trade-offs of adding multiple UNIQUE constraints to a table, as each additional constraint requires more resources to maintain.


The UNIQUE constraint is a powerful tool for maintaining data integrity and improving query performance in PostgreSQL. By understanding its functionality and limitations, you can make more informed decisions when designing your database schema. Keep in mind the best practices and common errors, and you’ll be well on your way to mastering this essential feature.

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