Constraints in PostgreSQL

An Introduction to Constraints in PostgreSQL: Your Roadmap to Data Integrity


Constraints are the backbone of any relational database, ensuring that data stays accurate and reliable. This article serves as an introductory overview of constraints in PostgreSQL, setting the stage for future deep dives into each specific type of constraint.

What Are Constraints?

Constraints are rules applied to table columns that enforce different kinds of checks on data. By defining constraints, you can maintain the accuracy and reliability of your data within the database.

Types of Constraints in PostgreSQL


Ensures that each record in a table is unique and not NULL.


Ensures that the value in a column or a set of columns matches the value in a column of another table.


Ensures that all values in a column are unique across the dataset.


Enforces specific conditions that each row must fulfill to be inserted into the table.


Ensures that a column cannot contain a NULL value.


Ensures that any two rows are distinct based on a specified set of columns.


Sets a default value for the column when no value is specified during data insertion.

Example Using the tv_series Table

Let’s take a look at how constraints are applied in a table. Here’s the tv_series table as an example:

CREATE TABLE public.tv_series (
    id serial4 NOT NULL,
    "name" text NOT NULL,
    genre text NOT NULL,
    seasons int4 NULL DEFAULT 1,
    is_running bool NULL DEFAULT true,
    release_year int4 NULL,
    CONSTRAINT tv_series_pkey PRIMARY KEY (id)

In this example, the id column has a PRIMARY KEY constraint, ensuring that each TV series has a unique identifier.

Common Errors and Solutions

Error Message:

ERROR: duplicate key value violates unique constraint "tv_series_pkey"


This error occurs when you attempt to insert a duplicate value into a PRIMARY KEY or UNIQUE column.


To resolve this, you can either:

  • Use a different unique value.
  • If the column is set to auto-increment, you can allow PostgreSQL to automatically generate a unique value for you.

Best Practices and Tips

  • Constraints can be set during the table creation process using CREATE TABLE, or they can be added later with ALTER TABLE.
  • Naming your constraints can simplify debugging and make your SQL code more self-explanatory.
  • Use constraints judiciously to prevent unnecessary performance overhead.


Constraints are a powerful feature in PostgreSQL, offering a wide array of options to ensure data integrity and quality. Although this overview briefly touched upon the basics, it lays the groundwork for more in-depth examinations of each constraint type. Understanding how to effectively apply constraints in your PostgreSQL environment can dramatically enhance data integrity and facilitate reliable queries.

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