Unique Usernames Guaranteed: Step-by-Step Guide to Enforcing Uniqueness in PostgreSQL

sql postgresql Making a Column Unique in PostgreSQL: A Beginner's Guide

Understanding the Problem:

  • SQL: Structured Query Language, used to interact with databases like PostgreSQL.
  • PostgreSQL: A popular relational database management system.
  • Unique Constraint: Ensures each value within a specified column is distinct.


The ALTER TABLE command in PostgreSQL empowers you to modify existing tables. To add a unique constraint, use the following syntax:

ALTER TABLE table_name ADD CONSTRAINT constraint_name UNIQUE (column_name);


  • table_name: Replace with the actual name of your table.
  • constraint_name: Choose a descriptive name for the constraint (optional).
  • column_name: Specify the column you want to enforce uniqueness on.


Assume you have a table named users with columns id (primary key), username, and email. To ensure unique usernames:

ALTER TABLE users ADD CONSTRAINT unique_username UNIQUE (username);

Important Notes:

  • This operation will succeed only if existing data adheres to the uniqueness rule. If duplicates exist, the command will fail.
  • Adding a unique constraint automatically creates an index on the specified column. This can improve performance for queries that utilize the unique column but might impact write performance slightly.
  • You can also create a unique constraint along with adding a new column:
ALTER TABLE products ADD COLUMN product_code VARCHAR(20) UNIQUE;

Related Issues and Solutions:

  • Duplicate Data: Before adding the constraint, identify and remove or modify duplicate values using queries like SELECT username FROM users GROUP BY username HAVING COUNT(*) > 1;.
  • Performance Impact: Consider creating a partial unique constraint if only specific combinations of values within the column need to be unique.

Additional Tips:

I hope this explanation, along with the examples, empowers you to confidently make columns unique in your PostgreSQL tables!