How to fix Undefined column in PostgreSQL

First, check your query for typos. Review the exact spelling of the column name you're referencing:

SELECT * FROM users WHERE user_name = 'john';

Compare it carefully with the actual column name in your table. Even one character difference will cause the error.

Use the information_schema to list all columns in your target table:

SELECT column_name 
FROM information_schema.columns 
WHERE table_name = 'users';

Alternatively, use the psql command line tool:

\d users

This will show you all columns with their exact names and types. Verify that the column you're trying to access actually exists.

If your column was created with mixed case (e.g., "UserName"), you must use double quotes to reference it:

SELECT "UserName" FROM users WHERE id = 1;

Without quotes, PostgreSQL converts unquoted identifiers to lowercase, resulting in the undefined column error. This is PostgreSQL's standard behavior for identifier case handling.

When using multiple tables, always prefix column names with the appropriate table alias:

SELECT u.id, u.name, p.title 
FROM users u
JOIN posts p ON u.id = p.user_id
WHERE u.id = 1;

Without the table prefix (u., p.), PostgreSQL cannot determine which table the column belongs to, causing the undefined_column error.

If the column truly doesn't exist and you need to add it, use ALTER TABLE:

ALTER TABLE users ADD COLUMN email VARCHAR(255);

After adding the column, update your queries to reference it correctly. For optional columns that may not have values yet, consider adding a DEFAULT value or making the column nullable.