I'm storing user data (username, email, password hash) in PostgreSQL, and I can only enforce uniqueness on username. I'd like to let users provide either their username or their email when they login. However, when matching their input against usernames and email addresses I want to ensure that a username match gets priority over an email match.

For example, in the following contrived instance:

id | username        | email address    | pw_hash
1    john_doe          [email protected]    x1j34
2    [email protected]   [email protected]    x1j34

if both users enter their username, I want to guarantee that user 2 gets logged in.

If I use:

SELECT * FROM member WHERE (username='jo[email protected]' OR email='[email protected]') AND pw_hash='x1j34';

this matches both users but provide no guarantee on order.

Is there a way to have to conditions, and only check the second condition if the first isn't satisfied? Can this be done in a single query?

1 Answer 1


You have two conditions to check during the authentication:

  1. There is a user with matching username; do not need to match by email.

  2. There are no users with matching usernames; need to match by email.

So the query is:

WITH matching_username AS (SELECT * FROM member WHERE username = '[email protected]')
SELECT * FROM matching_username WHERE (SELECT COUNT(*) FROM matching_username) = 1
                                   AND pw_hash = 'x1j34'
SELECT * FROM member            WHERE (SELECT COUNT(*) FROM matching_username) = 0
                                   AND email = '[email protected]' 
                                   AND pw_hash='x1j34';

On the other hand, there is a problem with your design of the table.

What if two users have have the same email, but neither of them have email as their username? If one of them tries to authenticate with an email, you will not be able to tell, which user record to authenticate against.

  • Yes, that would be an issue. I'm modeling off a prominent site that allows duplicate emails. I've found that in their case, if two users share an email and pass, logging in with email fails without an explanation. The user must use his username in that case. Certainly not ideal, but perhaps this is enough of an edge case (only applying to users who are playing around with the system) that it's sufficient. Jun 9, 2014 at 8:21
  • @LorenzForvang Surely, if you are okay with these unlikely but possible issues, it's a fine scheme. By the way, the query is what you needed, right? Jun 9, 2014 at 8:24
  • The suggested query returns both users, but I'd like only User 2 returned. Jun 9, 2014 at 8:26
  • @LorenzForvang Yes, I haven't looked into the query in depth at first. Check the updated one. Jun 9, 2014 at 8:47

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