How to Join a Table Twice on Different Columns to Display Usernames Instead of User IDs?

Joining the Same Table Twice on Different Columns

Question:

A user and complaint table are established, where users can raise and resolve complaints. The complaint table includes the user_id of both the complainant and the resolver. The goal is to write a query that displays the usernames of both parties instead of their user_ids.

Answer:

To join the user table twice on different columns and retrieve the usernames, the following query can be used:

SELECT 
     complaint.complaint_text, 
     A.username AS OpenedBy, 
     B.username AS ClosedBy
FROM 
     complaint 
     LEFT JOIN user A ON A.user_id=complaint.opened_by 
     LEFT JOIN user B ON B.user_id=complaint.closed_by

In this query:

• The LEFT JOIN clause is used to join the complaint table with the user table twice.
• Alias A is used for the first join, referencing the complainant's username.
• Alias B is used for the second join, referencing the complaint-resolver's username.
• The user table is joined to the complaint table based on the opened_by and closed_by columns, retrieving the relevant usernames.

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