How to Identify Both Complainant and Complaint Resolver in a Single Query?

Joining the Same Table Twice on Different Columns for User Identification

Consider a database scenario involving a user table and a complaint table. The complaint table includes information such as the user who opened the complaint, the complaint text, and the user who closed it. All users involved in complaints (complainers and complaint resolvers) are stored in the user table.

To identify both the complainant and complaint resolver usernames, we need to join the complaint table with the user table twice, once for each user column. The following query accomplishes this:

<code class="sql">SELECT 
     complaint.complaint_text, 
     A.username, 
     B.username
FROM 
     complaint 
     LEFT JOIN user A ON A.user_id=complaint.opened_by 
     LEFT JOIN user B ON B.user_id=complaint.closed_by</code>

In this query:

• We start by selecting the complaint text as well as the usernames for both the opened_by and closed_by users.
• We left join the complaint table with the user table twice, assigning the aliases "A" and "B" to distinguish between the two joins.
• The ON clause ensures that the appropriate user rows are joined based on the opened_by and closed_by columns from the complaint table.

Executing this query will retrieve the complaint text along with the usernames of both the complaint opener and the complaint resolver, providing a comprehensive view of the complaint data with respect to user identification.

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