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Databases

Posted by Mohammad Aslam
Mohammad Aslam
6,053 Points

How to query for patrons with no loans?

In a library database, there are three tables for books, loans (with dates for loans) and patrons. It is easy to query for the patron info for patrons who already have outstanding loans. But how do we select for patrons who don't have any loans and hence, whose id won't be in the loans table? You have to use JOIN clauses.

2 Answers

Steven Parker
Steven Parker
231,269 Points
Steven Parker
Steven Parker
231,269 Points

If you use a "LEFT OUTER JOIN", the patrons will still be included but any field from the loans table will be null. So you can select just those rows "WHERE loans.patron_id IS NULL".

For more specific answers, always show your query code and include a link to the course page you are working with.

Mohammad Aslam
Mohammad Aslam
6,053 Points
Mohammad Aslam
Mohammad Aslam
6,053 Points

Thanks