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Databases Querying Relational Databases Joining Table Data with SQL Review and Practice

Posted by Yingzi Huang
Yingzi Huang
3,216 Points

Task #1: How to count number of outstanding books each patron has and add this "COUNT" to a third column in same query?

In the practice problems for this section, after finishing the query for selecting all patrons with outstanding books, I can display the result with or without "DISTINCT" to show which patrons still needs to return books.

SELECT DISTINCT first_name, email FROM patrons INNER JOIN loans ON patrons.id = loans.patron_id WHERE returned_on IS NULL

Is there a method within the same query to "COUNT" the number of books per patron and write the result in the 3rd column?

I have tried using "GROUP BY" and "COUNT" but am unable to narrow down to the correct solution, likely due to how NULL entries are involved in this calculation.

1 Answer

Steven Parker
Steven Parker
231,271 Points
Steven Parker
Steven Parker
231,271 Points

You just need to add a count column and a GROUP BY clause.

Your WHERE clause is fine as is. But you won't use DISTINCT when you're grouping:

SELECT first_name, email, count(*) AS books  -- added 3rd column
FROM patrons 
INNER JOIN loans ON patrons.id = loans.patron_id 
WHERE returned_on IS NULL
GROUP BY first_name, email                   -- added group clause
Yingzi Huang
Yingzi Huang
3,216 Points

Thanks for clarifying, Steven! This logic makes perfect sense.