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Start your free trialYingzi Huang
3,216 PointsTask #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
231,236 PointsYou 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
3,216 PointsYingzi Huang
3,216 PointsThanks for clarifying, Steven! This logic makes perfect sense.