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Stephen Funk
Courses Plus Student 2,650 PointsJOINS Review & Practice
-- Use a JOIN to select all patrons with outstanding books. Select their first name and email address. loans table columns: id, patron_id, book_id, loaned_on, return_by, returned_on. patrons table columns: id, first_name, email
SELECT pt.first_name, pt.email FROM patrons AS pt
INNER JOIN loans AS ln ON pt.id = ln.patron_id
WHERE ln.returned_on = NULL;
I keep getting errors that no RIGHT and FULL OUTER JOINS are in this pre-programmed objective. The only criteria that both tables share are IDs.
Do I need a sub-query? I have wasted about about a day.
4 Answers
Steven Parker
231,236 PointsYou're really close.
When testing for NULL, instead of the standard equality operator, you use the word "IS".
... WHERE ln.returned_on IS NULL;
Stephen Funk
Courses Plus Student 2,650 PointsI lost a day on the word IS. Wish I posted earlier when you were helping me with the other stuff.
Dwayne Pate
12,249 PointsYou've summed up my programming career in one sentence!
Oğulcan Girginc
24,848 PointsUpdate: Ignore this answer...
~Also, I think, you are suppose to use LEFT OUTER rather than INNER for your JOIN.~
Steven Parker
231,236 PointsYou are told to "select all patrons with outstanding books". Since having outstanding books guarantees that there will be a record in the loans table for the patron, then you don' t need a LEFT OUTER join here.
Oğulcan Girginc
24,848 PointsSteven Parker I thought, this one was the second task, not first! My bad! :)
Luke Vaughan
15,258 PointsCouldn't you use
Select pt.first_name, pt.email from patrons as "pt"
INNER JOIN loans as "ln" on pt.id = ln.patron_id where ln.returned_on > ln.return_by;
No need for is NULL right?
Luke Vaughan
15,258 PointsOr is it asking to check and see who has not returned their book?
I am reading it as select all patrons that have a book that was returned late.
Steven Parker
231,236 PointsI took the phrase "outstanding books" in the instructions to mean "loans that have not been returned".