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Databases

mykolash
mykolash
12,955 Points
Posted by mykolash
mykolash
12,955 Points

Why can't we use two embedded SELECT + WHERE + IN requests instead of two JOINs as teacher Ben Deitch did?

I've solved the task in a bit different way. Can smbd say which way is better - joining tables or using SELECT request multiple times?

-- Which teachers teach only students in 8th grade?
SELECT Teachers.first_name, Teachers.last_name FROM TEACHERS WHERE Teachers.id IN
  (
  SELECT Classes.teacher_id FROM CLASSES WHERE Classes.subject_id IN
    (
      SELECT id FROM Subjects WHERE grade=8
    )
  );

Ben Deitch

And thanx in advance.

PS: Oops. here is link to the lesson

1 Answer

Steven Parker
Steven Parker
231,128 Points
Steven Parker
Steven Parker
231,128 Points

Your alternative does a good job of providing the same result set. And by filtering using IN, you eliminated the need for the DISTINCT restriction.

But the additional filters and sub-queries might cause this query to be a bit less efficient than the original. Benchmark testing might be needed to be certain.

The original may also be easier to read and comprehend, but that could be a matter of personal preference.

But as far as getting the job done, this is clearly an equally valid solution. :+1:

mykolash
mykolash
12,955 Points
mykolash
mykolash
12,955 Points

Thanx a lot, Steven Parker

I've used multiple-SELECTs with such a logic. Using JOINs we add more and more data to the request and create bigger JOINed table and filtering it after everything is added. While using multiple-SELECTs we request separated tables one by one, prolonging request in time, but decreasing processor calculations at each moment. What, I think/hope will allow to provide requests on other threads. Not sure, I've explained it in clear manner :-(

Anyway, thanx for your answer!

Steven Parker
Steven Parker
231,128 Points
Steven Parker
Steven Parker
231,128 Points

As I said, it might take benchmark testing to be sure, but the database may quite likely perform optimizations that would discard columns not used in the final SELECT. I would expect sub-queries and filters to be more costly in terms of performance than JOINs.