Introduction
Simple CASE WHEN
Searched CASE WHEN
CASE WHEN with aggregates
18. CASE WHEN with COUNT DISTINCT
CASE WHEN with GROUP BY
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Instruction

Good! Now, in certain situations you may also want to include DISTINCT in your CASE WHEN statements with COUNT. Take a look:

SELECT
  COUNT(DISTINCT CASE WHEN pay_date BETWEEN
    '2015-06-01' AND '2015-06-05'
    THEN candidate_id END) AS accepted_student,
  COUNT(DISTINCT CASE WHEN pay_date = '2015-06-06'
    THEN candidate_id END)
    AS conditionally_accepted_student,
  COUNT(DISTINCT CASE WHEN pay_date > '2015-06-06'
    THEN candidate_id END) AS not_accepted
FROM application;

What changed here? Two things. First of all, we included the keyword DISTINCT in each COUNT. Second of all, we now count student_ids and not pay_dates. What is the meaning of these changes?

We previously counted the number of accepted payments, conditionally accepted payments and not accepted payments. Now, we count the number of candidates who were accepted for at least one degree course, the number of candidates who were conditionally accepted for at least one degree course and not accepted for at least one degree course.

We had to include the keyword DISTINCT because a single candidate can apply for more than one degree course, so if a candidate made two payments for two courses on time, that candidate would be counted twice (the candidate can be still counted twice in two different columns if the candidate paid for one degree course and didn't pay for another, but that's a different story).

Exercise

Show how many students paid the full fee of 50 (full_fee_sum) and the reduced fee of 10 (reduced_fee_sum), but if a certain student paid the same amount for more than one degree course, count them only once.

Stuck? Here's a hint!

One of the counts:

COUNT(DISTINCT CASE WHEN fee = 50 THEN candidate_id END) AS full_fee_sum

Console

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Result

TableConsole