SQL query to count number of times certain values occur in multiple rows

Question

Suppose I have a table of election data, call it ELECTIONS, with one row per voter per election, like so:

VoterID ElectionID
A           1
A           2
B                


        

Answer 1

Yes. what you have should work. (You will need to add an alias on the derived table, the error messsage you get should be self explanatory. Easy to fix, just add a space and the letter c (or whatever name you want) at the end of your query.

There's one caveat regarding the potential for duplicate (VoterID, ElectionID) tuples.

If you have a unique constraint on (VoterID, ElectionID), then your query will work fine.

If you don't have a unique constraint (which disallows duplicate (VoterID, ElectionId)), then there's a potential for a voter with two (2) rows for ElectionID 1, and no rows for ElectionID 2... for that voter to get included in the count. And a voter that voted twice in ElectionID 1 and only once in ElectionID 2, that voter will be excluded from the count.

Including the DISTINCT keyword inside a COUNT would fix that problem, e.g.

HAVING COUNT(DISTINCT ElectionID) = 2

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I'd write the query differently, but what you have will work.

To get the count of VoterID that participated in both ElectionID 1 and ElectionID2, for improved performance, I'd avoid using an inline view (MySQL calls it a derived table). I'd have the query use a JOIN operation instead. Something like this:

SELECT COUNT(DISTINCT e1.voterID) AS NumVoters
  FROM elections e1
  JOIN elections e2
    ON e2.voterID = e1.voterID
 WHERE e1.electionID = 1
   AND e2.electionID = 2

If you are guaranteed that (voterID, ElectionID) is unique, then the select could be simpler:

SELECT COUNT(1) AS NumVoters
  FROM elections e1
  JOIN elections e2
    ON e2.voterID = e1.voterID
 WHERE e1.electionID = 1
   AND e2.electionID = 2