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

This is as simple as the following

SELECT voterid, COUNT(DISTINCT electionid) AS electioncount
FROM table
WHERE electionid IN (1, 2) /* substitute elections you are interested in here */
GROUP BY voterid
HAVING electioncount = 2 /* substiture number of election listed in where condition above

The result set size would provide the number of voters that meet your criteria (i.e. there is no reason to aggregate down furter (i.e. like with subselect) to get to that data.