Question

I have 3 tables:

User (PK userid, ...)  
Computer (PK computerid, FK userid, FK cpuid, ...)   
CPU (PK cpuid, ...)

So, a user can have multiple computers, and each computer has exactly one CPU. Same cpu can appear in different computers.

I would like to get the number of computers and unique CPUs per user.

userid | Number of computers | Number of CPUs
---------------------------------------------
     1 | 3                   | 1     <- has 3 comps all with the same cpu
     2 | 13                  | 4     <- has 13 comps with 4 different cpus
...and so on

I dont have the problem with counting computers, but I stuck on counting the CPUs. Joining tables causes bad results because if the same cpu appears in more then one computer – count() returns total number of records as if they were not grouped…

Answer 1

You don’t need a join, because you can count the IDs instead of using COUNT(*). In this case, your query would be:

SELECT userid, COUNT(DISTINCT computerid) AS Computers, COUNT(DISTINCT cpuid) AS CPUs
FROM Computer
GROUP BY userid