Question

Table 1:
AccountId, ReferenceId, Name, (lots of other columns)

Table 2:
AccountId, ReferenceId, (other columns)

How can I do a select to get the following:

AccountId, ReferenceId, [Count(*) in Table2 where accountId and reference ID match.]
1, AB, 1
1, AC, 0
2, AD, 4
2, EF, 0

etc

Guessing a join, but that gives me values, not a count?
Tried adding a count, but get errors?

Answer 1

Something like:

SELECT t1.AccountId, t1.ReferenceId, COUNT(t2.AccountId)
FROM Table1 t1
LEFT JOIN Table2 t2 ON t1.AccountId = t2.AccountId AND
                       t1.ReferenceId = t2.ReferenceId
GROUP BY t1.AccountId, t1.ReferenceId

should work. The trick is to group by both key values so you can aggregate over other values. In this case you want to simply count values from other rows (you could also sum or average values from the grouped-by rows.).