JOIN and aggregate problem

I have 2 tables T1 and T2

T1 has the columns: D, S, C. The combination of D,S,C is unique.
T2 has the columns: D, S, C, and boolean X. The combination of D,S,C is
not unique. 

I need to produce the following result for every occurrence of T1:
D,S,C, COUNT

COUNT is the number of matching D,S,C combinations in T2 where X = true.
There might be no matching pair in T2 or there might be match but X
is false.

How can I express this?



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Best Regards,

Tarlika Elisabeth Schmitz