I need to avoid a cartesian product in Informix SQL

Question

I'm getting a cartesian product that I don't want in a complex Informix SQL query.

I know which of the tables is causing the problem, but I can't figure out a way around the cartesian product.

The query I refer to is very long, but I am going to exemplify the problem with only two tables:

SELECT ROUND( SUM( CASE WHEN ccp.IdConcepto IN ( 60, 62 ) THEN ccp.Importe END ), 2 ) TotalInvoiced
    FROM Cbts cc, Cbts_Cpts ccp
    WHERE cc.CbteId = ccp.CbteId
    AND ccp.IdConcepto IN ( 10, 30, 31, 32 )

The table called Cbtes has a primary key called CbteId, which is not the primary key of Cbtes_Cptos. Cbtes_Cptos also has other columns, including IdConcepto (which I use in the query), IdSubConcepto, Unidades, Importe, etc.).

The problem is that in Cbtes_Cptos for the value 10 there are from 1 to 4 different records, and this difference is marked by the "IdSubConcepto" column, which can range from 1 to 4. The relationship can be the following:

Cbtes          Cbtes_Cptos
IdCbte        IdCbte  IdConcepto   IdSubConcepto
1               1        10              1
                1        10              2
                1        10              3
                1        10              4
                1        60              0
                1        62              0

So, the query returns the sum of several times the value of "Importe".

And I need to use the IdConcepto in the WHERE to speed up the query since it can have more than 5000 different values.

I hope I have explained the problem well, and I appreciate your attention.