Select syntax - Subtable

Hello, 

I am stuck with the select syntax. 

I have a table and a subtable (three columns : Year, Month, Index). I woudl like to get a formula to pickup the index of the substable with the same year and month that is present in the record of the maintable. 

If I do (select SUBTABLE).INDEX, I received the list of the INDEX. Each time, I am using the function where to reduce the result it does not work. 

Thanks in advance for your help.