I am having a problem in a table that calculates P&L on each security in the
database. It would not distinguish between different classes of security even
though each security has a unique identifier.

For instance, it calculates SBER and SBERP as having identical P&L in a
month where only SBERP has produced P&L

In months where both securities produced P&L it calculates the results
accurately.

What am I doing wrong?