GetSum count on combined fields

My database is a list of attendance. Field 1 is the key from the 'type of class' related database and field 2 is from a value list defining if they were 'booked' or 'attended' the class. I have to generate two fields with the result to use in a portal in a related 'student' database.

I'm trying to get a count of the different contents of a field (field 2 in the example) only when this field is either "a" or "b" but this count is only to occour when another field (field one) is the same.

i.e.(field 2&3 are the result)

record 1 - field 1="a01" / field 2="a" / field 3="2" / field 4="1"

record 2 - field 1="a01" / field 2="a" / field 3="2" / field 4="1"

record 3 - field 1="a01" / field 2="b" / field 3="2" / field 4="1"

record 4 - field 1="a02" / field 2="a" / field 3="1" / field 4="0"

record 4 - field 1="a03" / field 2="b" / field 3="0" / field 4="1"

I tried combining field1&2 in a calc and then did a gSum on this calc (with logic opps to exclude any result when field 2 is not "a" or "b". And I tried using self joins but I cant work it out simply....

However I have worked it out if I create a field in the 'Type of class" database that calculates on thier relationship the max(getSUM) and then in the original "attandance" database have a calc that dumps that field back into a new pair of fields. This is very messy and long winded -anyone else have a better idea?