updating table based on other table values

[DebugasRu]

Hello i can't figure out how to update the table tmpISM based on other
table ta values. What i need i can intuitivly express as follows:

update tmpISM
set tmp.SUMA=tmp.SUMA-ta.SUMA
where (tmp.PID=ta.PID) and (tmp.PAJ=ta.PAJ)

index PID+PAJ is unique both for tmpISM and ta
Any suggestions how the update should look like (i suspect it will
have to have sub-select)