How can I further optimise this query?
Is it to do with the Count(*), the OR joins, or the IS (NOT) NULLs? Or something else?
How would you optimise this or rewrite the query?
Since this query runs in 43 milliseconds, I'm not sure how much time and effort it's worth to try and make it run faster. It's possible that an index on contno, divno and WFMClientID might reduce the I/O cost, but that's just a guess – I have no idea how many other columns are in that table, and whether the added maintenance from that index would be worth shaving a few milliseconds off the query.