Question

I’m doing some SQL optimizations but my SQL fu could use some improvement…

Why do a join on a subquery, could this just be a straight inner join, or am I missing something?

        -- Why do a join and a subquery?
        -- INNER JOIN  (
        --          SELECT client_id, session_id, id, customer_id, validity, machine_id, 
        --                  utc_date_completed, itemStatus, totalValue, utc_date_updated, 
        --                  itemValue, itemId, itemName, customField1, customField2, itemPage, itemCurrency
        --          FROM data.dashboard WITH(NOLOCK)
        --          WHERE client_id = @client_id
        --          ) d  ON cm.client_id = d.client_id

        INNER JOIN data.dashboard AS d ON cm.client_id = d.client_id AND d.client_id = @client_id 

Answer 1

Yes, functionnally they are the same, and you should have better performance doing an INNER JOIN. Check on the execution plan for performance.

And it is far more readable and maintainable.