Question

I stumbled upon this T-SQL query created on a SQL Server 2005 database:

Select s_bs.ScheduleHeaderId, 
       s_bs.ScheduleBatchScheduleId, 
       FlowRateOperational.FlowRate, 
       BatchScheduleFlowRateOperational.EffectiveStartTime, 
       BatchScheduleFlowRateOperational.EffectiveEndTime
From BatchSchedule as bs 
Inner Join ConnectionPoint as cp on bs.ConnectionPointId = cp.ConnectionPointId 
Inner Join ScheduleBatch as s_b 
Inner Join ScheduleConnectionPoint as s_cp 
Inner Join ScheduleBatchSchedule as s_bs 
            on s_cp.ScheduleConnectionPointId = s_bs.ScheduleConnectionPointId 
            on s_b.ScheduleBatchId = s_bs.ScheduleBatchId 
            on cp.ConnectionPointName = s_cp.ConnectionPointName and bs.BatchID = s_b.BatchID 
Inner Join BatchScheduleFlowRateOperational on bs.BatchScheduleId = BatchScheduleFlowRateOperational.BatchScheduleId 
Inner Join FlowRateOperational on BatchScheduleFlowRateOperational.FlowRateOperationalId = FlowRateOperational.FlowRateOperationalId

I am not a SQL expert by far but at least I think I know how to join tables and I had never seen this way of joining tables before.

Is having several ON clauses together after their JOINS producing different results or increasing performance?

Why couldn’t this person just move the joins around and keep the ON clauses beside their corresponding JOIN?

Thanks for any light you can shed on this “mystery” 🙂

Answer 1

The ON clause can be placed right after each INNER JOIN (which looks nicer to me). But sometimes it’s inevitable to put the ON clauses for the first couple of joins after a later join. For example the table which provides the join condition is not yet available in the JOIN chain, for example:

SELECT * FROM 
A INNER JOIN 
B INNER JOIN 
C 
   ON B.Col1 = C.Col1 
   ON A.Col2 = C.Col2

Look that both ON clauses are referring to C so the associated ON clause could not appear earlier. It can be prevented by putting C first, but depending on the usage, one may find it less readable.

The above is just from the syntax point of view. There should be some performance considerations as well, which I’m not aware of.