More adventures in T-SQL

I often see overly complicated where clauses in procedures or views that could have been simplified with a little trick I have been using for years.

Here is an example of a where clause which is hard to read, and therefore hard to debug and modify:

WHERE (SV00700.Service_Call_ID = @Service_Call_ID OR
@DocumentNumber <> '' OR
@BatchNum <> '') AND (SV00700.Call_Invoice_Number = @Call_Invoice_Number OR
@DocumentNumber <> '' OR
@BatchNum <> '') AND (@DocumentNumber = '' OR
@DocumentNumber = rtrim(SV00700.Call_Invoice_Number)) AND (@BatchNum = '' OR
SV00700.BACHNUMB = @BatchNum)

The intent here was to filter based on a combination of parameters.
The trick I like to use is simple:  Assume all parameters are empty and temporarily replace them with the values.  If the statement still looks complicated, you need to refactor.

In this case:
WHERE (SV00700.Service_Call_ID = '' OR

'' <> '' OR
'' <> '') AND (SV00700.Call_Invoice_Number = '' OR
'' <> '' OR
'' <> '') AND '' = '' OR
'' = rtrim(SV00700.Call_Invoice_Number)) AND '' = '' OR
SV00700.BACHNUMB = '' ) AND remit2.INTERID = rtrim(db_name())

Notice all those instances of '' <> ''.  That is what SQL is actually seeing at run-time if the parameter is an empty string. 

We can make this much easier to read by figuring out how to evaluate each parameter individually, rather than trying to evaluate all possible combinations.

Consider this:
WHERE SV00700.Service_Call_ID LIKE CASE RTRIM(@Service_Call_ID) WHEN '' THEN '%' ELSE RTRIM(@Service_Call_ID) END
  AND SV00700.Call_Invoice_Number LIKE CASE RTRIM(@Call_Invoice_Number) WHEN '' THEN '%' ELSE RTRIM(@Call_Invoice_Number) END
  AND SV00700.Call_Invoice_Number LIKE CASE RTRIM(@DocumentNumber) WHEN '' THEN '%' ELSE RTRIM(@DocumentNumber) END
  AND SV00700.BACHNUMB LIKE CASE RTRIM(@BatchNum) WHEN '' THEN '%' ELSE RTRIM(@BatchNum) END
It accomplishes the same thing, but is much easier to read.  Further, each parameter can be easily removed without reworking the entire statement.
 
When we apply the trick to it:
WHERE SV00700.Service_Call_ID LIKE CASE RTRIM('') WHEN '' THEN '%' ELSE RTRIM('') END

AND SV00700.Call_Invoice_Number LIKE CASE RTRIM('') WHEN '' THEN '%' ELSE RTRIM('') END
AND SV00700.Call_Invoice_Number LIKE CASE RTRIM('') WHEN '' THEN '%' ELSE RTRIM('') END
AND SV00700.BACHNUMB LIKE CASE RTRIM('') WHEN '' THEN '%' ELSE RTRIM('') END

It is still easy to read. 

I hopes this helps someone else to go forth and simplify.



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