CASE 表达式在 SQL 服务器中有效，但在 Visual Studio Reporting Services 中无效

Question

There is a where statement:

WHERE (@obsolete=(case  when part.drawissno = 'OBS' then 'OBS' else 'NO' end) or @obsolete is null)
and ((CASE WHEN part.sm = 'MANUFACTURED' THEN mpfmain.mpflang END) = @country or @country IS NULL)

The parameters work correctly in SQL Server, but when added to the Report, all values, except null, don't work. Parameter @country has values: GERMANY, SPAIN, FRANCE, ITALY. The parameter in report works only when the user has to input the country himself. If write in parameter properties the available values, they do not work. Other parameters, which are not included here, work correctly, but they don't have CASE within themselves. Could it be the problem with the CASE expression?

Answer 1

case expressions just make where clauses harder to follow and harder to optimize. They can usually be rewritten without case :

where (@obsolete is null or part.drawissno = @obsolete) and
      (@country is null or (part.sm = 'MANUFACTURED' and mpfmain.mpflang = @country))

Answer 2

I see 2 possible problems with the second CASE expression:

1) If mpfmain.mpflang is NULL then you compare with NULL with the equal sign. This will not work even if @country is also NULL.

2) You are missing an ELSE part. It is of course not mandatory, but consider what happens if either part.sm is NULL, or part.sm has another value than 'MANUFACTURED'.

You might benefit from the ISNULL method around columns or parameters than can be NULL. You can read more about the ISNULL method here:

https://docs.microsoft.com/en-us/sql/t-sql/functions/isnull-transact-sql