Using CASE in WHERE Statement when parameter has multiple values

Question

I have a problem which I think relates to having a multiple value parameter.

In my TblActivity there are two fields TblActivity.ActivityServActId

Answer 1

In SQL, the IN clause does not support parameters the way you are using them. The general syntax is

IN (1, 2, 3, 4)

you have

IN (@Param)

where something like @Param = '1, 2, 3, 4'

Internally, SQL will turn this into

IN ('1, 2, 3, 4')

Note the quotes... you are now matching against a string!

There are a number of ways to address this. Search SO for "sql in clause parameter", pick one that works for you, and upvote it.

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(Added)

Parameterize an SQL IN clause seems pretty definitive on the subject. While long ago I upvoted the third reply (the one with table-value parameters), any of the high-vote answers could do the trick. The ideal answer depends on the overall problem you are working with. (I am not familiar with SSRS, and can't give more specific advice.)