I am having difficulty understanding a query here. My query ultimately returns a result set in a report where you can specify a Person in the parameter to view their data. I am joining my Dimension table to my Employee table to return the Name from the Employee table. It looks something like this

Declare @PM varchar(30)
Set @PM = 'John Smith'

SELECT....FullName, EmployeeID, .... 
FROM...
Inner Join EmployeesT on emp.EmployeeNumber = DimP.PersonID
WHERE FullName in (@PM)

Note: My Employee table is in nvarchar and Dimension is is varchar but i dont think that matters as the join still works.

Now, I set a parameter up at the top for testing.

Here is my issue: If I switch the WHERE clause to say WHERE DimP.PersonID IN ('12345') my query takes 3 seconds to run. When I change the query to WHERE FullName in (@PM) the query is taking forever to run; it hangs and runs for 5+ minuets. Has anyone experienced a similar issue?

The result set produces the correct data with multiple "people" and I want to test this by specifying a person, not an id, but when I change the parameter to the Name the query hangs..