Using Declare and Set in SQL statement

[fmsavey]

To make my lengthy calculated SQL statements easier to read I tried using the delcare and set functions (see example below ) so that I would only have to do one concatenation for each variable passed to the SQL statement from FM. Now when I execute this statement I get an error that says there were "No variables defined in the data source". What does this mean and how do I fix it?

"Declare @startdate as datetime

Set @startdate = '" & table_name::field_name & "'...