To do something similar on a expression level use the conditional operator ( ? : ) instead. On the fly variables and functions evaluation In addition ... evaluated before but not cancelable In scripts ...

Transact-SQL reference for using DECLARE to define local variables for use in a batch ... The expression can be a noncomputed column name, constant, built-in function, variable, or any combination of ...

or symbol substitutions in the host variable declare section or in any SQL statements. You can use these constructs elsewhere in your application. The SQL precompiler does not handle preprocessor ...