The other day, my friend Jonathan (of Camenisch Creative) surprised me by pointing out that SET FMTONLY ON changes the way Microsoft SQL Server processes control-of-flow statements (IF…ELSE…END, GOTO, RETURN, etc.)
When SET FMTONLY is OFF (the default), SQL Server processes control-of-flow statements normally. In the case of an IF statement, SQL Server determines which conditional expression evaluates to true and then executes the code associated with that expression. Turn FMTONLY to ON and SQL Server executes every conditional branch, even those associated with conditions evaluating to false!