IF (SELECT CASE WHEN b=1 THEN 1 WHEN c=dbo.Calc_f(x) THEN 1 ELSE 0 END)=1 ... ELSE ...
IF CASE WHEN b=1 THEN 1 WHEN c=dbo.Calc_f(x) THEN 1 ELSE 0 END=1 ... ELSE ...
IF CASE WHEN b=1 THEN 1 WHEN c=dbo.Calc_f(x) THEN 1 END=1 ... ELSE ...