Condition for generating NULL - response (1) by msk.khurram

when you run the above query, the when condition is checked and as the argument is invalid, the condition becomes false and it goes to the else part. 
SELECT CASE WHEN (ACOS(2.3) IS NULL) THEN 'T' ELSE 'F' END AS C0
try it without the else part and you will get what is actually happining.
SELECT CASE WHEN (ACOS(2.3) IS NULL) THEN 'T' END AS C0