In a MySQL query, I am trying to select either a DATEDIFF() or NOW() based on an IF() condition. The behavior of MySQL is confusing me here.

This query run in phpMyAdmin has some strange results. The values should be identical, but they are not.

SELECT
                 TIMEDIFF(DATE_ADD(NOW(), INTERVAL 1 HOUR), NOW())  AS `A`,
IF(FALSE, NOW(), TIMEDIFF(DATE_ADD(NOW(), INTERVAL 1 HOUR), NOW())) AS `B`

A ----------------- | B -----------------
01:00:00 ---------- | 2001-00-00 00:00:00

If I try this query, changing NOW() to NULL in the first option of the IF condition, it shows the proper result for the TIMEDIFF. Why is this?

SELECT
                TIMEDIFF(DATE_ADD(NOW(), INTERVAL 1 HOUR), NOW())  AS `A`,
IF(FALSE, NULL, TIMEDIFF(DATE_ADD(NOW(), INTERVAL 1 HOUR), NOW())) AS `B`

A ----------------- | B -----------------
01:00:00 ---------- | 01:00:00

So then my question is: how is NOW() versus NULL affecting the IF return?

May be NOW() or other date functions have to return a value in all cases, because I was testing the same query with RAND() and SQRT() functions but in that case it is returning the correct value, so may be the issue is with NOW() function return value

TRY using NOW() with a blank CONCAT(),it won't manipulate your return value also, and it worked well both with TRUE and FALSE

 SELECT
      TIMEDIFF(DATE_ADD(NOW(), INTERVAL 1 HOUR), NOW())  AS `A`,
 IF(TRUE, CONCAT(NOW()), TIMEDIFF(DATE_ADD(NOW(), INTERVAL 1 HOUR), NOW())) AS `B`

SELECT TIMEDIFF( DATE_ADD( NOW( ) , INTERVAL 1 HOUR ) , NOW( ) ) AS  `A` ,
CASE WHEN TRUE THEN TIMEDIFF( DATE_ADD( NOW( ) , INTERVAL 1 HOUR ) , NOW( ) )
ELSE CONCAT( NOW( ) )
END AS  `B

both gave correct result