Question

I have a MySQL (v 5, MyISAM) query that returns different rows depending on date string format.

(1) IFNULL(date1, ADDDATE('2008/10/31 23:59:59',INTERVAL 1 DAY)) > '2008-10-31 23:59:59' (2) IFNULL(date1, ADDDATE('2008/10/31 23:59:59',INTERVAL 1 DAY)) > '2008/10/31 23:59:59'  (3) date1 > '2008-10-31 23:59:59' (4) date1 > '2008/10/31 23:59:59' 

‘/’ vs ‘-‘ on RHS of ‘>’ comparisson operator.

(1) 75,098 rows *expected* (2) 0 rows *DIFFERENCE* (3) 199 rows *simple case as expected* (4) 199 rows *simple case as expected* 

Question – Why ?

Answer 1

Short answer: Use CAST(... AS DATE)

Long answer:

From MySQL DATE type:

A “relaxed” syntax is allowed: Any punctuation character may be used as the delimiter between date parts or time parts. For example, ’98-12-31 11:30:45′, ‘98.12.31 11+30+45′, ’98/12/31 11*30*45′, and ’98@12@31 11^30^45’ are equivalent.

In your examples 1 and 2 you’re using ADDDATE… this will always return a date with a ‘-‘ separator, no matter what the input format:

+-----------------------------------------------+ | ADDDATE('2008/10/31 23:59:59',INTERVAL 1 DAY) | +-----------------------------------------------+ | 2008-11-01 23:59:59                           | +-----------------------------------------------+ 

You’re also using IFNULL, which loses the type information, so when you compare this it’s comparing as strings.

What you can do is cast this back to a date:

mysql> SELECT IFNULL(null, ADDDATE('2008/10/31 23:59:59',INTERVAL 1 DAY)) > '2008/10/31 23:59:59'; +-------------------------------------------------------------------------------------+ | IFNULL(null, ADDDATE('2008/10/31 23:59:59',INTERVAL 1 DAY)) > '2008/10/31 23:59:59' | +-------------------------------------------------------------------------------------+ |                                                                                   0 | +-------------------------------------------------------------------------------------+ 1 row in set (0.00 sec)   mysql> SELECT CAST(IFNULL(null, ADDDATE('2008/10/31 23:59:59',INTERVAL 1 DAY)) AS DATE) > '2008/10/31 23:59:59'; +---------------------------------------------------------------------------------------------------+ | CAST(IFNULL(null, ADDDATE('2008/10/31 23:59:59',INTERVAL 1 DAY)) AS DATE) > '2008/10/31 23:59:59' | +---------------------------------------------------------------------------------------------------+ |                                                                                                 1 | +---------------------------------------------------------------------------------------------------+