max vs order by example

Let me create some sample data, to show you why:

mid :: mday_no :: something
1 :: 37 :: apple
2 :: 37 :: banana
2 :: 38 :: canteloupe

Now consider the first query:

SELECT * FROM cb_main WHERE mid = '1' AND mday_no=(select max(mday_no))

Okay, the select max(mday_no) will return a value of 38, right? So then that query becomes the same as

SELECT * FROM cb_main WHERE mid = '1' AND mday_no = 38

And you can clearly see that there are NO RECORDS AT ALL that match that query. So you will get a blank recordset.

Now compare that to:

SELECT * FROM cb_main WHERE mid = '1' ORDER BY mday_no DESC limit 1

That *WILL* return the record
1 :: 37 :: apple