Question

I have a terrible doubt.

I have a query like this:

SELECT id,fatherID FROM comments WHERE fatherID IS NULL 
 UNION
SELECT id,fatherID FROM comments WHERE fatherID IS NOT NULL 
 LIMIT 1

(Note the limit 1 applies to the union, not to only the second query)
(Note2: IS NULL, and IS NOT NULL are just an example, they can be a random string too)

With this limit 1 I am sure that this kind of query will return the row from the first query all the time?

Or even if both query returns something it can happen that the LIMIT 1 get the row coming from the second query?

I am asking this because If i run that query (wtihtou limit 1) I get this results

http://img856.imageshack.us/img856/5212/immaginejv.jpg

When I would except something like this (First all the rows with fatherid = null, then the other):

http://img847.imageshack.us/img847/479/immaginehx.jpg

Answer 1

Order of returned results is (by default) undefined. If you want them ordered in a specific manner, use ORDER BY, it was made to do exactly this.