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There is a difference between MariaDB's behavior and MySQL 5.5 when a query is doing select @var := count(*). MySQL would return the number of rows matched while Maria returns 0.. Its unclear to me which behavior is actually correct.
While I personally wouldn't have used the := syntax here, someone else did and it broke our application a little bit..
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Minimal Example:
drop database if exists `test_example`;
create database test_example;
use test_example;
CREATE TABLE `blarg` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`v` varchar(10),
PRIMARY KEY (`id`)
) ENGINE=InnoDB;
insert into blarg (`v`) values ('AAA');
insert into blarg (`v`) values ('AAA');
insert into blarg (`v`) values ('AAA');
insert into blarg (`v`) values ('BBB');
insert into blarg (`v`) values ('BBB');
insert into blarg (`v`) values ('CCC');
MariaDB [test_example]> select count(*), @a := count(*), v from blarg group by v;
+----------+----------------+------+
| count(*) | @a := count(*) | v |
+----------+----------------+------+
| 3 | 0 | AAA |
| 2 | 0 | BBB |
| 1 | 0 | CCC |
+----------+----------------+------+
3 rows in set (0.00 sec)
mysql> select count(*), @a := count(*), v from blarg group by v;
+----------+----------------+------+
| count(*) | @a := count(*) | v |
+----------+----------------+------+
| 3 | 3 | AAA |
| 2 | 2 | BBB |
| 1 | 1 | CCC |
+----------+----------------+------+
3 rows in set (0.00 sec)
mysql>
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It was fixed in 10.0.5. I can't point at the particular revision, as it was fixed by a big merge of 10.0-base (revno 3817 on 10.0 tree).
It doesn't look critical enough to backport the fix to 5.5; please comment if you disagree.
(I've set 'Fix Version' to 10.0.6 because 10.0.5 is not on the list).