set @num=0, @type := ”;
select * from fruits
where (@num := if(type = @type, @num + 1, 1))
and (@type := type)
and (@num <= 2);
Empty set (0.00 sec)

select @num, @type;
+——+——–+
| @num | @type |
+——+——–+
| 9 | cherry |
+——+——–+

On MySQL 5.1.41-3ubuntu12.10 the result is:

select @num, @type;
+——+——–+
| @num | @type |
+——+——–+
| 2 | cherry |
+——+——–+

I don't know how the heck you got 9 there.
Your article is excellent though.

I found your blog when searching for an odd side-effect of using user variables in (rather simple) queries -they get very slow.

As way of example, we join a couple of tables, one of them with about a million recs the other one < 300 recs.

Here is the original version:

set @version = '100952';
select j.wo, t.wo from
(
select distinct wo from t1.details where version = @version and reg_id ‘foo’
) t
right join table2.wo2 j on locate(j.wo,t.wo);

This takes about 12 secs, now, if I get rid of the set @version…

select j.wo, t.wo from
(
select distinct wo from t1.details where version = ’100952′ and reg_id ‘foo’
) t
right join table2.wo2 j on locate(j.wo,t.wo);
This takes < 1 sec!

These are InnoDBs on a 5.1 server on an XP SP3 machine 2GHz 2GB that pretty much only hosts the MySQL server.

I can repeat these results at anytime. On more complex queries I get similar improvements when getting rid of the user vars. What is it with these things?
Loved the your post btw. Thanks!

Instead of:

set @num := 0, @type := ”;
select …

You can use a UNION to do both in a single query:

select
null as type,
null as variety,
null as price,
null as num
from dual where ((@num := 0) and (@type := ”))
union
select …

The WHERE clause creates an “Impossible WHERE”, so it doesn’t return any results, but it does reset the variables each time the query is run.

The alternative is to tag an extra SELECT after the FROM:

from fruits, (select @num := 0, @type := ”) vars

But I think this creates a subquery, which is what you are trying to avoid in the first place.