MySQL Ordering the Group By

Posted by Mr PHP on

A problem that I have faced with SQL many times is pre-ordering the GROUP BY clause.

When you GROUP BY, MySQL will return a set of records which I call representing records. These records represent the group that is selected.

For example select id from test group by class will return one representing record per unique value of the class field. The problem with this is that you have no control over which record will represent the group.

Lets setup an example so you can see the problem in more detail.

# setup the table and some data
  `class` varchar(64) DEFAULT NULL,
  `created` date DEFAULT NULL,
  PRIMARY KEY (`id`)
('1', 'aaa', '2009-01-01'),
('2', 'aaa', '2009-01-02'),
('3', 'bbb', '2009-01-03'),
('4', 'bbb', '2009-01-04'),
('5', 'ccc', '2009-01-05'),
('6', 'ccc', '2009-01-06');

Great, now we have some data that we can group. Now I want to find the latest created records, one per class. The records I need are 2, 4 and 6.

attempt 1 - group by

select id 
from test 
group by class

incorrect - returns the earliest created (1, 3 and 5)

attempt 2 - group by order by

select id 
from test 
group by class 
order by created desc

incorrect - returns the earliest created, only the final result is ordered (5, 3 and 1)

attempt 3 - subselect a field

select id, class class1, (select id from test where class=class1 order by created desc limit 1) id2 
from test 
group by class

almost there - returns field id2 containing 2, 4 and 6 !

attempt 4 - joining using subselects

from test t1 
left join test t2 on = (select id from test where class=t1.class order by created desc limit 1) 
group by t1.class

Success! This returns representing record in table t2 containing 2, 4 and 6 !

Tagged with : MySQL