chrisdew chrisdew - 6 months ago 10
MySQL Question

Returning the 'last' row of each 'group by' in MySQL

Is there a more efficient way of doing the following?

select *
from foo as a
where a.id = (select max(id) from foo where uid = a.uid group by uid)
group by uid;
)


This answer looks similar, but is this answer the best way of doing this - How to select the first row for each group in MySQL?

Thanks,

Chris.

P.S. the table looks like:

CREATE TABLE foo (
id INT(10) NOT NULL AUTO_INCREMENT,
uid INT(10) NOT NULL,
value VARCHAR(50) NOT NULL,
PRIMARY KEY (`id`),
INDEX `uid` (`uid`)
)


data:

id, uid, value
1, 1, hello
2, 2, cheese
3, 2, pickle
4, 1, world


results:

id, uid, value
3, 2, pickle
4, 1, world


See http://www.barricane.com/2012/02/08/mysql-select-last-matching-row.html for more details.

Answer

Try this query -

SELECT t1.* FROM foo t1
  JOIN (SELECT uid, MAX(id) id FROM foo GROUP BY uid) t2
    ON t1.id = t2.id AND t1.uid = t2.uid;

Then use EXPLAIN to analyze queries.


SELECT t1.* FROM foo t1
  LEFT JOIN foo t2
    ON t1.id < t2.id AND t1.uid = t2.uid
WHERE t2.id is NULL;
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