Mysql GROUP BY and ORDER BY DESC [duplicate] - php

Question

This question already has answers here:
Get top n records for each group of grouped results
(12 answers)
Closed 9 years ago.
This is my full query:
SELECT * FROM `clientgroupassign`
LEFT JOIN `clients` ON `clientgroupassign`.clientId = `clients`.clientId
LEFT JOIN `users` ON `carerId` = `userId`
LEFT JOIN
(SELECT * FROM
(SELECT * FROM `contacts` WHERE `contactGroup` = 4 ORDER BY `contactId` DESC)
as `contacts`
GROUP BY (`contactClientId`)
) AS `contacts` ON `contactClientId` = `clients`.clientId
WHERE groupId = 4
ORDER BY `clients`.clientId
There is a problem with third join causing the script to execute for about 1 minute. When I run it separattly in PMA:
SELECT * FROM (SELECT * FROM `contacts` WHERE `contactGroup` = 4 ORDER BY `contactId` DESC) AS `contacts` GROUP BY (`contactClientId`)
it still gets very long to execute.
What I want is to get one, last added row from contacts for each client who is in the group 4(client can be in various groups).
Thanks.

Answer 1

To get "last added row from contacts for each client who is in the group 4" try this:
SELECT
c.*
FROM(
SELECT
contactClientId,
MAX(contactId) as cid
FROM
contacts
WHERE
contactGroup = 4
GROUP BY
contactClientId
ORDER BY
NULL
) as tmp
INNER JOIN contacts as c
ON c.contactId = tmp.cid
AND c.contactClientId = tmp.contactClientId
If contactId is PK in contacts then the second join clause is not needed.
By default, MySQL sorts all GROUP BY col1, col2, ... queries as if you
specified ORDER BY col1, col2, ... in the query as well. If you
include an ORDER BY clause explicitly that contains the same column
list, MySQL optimizes it away without any speed penalty, although the
sorting still occurs. If a query includes GROUP BY but you want to
avoid the overhead of sorting the result, you can suppress sorting by
specifying ORDER BY NULL.
The complete docs.
Full query:
SELECT * FROM `clientgroupassign`
LEFT JOIN `clients` ON `clientgroupassign`.clientId = `clients`.clientId
LEFT JOIN `users` ON `carerId` = `userId`
LEFT JOIN
(
SELECT
c.*
FROM(
SELECT
contactClientId,
MAX(contactId) as cid
FROM
contacts
WHERE
contactGroup = 4
GROUP BY
contactClientId
ORDER BY
NULL
) as tmp
INNER JOIN contacts as c
ON c.contactId = tmp.cid
AND c.contactClientId = tmp.contactClientId
) AS `contacts` ON `contactClientId` = `clients`.clientId
WHERE groupId = 4
ORDER BY `clients`.clientId