Join statment WITH Where and Order By clauses

[neophyte]

Hello All -

I'm trying to join all of the columns in one table (tbl_admin_users) with 2 fields in another table(tbl_access_log) using both a WHERE and ORDER BY clause on tbl_access_log.

So far I can't get my sql statement (shown below) to work.

$sql =
"SELECT *
FROM tbl_admin_users
LEFT JOIN tbl_access_log.fld_login, tbl_access_log.fld_logout
WHERE tbl_access_log.fld_user_id = '" . $adminId . "'
ORDER by tbl_access_log.fld_logout DESC limit 1";

What's going on is that there are multiple rows in tbl_access_log with the same foreign key...

(WHERE tbl_access_log.fld_user_id = '" . $adminId . "')

and what I need to get is a row matching $adminId which also contains the most recent unix timestamp which is found in the fld_logout...

(ORDER by tbl_access_log.fld_logout DESC limit 1)

Can someone show me where I'm going wrong with my sql statement?

All replies greatly appreciated!

[noyearzero]

I'm not sure if this is exactly right....but it should put you on the path. (replace `tbl_access_log.id` with the actual primary index for the table `tbl_access_log`)

SELECT
tbl_access_log.fld_login,
tbl_access_log.fld_logout,
tbl_admin_user.*
FROM tbl_admin_users
LEFT JOIN tbl_access_log ON tbl_access_log.id = (
SELECT
tbl_access_log.id
FROM tbl_access_log
WHERE
tbl_access_log.fld_user_id = '" . $adminId . "'
ORDER BY
tbl_access_log.fld_logout DESC
LIMIT 1
)

Additionally, if your $adminId variable actually refers to the current row in the outer table (tbl_admin_users) you could replace

'" . $adminId . "'

with

tbl_admin_users.fld_user_id

(assuming fld_user_id is the name of the same field)

That would be more appropriate if your outer table is meant to pull back more than 1 row

[rocknbil]

I always get this wrong off the top of my head and have to mantra, it goes in this order:

select/from/join/where/order/limit

I'm guessing at a few of the field names.

$sql =
"select tbl_admin_users.username, tbl_access_log.login,tbl_access_log.logout
from tbl_admin_users
left join tbl_access_log on tbl_access_log.fld_user_id = tbl_admin_users.fld_user_id
where tbl_access_log.fld_user_id = '" . $adminId . "'
order by tbl_access_log.fld_logout desc limit 1";

Some notes:

- Do not use * unless you specifically need all fields, it's a lot of overhead that will cost you on complex multi table joins.
- If admin ID is numeric, which it should be, it doesn't need to be quoted (and shouldn't.)

where tbl_access_log.fld_user_id = $adminId
If it's just a single table select, and you're sure it always will be (i.e., no other potential joins in the near future) you can move the "joining" to the where clause:

$sql =
"select tbl_admin_users.username, tbl_access_log.login, tbl_access_log.logout
from tbl_admin_users,tbl_access_log
where tbl_access_log.fld_user_id = $adminId
and tbl_access_log.fld_user_id = tbl_admin_users.fld_user_id
order by tbl_access_log.fld_logout desc limit 1";

[neophyte]

Noyearzero and Rockinbil -

Thanks to you both for your input on this - I was struggling for so long on this that I just settled for doing a multiple query routine that - while it works - is really inefficient. Now I can get back to doing it the right way.

Rockinbil - thanks also for the Mantra... I'm sure this will help others as well.

[noyearzero]

rocknbil:

Is there any speed advantage to 'move the "joining" to the where clause' as opposed to what i suggested? I always thought it was faster to filter out as many rows from the join table in the ON clause rather than in the FROM table's WHERE? But to be honest i could be completely wrong.

[rocknbil]

I don't know - bench test it. :-) My **guess** would be that your approach contains a nested select, and may traverse more rows because it's essentially two selects. The approach I suggested is certainly a little less complex in the coding part.