根据另一个SQL PHP的结果从一个表获取数据

Question

I know this involves JOINS but I can't seem to find a working solution to what I'm trying to do.

I have 2 custom tables :

table1    |    table2
---------------------
id             id
uid            uid
track_id       track_id
date           date
art            active
info
blah
blah2

First I want to select everything WHERE uid =55 AND active =1 from table2 :

$tracks = $wpdb->get_results( "SELECT * FROM table2 WHERE uid = 55 AND active = 1");

And then match the track_id from table2 with results from table1 so I can traverse the table1 data.

I know I can do it like this :

foreach( $tracks as $track ) {

$this_track = $track->track_id;

$results = $wpdb->get_results( "SELECT * FROM table1 WHERE track_id = $this_track");

// Do stuff here

}

But this is the part where it gets tricky...

I then want to ORDER the $results from table1 by date DESC from table2

And this is where I'm lost...

Effectively I want (pseudo code) :

$results = $wpdb->get_results( "SELECT * FROM table1 WHERE track_id = $this_track" ORDER BY date DESC FROM table2);

As well as that last bit, I know I can do this entire routine with JOINS to keep this all in one query and make it way more efficient but I just don't know how.

So just to be clear, my overall routine should be like this :

Get all instances of track_id from table2 where user_id=55 and active=1, then use those results to match the track_id to every result in table1 with the same track_id and then sort the results by date back over from table2

Psuedo code, I know it contains nonsense :

$finalresults = $wpdb->get_results( "SELECT * FROM table2 where uid=55 AND active=1 THEN SELECT * FROM table1 WHERE track_id = "the track_id from the first query" THEN ORDER BY date DESC FROM table2);

Answer 1

Try with this query

SELECT t1.* ,t2.date AS t2date, t2.active FROM table2 AS t2 INNER JOIN table1 AS t1 ON (t1.track_id = t2.track_id) WHERE t2.uid=55 AND t2.active=1 ORDER BY t2.date DESC;

Edit : Explanation of what this query is doing. and inverted the order of the tables retrieved in the query (this don't affect the final datatset, i did this to make to follow the logic of the explanation.

1.- Begin with retrieving all rows from table2 (theres is no specific reason because i used table2 over table1 , I'm only following an logical order), using the criteria that you specified iud=55 and active=1

SELECT * FROM table2 WHERE uid=55 AND active=1;

2.- but as you said you need to expand the data retrieved in table2 with some information in table1 , that's exactly what it is the directive JOIN made, and we are using INNER JOIN because this type of JOIN will show rows ONLY if data for the uid=55 is present on table1, if there is NO data for the uid=55 present on both TABLES then mysql wil show empty the recordset (0 Rows selected).

in the ON(...) part I specify which criteria mysql will use to compara both tables for match in this case will compare that track_id on table2 it is the same that the specified on table1, if this codition is met then mysql considers it as a match.

anly for convenience and because i'm adding a Second table i gave an Alias to each one t1 and t2.

then the query now seems like this

SELECT * FROM table2 AS t2 INNER JOIN table1 AS t1 ON(t1.track.id = t2.track_id) WHERE t2.uid=55 AND t2.active=1;

3.- but then raise a problem, both tables has rows with the same field names, and this is something that DBMS don't like in their queries, to avoid this situation in the query i only show the fields (id, uid and track_id) from one table in this case t1 (t1.*) and only show the fields that doesn't have this problem from t2 (t2.date AS t2date, t2.active). in this way mysql won't throw any error.

SELECT t1.* ,t2.date AS t2date, t2.active FROM table2 AS t2 INNER JOIN table1 AS t1 ON (t1.track_id = t2.track_id) WHERE t2.uid=55 AND t2.active=1;

4.- for the final step i specify to mysql that i want all found rows ordered descent by a field in the table2;

ORDER BY t2.date DESC; 

then this criteria will be applied to the whole selected rows. and the final query has this form.

SELECT t1.* ,t2.date AS t2date, t2.active FROM table2 AS t2 INNER JOIN table1 AS t1 ON (t1.track_id = t2.track_id) WHERE t2.uid=55 AND t2.active=1 ORDER BY t2.date DESC;

if is not completely clear you can ask ...