在MySQL的同一张表上使用内部联接从两列中选择DISTINCT值

Question

I have a query that selects 20 rows from a table, loops, and pulls a single row from the same table that falls in the desired score range. The found row is then deleted so that it will not be selected again.

user_id is unique and some rows have col1=0 and others have col1=1, therefore the second query will never select a row from the first query.

The temp table looks like this:

user_id  col1
-------------------
1        0
2        0
3        1
4        1

The user table looks like this:

user_id   score
-----------------
1         1000
2         2000
3         3000
4         4000

$res = do_query("SELECT temp.user_id,user.score 
        FROM temp,user
        WHERE temp.col1=0 AND temp.user_id=user.user_id LIMIT 20");

while (($row = mysql_fetch_row($res))) {
    $score = $row[1];

    $alt_res = do_query("SELECT temp.user_id, user.score
        FROM temp,user
        WHERE temp.col1=1 AND temp.user_id=user.user_id 
        AND user.score<$score AND user.score>$score*0.66 LIMIT 1");

    $alt_row = mysql_fetch_row($alt_res)
    $user_id = $alt_row[0];

    do_query("DELETE FROM temp WHERE user_id=$user_id");
}

This works just fine, however I was trying to turn this into a single query, but I keep getting duplicate values, and I can't seem to weed them out.

 SELECT temp.user_id,t1.user_id,t1.score FROM (
 SELECT temp.user_id,user.score
 FROM temp,user
 WHERE temp.col1=0 AND temp.user_id=user.user_id LIMIT 20) AS t1,temp,user
 WHERE temp.col1=1 AND temp.user_id=user.user_id 
 AND t1.score<user.score AND t1.score>user.score*0.66 GROUP BY temp.user_id

I get 20 rows with temp.user_id being unique, but duplicates with t1.user_id.

For example:

temp.user_id    t1.user_id
----------------------------
1               6
2               7
3               7 
4               7
5               8

and I want:

temp.user_id      t1.user_id
-----------------------------
1                 6
2                 7
3                 8 
4                 9
5                 10

Any idea how to make it so that no user_id is repeated in either column?

Answer 1

也许您可以在子查询中将LIMIT 20更改为LIMIT 1 ？