幫助限制內部連接

Question

我需要幫助限制內部連接。

表：

---

用戶：

• UID
• 左
• 活性

---

個

文件：

• FID

---

UID = FID

---

SELECT uid 
FROM users AS u 
INNER JOIN files AS x ON u.uid = x.fid 
WHERE u.left = '0' 
AND u.active = '1' 
ORDER BY `u`.`uid` ASC 

結果：

uid 
3 
3 
3 
3 
7 
47 
47 
47 
47 
47 
47 
47 
47 
47 
47 

我需要限制INNER JOIN 5。所以「uid」不會出現超過5次。像這樣：

uid 
3 
3 
3 
3 
7 
47 
47 
47 
47 
47 

---

更新：

這裏是PHP代碼

$res = do_sqlquery("SELECT uid FROM users as u INNER JOIN files as x ON u.fid=x.fid WHERE u.left = '0' AND u.active='1'"); 
    if (mysql_num_rows($res) > 0) 
    { 
     while ($arr = mysql_fetch_assoc($res)) 
     { 
     $x=$arr["uid"]; 
     quickQuery("UPDATE users SET pots = pots+".$GLOBALS["bui"]."*".$cleaint."/900 WHERE id = '$x'"); 
     } 
    } 

Answer 1

在MySQL：

SELECT CONCAT(`uid`, ":", MAX(5, COUNT(`fid`))) `uid_count` 
FROM ... 
GROUP BY `uid` 

給你類似的結果：

uid_count 
3:4 
7:1 
47:5 

在PHP中，例如，您可以獲取拆分後：

while (FALSE !== ($row = mysql_fetch_assoc($query))) { 
    ($uid, $count) = split(':', $row['uid_count'], 2); 
} 

Answer 2

一個MySQL唯一的解決方案，而無需使用PHP

SELECT uid 
from (
    SELECT u.uid, @r:= if(@u=u.uid,@r+1,1) r, @u:=u.uid 
    FROM users AS u 
    CROSS JOIN (select @u:=null) g 
    INNER JOIN files AS x ON u.uid = x.fid 
    WHERE u.`left` = '0' 
    AND u.active = '1' 
    ORDER BY u.uid ASC 
) U 
WHERE U.r <= 5 
ORDER BY uid ASC 

或更一般地，只是沒有MySQL變量的SQL

SELECT u.uid 
from (
    SELECT u.uid, count(*) C 
    FROM users AS u 
    INNER JOIN files AS x ON u.uid = x.fid 
    WHERE u.`left` = '0' 
    AND u.active = '1' 
    GROUP BY u.uid 
) U inner join (
    select 1 n union all select 2 union all select 3 
    union all select 4 union all select 5) V ON U.C>=V.n 
ORDER BY U.uid ASC