將子查詢轉換爲加入

Question

是否可以將以下子查詢轉換爲JOIN，最好是沒有派生表？

SELECT * FROM users u 
LEFT JOIN user_groups ug ON u.usergroupid=ug.groupid 
WHERE 
u.userstatus=1 AND 
ug.groupstatus=1 AND 
ug.grouprank>=(SELECT grouprank FROM user_groups WHERE groupkey='users') 

的user_groups表如下所示：

+-------------+---------------------+------+-----+---------+----------------+ 
| Field  | Type    | Null | Key | Default | Extra   | 
+-------------+---------------------+------+-----+---------+----------------+ 
| groupid  | tinyint(3) unsigned | NO | PRI | NULL | auto_increment | 
| groupkey | varchar(8)   | NO |  | NULL |    | 
| grouprank | smallint(6)   | NO |  | NULL |    | 
| groupstatus | tinyint(1) unsigned | NO |  | NULL |    | 
+-------------+---------------------+------+-----+---------+----------------+ 

的用戶表看起來像：

+--------------+---------------------+------+-----+---------+----------------+ 
| Field  | Type    | Null | Key | Default | Extra   | 
+--------------+---------------------+------+-----+---------+----------------+ 
| userid  | int(10) unsigned | NO | PRI | NULL | auto_increment | 
| username  | varchar(32)   | NO |  | NULL |    | 
| userpassword | varchar(32)   | NO |  | NULL |    | 
| usergroupid | tinyint(3) unsigned | NO |  | NULL |    | 
| userstatus | tinyint(1) unsigned | NO |  | NULL |    | 
+--------------+---------------------+------+-----+---------+----------------+ 

Answer 1

我認爲這是你在找什麼 - 給它一個測試。

SELECT * FROM users u 
LEFT JOIN user_groups ug ON u.usergroupid=ug.groupid 
JOIN user_groups ug2 ON ug.grouprank >= ug2.grouprank AND ug2.groupkey = 'users' 
WHERE 
u.userstatus=1 AND 
ug.groupstatus=1 

Answer 2

假設只有一個user_groups記錄具有groupkey = 'users'（否則您的查詢是無效的），您的查詢是相同的：

SELECT u.*, 
     ug1.* 
    FROM users u 
    LEFT 
    JOIN user_groups ug1 
    ON u.usergroupid = ug1.groupid 
    LEFT 
    JOIN user_groups ug2 
    ON ug2.groupkey = 'users' -- not a real join condition 
WHERE u.userstatus = 1 
    AND ug1.groupstatus = 1 
    AND ug1.grouprank >= ug2.grouprank 
; 

但是注意，LEFT JOIN 5事實上最終工作作爲INNER JOIN s，因爲您的WHERE子句取決於成功的連接。所以你可能真的想要這樣的事情：

SELECT u.*, 
     ug1.* 
    FROM users u 
    LEFT 
    JOIN user_groups ug1 
    ON u.usergroupid = ug1.groupid 
    AND ug1.groupstatus = 1 
    LEFT 
    JOIN user_groups ug2 
    ON ug2.groupkey = 'users' 
    AND ug1.grouprank >= ug2.grouprank 
WHERE u.userstatus = 1 
;