SQL子查询：行中找不到

Question

我有以下SQL查询：

SELECT t1.`userID`,`name`, COUNT(*) AS `count`, (SELECT `val` FROM `user-data` WHERE `userID` = t1.userID AND `keyID` = 2 LIMIT 1) AS `staff` 
FROM `activity` t1 
LEFT JOIN `users` ON `users`.`id` = t1.`userID` 
WHERE 
t1.`userID` <> 0 
GROUP BY t1.`userID` 
ORDER BY `count` DESC LIMIT 10 

其中一期工程，它需要大约2秒。但是，我想找个地方staff <> 1条目：

SELECT t1.`userID`,`name`, COUNT(*) AS `count`, (SELECT `val` FROM `user-data` WHERE `userID` = t1.userID AND `keyID` = 2 LIMIT 1) AS `staff` 
FROM `activity` t1 
LEFT JOIN `users` ON `users`.`id` = t1.`userID` 
WHERE 
t1.`userID` <> 0 
AND `staff` <> 1 
GROUP BY t1.`userID` 
ORDER BY `count` DESC LIMIT 10 

我得到的错误：Unknown column 'staff' in 'where clause'

我可以把子查询的WHERE子句中，但随后的查询需要70秒。

但我当然应该可以访问我在SELECT语句中引用的列吗？

Answer 1

由于选择列表是在where子句之后处理的，因此不能通过其别名引用where子句中的任何计算字段。

您可以将子查询移入加入，只要它返回一个只记录，即使没有限制条款：

SELECT t1.`userID`,`name`, COUNT(*) AS `count`, `user-data`.`val` as staff 
FROM `activity` t1 
LEFT JOIN `users` ON `users`.`id` = t1.`userID` 
LEFT JOIN `user-data` ON `user-data`.`userID`=t1.userID AND `user-data`.`keyID` = 2 
WHERE 
    t1.`userID` <> 0 
    AND `user-data`.`val` <> 1 
GROUP BY t1.`userID`, `user-data`.`val` 
ORDER BY `count` DESC 
LIMIT 10