SQL子查詢：行中找不到

Question

我有以下SQL查詢：

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，它需要大約2秒。但是，我想找個地方staff <> 1條目：

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 

我得到的錯誤：Unknown column 'staff' in 'where clause'

我可以把子查詢的WHERE子句中，但隨後的查詢需要70秒。

但我當然應該可以訪問我在SELECT語句中引用的列嗎？

Answer 1

由於選擇列表是在where子句之後處理的，因此不能通過其別名引用where子句中的任何計算字段。

您可以將子查詢移入加入，只要它返回一個只記錄，即使沒有限制條款：

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