SQL中的WHERE子句失败

Question

我有以下SQL：

SELECT `id`,`postcode`,
(SELECT COUNT(*) FROM `kids` WHERE `kids`.`bookingID` = `booking`.`id`) AS `k`,
(SELECT COUNT(*) FROM `youth` WHERE `youth`.`bookingID` = `booking`.`id`) AS `y`
FROM `booking`
WHERE `k` > 0 AND `y` > 0

但由于列k未知，它失败了。虽然它不是数据库中的真正列，但我在SELECT中定义了它，那么我哪里出错？

Answer 1

您无法引用SELECT子句中WHERE中定义的别名。一般而言，这不仅适用于MySQL，而且只适用于MySQL。

MySQL提供了扩展程序。在这种情况下，您可以使用HAVING子句来实现此目的：

SELECT `id`,`postcode`,
       (SELECT COUNT(*) FROM `kids` WHERE `kids`.`bookingID` = `booking`.`id`) AS `k`,
       (SELECT COUNT(*) FROM `youth` WHERE `youth`.`bookingID` = `booking`.`id`) AS `y`
FROM `booking`
HAVING `k` > 0 AND `y` > 0;

Answer 2

如果你使用count（），我认为你必须在这个sql中使用。