Mysql - 在where子句中忽略状态

Question

我尝试使用此查询从选定的健身房获取所有用户。

我的问题是查询忽略这一部分：ual.user_id = weekUsers.user_id

好像查询到符合我选择，没有如果用户在我要选择健身房检查日期的所有用户ID。

在图片上我可以看到，ual.user_id和weekUsers.user_id不相等，我仍然在结果中得到它们。

这里是我的查询：

SELECT count(ual.user_id), FROM_DAYS(TO_DAYS(ual.time) -MOD(TO_DAYS(ual.time) -1, 7)) as weekNum, 

     ual.user_id, weekUsers.user_id, u.id, u.gym 

     FROM user_activity_log ual 

     LEFT OUTER JOIN user u 

     ON u.gym = 3 

     LEFT OUTER JOIN (SELECT ualWeek.user_id FROM user_activity_log ualWeek 

     GROUP BY FROM_DAYS(TO_DAYS(ualWeek.time) -MOD(TO_DAYS(ualWeek.time) -1, 7)), ualWeek.user_id 

     HAVING count(ualWeek.user_id) > 1) weekUsers 

     ON u.id = weekUsers.user_id 

     WHERE 

     (ual.time BETWEEN '2014-02-09' AND '2015-02-09') OR (('2014-02-09' IS NULL) OR ('2015-02-09' IS NULL)) 

     AND ual.user_id = weekUsers.user_id 

     GROUP BY ual.user_id 

Answer 1

让我们来解释你的(ual.time BETWEEN '2014-02-09' AND '2015-02-09') OR (('2014-02-09' IS NULL) OR ('2015-02-09' IS NULL))表达。

表示(ual.time BETWEEN '2014-02-09' AND '2015-02-09')为A和(('2014-02-09' IS NULL) OR ('2015-02-09' IS NULL))为B我们得到A or B。我们有A or B AND ual.user_id = weekUsers.user_id。

由于默认操作的优先级会这样解释A or (B AND ual.user_id = weekUsers.user_id)所以如果A是真实的，那么整个逻辑表达式为真，ual.user_id = weekUsers.user_id未选中，简单地忽略。

您可以参考http://www.bennadel.com/blog/126-sql-and-or-order-of-operations.htm 和http://www.bennadel.com/blog/126-sql-and-or-order-of-operations.htm 到运算符优先级的更多细节。