Question

我有以下MySQL查询运行绝对正常：

SELECT a.id, a.event_name, c.name, a.reg_limit-e.places_occupied AS places_available, a.start_date
FROM nbs_events_detail AS a, nbs_events_venue_rel AS b, nbs_events_venue AS c,
(SELECT e.id, COUNT(d.event_id) AS places_occupied FROM nbs_events_detail AS e LEFT JOIN nbs_events_attendee AS d ON e.id=d.event_id GROUP BY e.id) AS e
WHERE a.id=b.event_id AND b.venue_id=c.id AND a.id=e.id AND a.event_status='A' AND a.start_date>=NOW()
ORDER BY a.start_date

但是，我尝试添加一个WHERE子句，以便过滤为减法创建的列中显示的结果：a.reg_limit-e.places_occupied AS places_available

到目前为止我所做的是添加如下WHERE条款： WHERE places_available>0

但是，如果我尝试使用此指令，查询将失败，并且不会显示任何结果。报告的错误如下：#1054 - Unknown column 'places_available' in 'where clause'

在a.reg_limit我有号码，e.places_occupied我的子查询中COUNT生成了数字。
我错过了什么？

Answer 1

WHERE子句在SELECT语句之前执行，因此它不知道新的别名places_available，the logical order of operations in Mysql是这样的：

FROM clause

WHERE子句

GROUP BY子句

HAVING条款

SELECT条款

ORDER BY子句

作为解决方法，您可以将其包装在这样的子查询中：

SELECT *
FROM
(
    SELECT 
      a.id, 
      a.event_name, 
      c.name, 
      a.reg_limit-e.places_occupied AS places_available,  
      a.start_date
    FROM nbs_events_detail AS a
    INNER JOIN nbs_events_venue_rel AS b ON a.id=b.event_id
    INNER JOIN nbs_events_venue AS c ON b.venue_id=c.id 
    INNER JOIN
    (
       SELECT e.id, 
         COUNT(d.event_id) AS places_occupied 
       FROM nbs_events_detail AS e 
       LEFT JOIN nbs_events_attendee AS d ON e.id=d.event_id    GROUP BY e.id
    ) AS e ON a.id=e.id
    WHERE a.event_status='A' AND a.start_date>=NOW()
) AS t
WHERE places_available>0
ORDER BY a.start_date;

还尝试使用ANSI-92 JOIN语法而不是旧语法，并使用显式JOIN而不是像我所做的那样混合WHERE子句中的条件。

Answer 2

can not use aliases，在查询中定义，位于WHERE子句中。为了使您的查询有效，请在a.reg_limit-e.places_occupied>0子句中使用条件WHERE。

Answer 3

首先：在第一次AND出现后，必须通过布尔逻辑运算符（OR或WHERE）添加另一个子句。含义如下：

SELECT * FROM yourtable
WHERE col1 = value
AND col2 = value

第二：你在你的减法上放了一个别名，但是你不会按照你想要的方式在WHERE子句中使用。你可以这样写（我的坏/坏建议）：

SELECT a.id, a.event_name, c.name, a.reg_limit-e.places_occupied AS places_available, a.start_date
FROM nbs_events_detail AS a, nbs_events_venue_rel AS b, nbs_events_venue AS c,
(SELECT e.id, COUNT(d.event_id) AS places_occupied FROM nbs_events_detail AS e LEFT JOIN nbs_events_attendee AS d ON e.id=d.event_id GROUP BY e.id) AS e
WHERE a.id=b.event_id 
AND b.venue_id=c.id 
AND a.id=e.id 
AND a.event_status='A' 
AND a.start_date>=NOW()
AND a.reg_limit-e.places_occupied > 0
ORDER BY a.start_date

或者您可以使用子查询 - 这将是更好的表现...因为WHERE子句中的函数方程式是邪恶的和性价比。请参阅Mahmoud Gamals对该查询的回答...不要再在这里输入它，也不想偷它; - ）

关于两列MySQL之间的减法的WHERE子句

3 个答案: