Question

我正在尝试对两个查询进行INNER JOIN。如果我对第一个组件有一个广泛的SELECT语句，它可以正常工作。如果我尝试向第一个组件添加WHERE ...，我会收到一个无法提供信息的语法错误，我不知道为什么。

这是有效的，但是很慢，因为我的桌子很大（〜1mil行）

SELECT a.RoomNum, a.EventName, a.EventStatus, a.EventDateTime FROM eventtable a
INNER JOIN
(
    SELECT `RoomNum`, eventname, MAX(`EventDateTime`) as MT
    FROM `eventtable`
    WHERE eventname = "DND"
    Group BY RoomNum
) b
ON a.RoomNum = b.RoomNum and a.EventDateTime = b.MT and a.eventname = b.eventname

但我真正需要的是：

SELECT a.RoomNum, a.EventName, a.EventStatus, a.EventDateTime from eventtable WHERE a.EventName = "DND" a
INNER JOIN
(
    SELECT `RoomNum`, eventname, MAX(`EventDateTime`) as MT
    FROM `eventtable`
    where eventname = "DND"
    Group by RoomNum
) b
on a.RoomNum = b.RoomNum and a.EventDateTime = b.MT and a.eventname = b.eventname

我想过将结果分配给一个表，但实际上并不想生成一个表然后放下它以确保下次进行查询时，将不会有表存在。

感谢。

Answer 1

您的查询应该是

SELECT a.RoomNum, a.EventName, a.EventStatus, a.EventDateTime from eventtable a
INNER JOIN
(
    SELECT `RoomNum`, eventname, MAX(`EventDateTime`) as MT
    FROM `eventtable`
    where eventname = "DND"
    Group by RoomNum
) b
on a.RoomNum = b.RoomNum and a.EventDateTime = b.MT and a.eventname = b.eventname
WHERE a.EventName = "DND"

观察 where即将结束。

Answer 2

Where子句出现在END ...然后你将它放在from和INNER之间，所以它会导致语法错误...所以你可以做这样的事情

    SELECT a.RoomNum, a.EventName, a.EventStatus, a.EventDateTime from eventtable a
    INNER JOIN
    (
        SELECT `RoomNum`, eventname, MAX(`EventDateTime`) as MT
        FROM `eventtable`
        where eventname = "DND"
        Group by RoomNum
    ) b
    on a.RoomNum = b.RoomNum and a.EventDateTime = b.MT and a.eventname = b.eventname
 WHERE a.EventName = "DND"

或者如果你不想使用where子句，你可以像这样直接检查连接中的条件..

SELECT a.RoomNum, a.EventName, a.EventStatus, a.EventDateTime from eventtable a
INNER JOIN
(
    SELECT `RoomNum`, eventname, MAX(`EventDateTime`) as MT
    FROM `eventtable`
    where eventname = "DND"
    Group by RoomNum
) b
on a.RoomNum = b.RoomNum and a.EventDateTime = b.MT and a.eventname = b.eventname and  a.EventName = "DND"

Answer 3

首先，JOIN可以在两个表上。在你写的时候，你不应该在这两个上添加条件。如果您愿意，最后添加它们。

将where子句放在最后，它将起作用。

SELECT * FROM mytable INNER JOIN等确定，但SELECT * FROM mytable WHERE ... INNER JOIN错误

3 个答案: