18

在调试一些 Oracle 代码时，我遇到了这个查询：

SELECT TPM_TASK.TASKID FROM TPM_GROUP 
INNER JOIN TPM_USERGROUPS ON TPM_GROUP.GROUPID = TPM_USERGROUPS.GROUPID 
INNER JOIN TPM_TASK
INNER JOIN TPM_GROUPTASKS ON TPM_TASK.TASKID = TPM_GROUPTASKS.TASKID 
INNER JOIN TPM_PROJECTVERSION ON TPM_TASK.PROJECTID = TPM_PROJECTVERSION.PROJECTID AND TPM_TASK.VERSIONID = TPM_PROJECTVERSION.VERSIONID 
INNER JOIN TPM_TASKSTAGE ON TPM_TASK.STAGEID = TPM_TASKSTAGE.STAGEID 
INNER JOIN TPM_PROJECTSTAGE ON TPM_PROJECTVERSION.STAGEID = TPM_PROJECTSTAGE.STAGEID 
ON TPM_GROUP.GROUPID = TPM_GROUPTASKS.GROUPID

我对这条线感到困惑：

INNER JOIN TPM_TASK

我以前没见过JOIN没有ON条款的。同样令人困惑的是这条线：

ON TPM_GROUP.GROUPID = TPM_GROUPTASKS.GROUPID

这似乎是一个ON没有任何匹配的随机子句JOIN。查询运行没有任何错误，并返回一堆数据，因此很明显语法是完全有效的。有人可以阐明这里到底发生了什么吗？

4

3 回答 3

22

Small universe... I ran across a tool generating this syntax yesterday and was rather flummoxed.

Apparently, 

FROM a 
     INNER JOIN b
     INNER JOIN c ON (b.id = c.id)
     ON (a.id = c.id)

is equivalent to a nested subquery

FROM a
     INNER JOIN (SELECT <<list of columns>>
                   FROM b
                        INNER JOIN c ON (b.id=c.id)) c
             ON (a.id = c.id)
于 2012-06-01T19:16:42.573 回答
5

I think that this is only a problem of ordering your query (since there are only INNER JOINs, the order of them is not really that important). I rearrenged your query and now it looks like this:

SELECT TPM_TASK.TASKID 
FROM TPM_GROUP 
INNER JOIN TPM_USERGROUPS 
    ON TPM_GROUP.GROUPID = TPM_USERGROUPS.GROUPID 
INNER JOIN TPM_GROUPTASKS 
    ON TPM_GROUP.GROUPID = TPM_GROUPTASKS.GROUPID
INNER JOIN TPM_TASK
    ON TPM_TASK.TASKID = TPM_GROUPTASKS.TASKID 
INNER JOIN TPM_PROJECTVERSION 
    ON TPM_TASK.PROJECTID = TPM_PROJECTVERSION.PROJECTID 
    AND TPM_TASK.VERSIONID = TPM_PROJECTVERSION.VERSIONID 
INNER JOIN TPM_TASKSTAGE 
    ON TPM_TASK.STAGEID = TPM_TASKSTAGE.STAGEID 
INNER JOIN TPM_PROJECTSTAGE 
    ON TPM_PROJECTVERSION.STAGEID = TPM_PROJECTSTAGE.STAGEID

Does it make more sense to you now?, it does to me.

于 2012-06-01T19:16:04.900 回答
4

如果里面有括号就好了...

SELECT TPM_TASK.TASKID 
FROM 
    TPM_GROUP 
    INNER JOIN TPM_USERGROUPS ON TPM_GROUP.GROUPID = TPM_USERGROUPS.GROUPID 
    INNER JOIN (
        TPM_TASK
        INNER JOIN TPM_GROUPTASKS ON TPM_TASK.TASKID = TPM_GROUPTASKS.TASKID 
        INNER JOIN TPM_PROJECTVERSION ON TPM_TASK.PROJECTID = TPM_PROJECTVERSION.PROJECTID 
            AND TPM_TASK.VERSIONID = TPM_PROJECTVERSION.VERSIONID 
        INNER JOIN TPM_TASKSTAGE ON TPM_TASK.STAGEID = TPM_TASKSTAGE.STAGEID 
        INNER JOIN TPM_PROJECTSTAGE ON TPM_PROJECTVERSION.STAGEID = TPM_PROJECTSTAGE.STAGEID 
    ) ON TPM_GROUP.GROUPID = TPM_GROUPTASKS.GROUPID

但由于它们都是内部连接，我同意 Lamak 的回答。

于 2012-06-01T19:22:05.643 回答