Can you have an INNER JOIN without the ON keyword?

Question

While debugging into some Oracle code, I came across this query:

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

I'm confused by the line:

INNER JOIN TPM_TASK

I haven't seen a JOIN without an ON clause before. Also confusing is the line:

ON TPM_GROUP.GROUPID = TPM_GROUPTASKS.GROUPID

This seems like a random ON clause without any matching JOIN. The query runs without any errors, and returns a bunch of data, so obvious the syntax is perfectly valid. Can someone shed some light on exactly what's going on here?

Answer 1

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)

Answer 2

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.

Answer 3

It'd look fine if it had parenthesis in there...

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

but since they are all inner joins I agree with Lamak's answer.