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)