Order of join operations: would these two FROM clauses produce the same results? [closed] - sql

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Closed 9 years ago.
Would these two FROM clauses produce the same results? And if not, is there a way to write the the first one so that no parenthesis are needed?
FROM SALESTAX
RIGHT JOIN ( ITEMS
RIGHT JOIN ( PINVOICE
INNER JOIN PINVDET ON PINVOICE.PNV_INVOICENO = PINVDET.PND_INVOICENO AND PINVOICE.PNV_Site = PINVDET.PND_Site
) ON ITEMS.ITE_INVNO = PINVDET.PND_INVNO
) ON SALESTAX.STX_GroupID = PINVDET.PND_TAX1
FULL JOIN ( CUSTMS
RIGHT JOIN CUSMER ON CUSTMS.TMS_CODE = CUSMER.CUS_TERM
) ON PINVDET.PND_CUSTID = CUSMER.CUS_CustID
FROM CUSTMS RIGHT JOIN
CUSMER ON TMS_CODE = CUS_TERM FULL JOIN
PINVDET ON PND_CUSTID = CUS_CustID LEFT JOIN
PINVOICE ON PNV_INVOICENO = PND_INVOICENO AND PNV_Site = PND_Site LEFT JOIN
SALESTAX on STX_GROUPID = PND_TAX1 left join
ITEMS on ITE_INVNO = PND_INVNO
EDIT: While I'd like to know the answer to the first question, I'm more immediately interested in just having a more straightforward version of the first FROM clause that doesn't need parenthesis so if you'd rather just rewrite it than compare the two then feel free to just do that.

Answer 1

I have no idea if the 1st is equivalent to the 2nd (first because the queries are unfriendly formatted, to say the least and second because RIGHT joins are kind of confusing, since many are used to write using LEFT joins.) But to answer the question:
Is there a way to write the the first one so that no parenthesis are needed?
Yes, you can simply remove the parentheses from the 1st query.
Keeping the parentheses and formatted with some white space:
FROM
SALESTAX
RIGHT JOIN
( ITEMS
RIGHT JOIN
( PINVOICE
INNER JOIN
PINVDET
ON PINVOICE.PNV_INVOICENO = PINVDET.PND_INVOICENO
AND PINVOICE.PNV_Site = PINVDET.PND_Site
)
ON ITEMS.ITE_INVNO = PINVDET.PND_INVNO
)
ON SALESTAX.STX_GroupID = PINVDET.PND_TAX1
FULL JOIN
( CUSTMS
RIGHT JOIN
CUSMER
ON CUSTMS.TMS_CODE = CUSMER.CUS_TERM
)
ON PINVDET.PND_CUSTID = CUSMER.CUS_CustID
Without parentheses and white-space formatted:
FROM
SALESTAX
RIGHT JOIN
ITEMS
RIGHT JOIN
PINVOICE
INNER JOIN
PINVDET
ON PINVOICE.PNV_INVOICENO = PINVDET.PND_INVOICENO
AND PINVOICE.PNV_Site = PINVDET.PND_Site
ON ITEMS.ITE_INVNO = PINVDET.PND_INVNO
ON SALESTAX.STX_GroupID = PINVDET.PND_TAX1
FULL JOIN
CUSTMS
RIGHT JOIN
CUSMER
ON CUSTMS.TMS_CODE = CUSMER.CUS_TERM
ON PINVDET.PND_CUSTID = CUSMER.CUS_CustID
To answer the other question, about the 2nd query, no it isn't equivalent. You missed the table aliases and changed an inner join to left join. This is equivalent to the 1st:
FROM CUSMER
LEFT JOIN
CUSTMS ON CUSTMS.TMS_CODE = CUSMER.CUS_TERM
FULL JOIN
PINVDET
INNER JOIN -- this is changed
PINVOICE ON PINVOICE.PNV_INVOICENO = PINVDET.PND_INVOICENO
AND PINVOICE.PNV_Site = PINVDET.PND_Site
LEFT JOIN
ITEMS ON ITEMS.ITE_INVNO = PINVDET.PND_INVNO
LEFT JOIN
SALESTAX ON SALESTAX.STX_GroupID = PINVDET.PND_TAX1
ON PINVDET.PND_CUSTID = CUSMER.CUS_CustID

Answer 2

I encourage you to write your from clauses by placing all left joins first followed by inner joins. This greatly simplifies trying to figure out what queries are doing. A series of left joins says "keep all the rows in the first table". A series of inner joins says "keep only rows where there are matches between tables." (Occasionally, you might need subqueries as in the first example.)
It is unlikely in this example that the two are the same. In the first, the full join is the "outermost" join. In the second, the full join is embedded in a series of joins. These are interpreted sequentially from the first to the last. One of these is probably converting the full join to an inner join or left join. Of course, the two could produce equivalent results if all the tables match.
The second example could probably be written as:
FROM CUSMER LEFT JOIN
CUSTMS ON TMS_CODE = CUS_TERM LEFT JOIN
PINVDET ON PND_CUSTID = CUS_CustID LEFT JOIN
PINVOICE ON PNV_INVOICENO = PND_INVOICENO AND PNV_Site = PND_Site LEFT JOIN
SALESTAX on STX_GROUPID = PND_TAX1 left join
ITEMS on ITE_INVNO = PND_INVNO
(assuming that something after the full join is converting it to a left join anyway).