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SQL Server SQL syntax =* and right joins

Posted on 2016-07-25
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Last Modified: 2016-07-26
Hi Folks.
I am struggling to understand the join conversion process, and how to nest right joins.

do the following have identical results?

thank you.

from APOPEN O,APPayments P, APNewPayables N  WHERE O.ApplytoId =* P.ApplytoId and N.voucherid =* P.applytoid

from (APOPEN RIGHT JOIN APPayments ON AOPEN.ApplytoId = APPayment.ApplytoId) RIGHT JOIN APNewPayables ON APNewPayables.voucherid = APPayment.applytoid
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Question by:COACHMAN99
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Expert Comment

by:Chris Luttrell
ID: 41728700
Close, SQL Server uses the ANSI JOIN syntax like you have in the second line. The =* is signifying RIGHT OUTER JOIN.  You don't have to have the Parenthesis either.
from APOPEN O RIGHT OUTER JOIN APPayments P ON O.ApplytoId = P.ApplytoId RIGHT OUTER JOIN APNewPayables N ON  N.voucherid = P.applytoid 

Open in new window

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Author Comment

by:COACHMAN99
ID: 41728729
Hi Chris
your text is jumping around so I cant read it.
Is right outer not the same as right? and
Do my two statements generate identical results?
thanks
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Author Comment

by:COACHMAN99
ID: 41728732
and can one not just use the table-name instead of the alias?
My objective is to use the second text/syntax in place of the first. I just need to know if the results (as I have phrased them) would be identical .

thanks
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Chris Luttrell earned 500 total points
ID: 41728735
Yes to both of you questions.  https://msdn.microsoft.com/en-us/library/ms177634(v=sql.130).aspx is the link to the T-SQL rules on FROM.
So OUTER is "OPTIONAL" but I am a fan of following the standards because it will transfer between databases better and you don't have to worry about old code breaking because they suddenly start enforcing the ANSI standards.  I also like table aliases over full table names, just a preference there.
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Expert Comment

by:Chris Luttrell
ID: 41728738
I should know about old code because I have been working with various database versions and vendors for 30 years now and have plenty of my own out there. :-)
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Author Comment

by:COACHMAN99
ID: 41728739
Excellent thanks! so my syntax is identical? I was concerned that the second RIGHT join was the wrong way around.
cheers.
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Expert Comment

by:PortletPaul
ID: 41729007
There is an unwritten convention: avoid right joins

Please don't get me wrong here. You are free to use any join you want at any time and the standards do allow you to do it. But many of us who write and maintain mountains of SQL prefer to avoid right joins completely making query logic much simpler to maintain.

Your query could be reversed in direction
FROM APNewPayables N
LEFT JOIN APPayments P ON  N.voucherid = P.applytoid 
LEFT JOIN APOPEN O ON P.ApplytoId = O.ApplytoId 

Open in new window

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Author Comment

by:COACHMAN99
ID: 41729658
Thanks Paul, makes sense.
So there are no efficiency issues, just 'trickier' to understand?
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Expert Comment

by:Chris Luttrell
ID: 41729662
Good point Paul.  I agree, I just did not want to push the issue.  Yes, they work the same, just trickier to think about the logic flow.
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Author Comment

by:COACHMAN99
ID: 41729676
Great to have consensus  :-)
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Expert Comment

by:PortletPaul
ID: 41730187
Right joins are indeed much trickier to follow, and they alter the precedence of tables, and in large multi-table from clauses it can be very hard to understand the author's intentions if precedence flips about like a trout on a river bank.

There should not be performance issues as it is actually the optimizer that determines the actual execution path.
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