RE: SQL View - Embedded Select vs Join

Luis Rodriguez wrote:

Hi,

Let's suppose three files:

----------
File1:
CustomerName, State_Code1, State_Code2, Cust_Code1, Cust_Code2

File2:
State_Code, State_Name (Primary Key: State_Code)

File3:
Cust_Code, Cust_Description (Primary Key: Cust_Code)
----------

I've been asked to write a view that shows the name or description for each one of the FileA codes.

As I see it, I can write the view as sequence of join files:

-----------------
SELECT A.*, B.State_Name as State_Name1, C.State_Name as State_Name2,
D.Cust_Description as Cust1, E.Cust_Description as Cust2
FROM FileA as A

LEFT OUTER JOIN File1, File2 B on A.State_Code1 = B.State_Code
LEFT OUTER JOIN File1, File2 C on A.State_Code1 = C.State_Code
LEFT OUTER JOIN File1, File3 D on A. Cust_Code1= C. Cust _Code
LEFT OUTER JOIN File1, File3 E on A. Cust_Code1= E. Cust _Code
-----------------

OR, I can write it using a Select for each .name. field:

-----------------
Select A.*,
(Select State_Name from File2 b where A.State_Code1 = B.State_Code) As State_Name1,
(Select State_Name from File2 c where A.State_Code2 = C.State_Code) As State_Name2,
.
etc., etc,

FROM File1 as A
-----------------------

Which option would you think is more efficient? Interestingly enough, a small test I did with only one field yielded exactly the same results on Visual Explain.

Thanks in advance,


Luis Rodriguez

I assume by "the same results on Visual Explain" you mean that you got
the identical access plan for each?

This doesn't surprise me. In my "15 second" analysis of the two
statements, they look functionally identical, right down to where nulls
are allowed or not.

If you did, in fact, get identical access plans, that's a pretty good
clue that they are identical.

It would then boil down to whether the optimizer continued to recognize
this (and whether it continues to be true) as you moved from a single
field example to something more complicated. Continuing with Visual
Explain seems like a good way to check it out.

See also:
http://social.msdn.microsoft.com/Forums/en-US/transactsql/thread/4f662bf7-0766-4201-a772-dcab5b9a5f08

. . .for an example I googled up that looks a lot like yours.


Larry Loen
www.applicationperformancegroup.com