RE: SQL View - Embedded Select vs Join

I think, with indexes as you have described, that the join approach would be most efficient. Have you tried both approaches to determine their relative performance? Visual explain could be helpful in this respect...

-Eric DeLong

-----Original Message-----
From: midrange-l-bounces@xxxxxxxxxxxx [mailto:midrange-l-bounces@xxxxxxxxxxxx] On Behalf Of Luis Rodriguez
Sent: Thursday, May 14, 2009 11:09 AM
To: Midrange-L
Subject: SQL View - Embedded Select vs Join


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 ânameâ 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,
â
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

IBM Certified Systems Expert
eServer i5 iSeries Technical Solutions