Aggregate Queries

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Aggregate Queries

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Kathi Kellenberger

Posted Tuesday, November 29, 2005 8:28 PM

Right there with Babe

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Comments posted to this topic are about the content posted at http://www.sqlservercentral.com/columnists/kKellenberger/aggregatequeries.asp

Aunt Kathi
Microsoft
(Former SQL Server MVP)

Post #240597

Mike Metcalf-160221

Posted Monday, December 19, 2005 6:06 AM

Mr or Mrs. 500

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nice one aunt kathi

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ivan gonzalez-195804

Posted Monday, December 19, 2005 6:30 AM

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Very nice, simple, and to the point. Appreciate it!!

Post #245137

Marie Wilkinson

Posted Monday, December 19, 2005 7:10 AM

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It was helpful, thank you. I am wondering about your comments at the end of the article. I think the derived table is easier to understand but did you mean that or did you mean the derived table would be more efficient? Thanks.

Post #245144

Gladys Clemmer

Posted Monday, December 19, 2005 7:34 AM

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Very helpful. I have struggled with aggregate queries in the past. Now I can just use your instructions!!

Thanks,

Gladys

Post #245152

Kathi Kellenberger

Posted Monday, December 19, 2005 8:28 AM

Right there with Babe

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Thanks, everyone.

From what I understand the derived table is more effiecient. It took me a while to understand them, though. The instructor I had when working on my degree taught us to do everything with sub-queries. The class was actually on PL SQL (Oracle's version) and some of the features are different.

Aunt Kathi
Microsoft
(Former SQL Server MVP)

Post #245197

Marie Wilkinson

Posted Monday, December 19, 2005 8:31 AM

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Thanks so much! I like derived tables also so it's nice to know they are efficient also.

Post #245199

R Wong

Posted Monday, December 19, 2005 9:31 AM

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Thank you! Easy to digest than BOL for a newbie like me.

Post #245221

Matthew Galbraith

Posted Monday, December 19, 2005 1:02 PM

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Marie, in this case, I think the derived table just makes it more obvious what the query is doing than the "WHERE...IN" version. SQL should only have to actualize the recordset once either way though the join might be a little more effecient; I'd have to test a few cases to see if it mattered. Note that I generally avoid the IN and NOT IN constructs where possible since it usually results in the query optimizer generating a giant "OR" block to test each condition.

When you use a correlated subquery (essentially a sub query that uses fields from outside its parenthesis), SQL may execute the statement once per row that it needs to filter. In these cases, joining a derived table should almost always be more efficient, though this isn't the example in the article.

There's one other case that she didn't mention... What if you were doing a query for last order by Customer Name and your system doesn't have a unique key on name? I don't know of a way to do this without either setting up a temp table before running your aggregate query, or using a derived table. Maybe this only comes up when the schema is poorly defined, but I have found it relevant when querying third-party databases. (Specific case in point: we have such a database that allows SSN's to be duplicated in an employee table; when looking up aggregate data per employee, I have to get the most recent employee id number for each SSN and use it when agregating data for each employee.)

Matthew Galbraith

Post #245312

Marie Wilkinson

Posted Wednesday, December 21, 2005 7:42 AM

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Thank you Matthew, your comments about correlated subqueries were also helpful, I had added just that type of subquery to a procedure and watched it almost grind to a halt. Went back and made it a derived table and it's flying again.

Marie

Post #245844

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