Group by and Where clause

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Group by and Where clause

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sanandh87

Posted Sunday, March 03, 2013 9:47 AM

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Hi,
Please see below,
Table name : Log_table
Columns: Logid int, databasename varchar(30), tablename varchar(30), logdate datetime

Query 1:
Select max(logdate) from log_table
where databasename='db1' and tablename='table1';
consider the Log_table is empty, when I execute Query 1, the result will be 'Null'.

Query 2 :
select max(logdate) from log_table
Where databasename='db1' and tablename='table1'
group by databasename,tablename;
Consider the log_table is empty, when I execute Query 2, the result will be empty.

Note : both the condition table is empty.

Please explain me what happening inside and why the result different?

Post #1425968

Jeff Moden

Posted Sunday, March 03, 2013 8:33 PM

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In the first query, you give SQL Server no choice. You say to find the MAX unconditionally for the given criteria and there isn't one so it returns a NULL.

In the second query, it's different. You're saying find the MAX for a given group which is actually a condition. There are no groups that match your criteria so it returns nothing.

--Jeff Moden

"RBAR is pronounced "ree-bar" and is a "Modenism" for "Row-By-Agonizing-Row".

First step towards the paradigm shift of writing Set Based code:
Stop thinking about what you want to do to a row... think, instead, of what you want to do to a column."

For better, quicker answers on T-SQL questions, click on the following...
http://www.sqlservercentral.com/articles/Best+Practices/61537/

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Post #1426065

sanandh87

Posted Sunday, March 03, 2013 11:04 PM

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Thanks for your reply Jeff.

I got clear pic .. thanks a lot...

one more doubt Jeff,

both the condition table is empty,

then why the where condition returns 'null'
and
group by condition returns nothing.

can you plz explain me ?

Post #1426089

GilaMonster

Posted Monday, March 04, 2013 12:43 AM

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http://sqlblog.com/blogs/paul_white/archive/2012/03/12/fun-with-aggregates.aspx

Gail Shaw
Microsoft Certified Master: SQL Server 2008, MVP
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability

We walk in the dark places no others will enter
We stand on the bridge and no one may pass

Post #1426104

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