Delete with left join - SQL Server Forums

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Delete with left join

Author

Topic

Suhi100
Starting Member

14 Posts

Posted - 2015-02-20 : 13:15:56

Hi,

can someone tell the logic of the below DELETE statement?
It deletes all records where the file_modified column value is smaller then the maximum value in the same column, but not crystal clear how...

delete a
from Staging a
left join (select max(file_modifieddate) as file_modifieddate from Staging) b on
a.file_modifieddate = b.file_modifieddate
where b.file_modifieddate is null

or if someone knows links to similar topics would be helpful

Thanks!

tkizer
Almighty SQL Goddess

38200 Posts

Posted - 2015-02-20 : 14:32:23

It's due to the use of LEFT JOIN with the IS NULL option in the WHERE clause. For the joined columns in a LEFT JOIN, any rows where there wasn't a match will have a NULL value. Any rows where there was a match will have a non-NULL value. So LEFT JOIN/IS NULL is a way to get the rows that would not show up with an INNER JOIN.

Tara Kizer
SQL Server MVP since 2007
http://weblogs.sqlteam.com/tarad/

Suhi100
Starting Member

14 Posts

Posted - 2015-02-23 : 05:20:27

Perfect, it is clear now
Thanks

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