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Tip of the Day
Language: SQL Server
Expertise: Beginner
Aug 5, 1998

Unmatched Query

Question:
When one is working with large amounts of data, what is the quickest way of displaying rows not found when comparing two tables, without doing the following:

SELECT table1.field1 FROM Table1 WHERE Table1.field1 NOT IN (SELECT Table2.field1 FROM Table2)

Answer:
When possible, you should always use the exists clause instead of the IN clause. Exists is more efficient.

 
So 
select * 
from table1 
where
not exists
(select * 
from table2 
where
table1.col1 = table2.col2
)
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