Q:

How to avoid duplicate rows in JOIN?

I have two identical tables (one with data from 2000-2009 and the other with data 2000-2013), and I am comparing both tables by left Join old with new on ID column and getting many-to-many mapping.
Table Structure(both)
ID  Date  Result1  Result3  Result3 Result10

Expected result is 
ID  new_Date Old_Date new_Result1 old_Result1 new_Result10  old_Result10

How can I avoid many to many relation?
ID  new_Date    Old_Date 
--  ----------  ----------
15  2005-05-04  2005-05-04 
15  2006-11-23  2005-05-04 
15  2005-05-04  2006-11-23 
15  2006-11-23  2006-11-23 

EXPECTED
ID  new_Date    Old_Date 
--  ----------  ----------
15  2005-05-04  2005-05-04 
15  2006-11-23  2006-11-23

A:

Have you considered using a DISTINCT?
As in:
SELECT DISTINCT id, name, lastName FROM Person

This will ensure unique results. Also you may consider doing a UNION on two queries as this also ensures uniqueness.
As in:
SELECT DISTINCT id, name, lastName FROM Person
UNION    
SELECT DISTINCT id, name, lastName FROM Person2

