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DB2 Join shortcuts - isn't there a special way you can join if the column name is the SAME in the 2 tables being joined

Posted on 2014-09-24
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Last Modified: 2014-09-25
I seem to recall reading there is a special way you can join if the column name is the SAME in the 2 tables being joined.  The perk to using this method is that when joining on this same column, the result set only gets the column one time.  So if you used this other syntax instead of "from t1 left join t2 on t2.mycol = t1.mycol" then "mycol" would only exist once in the joined result set.

Or maybe I just dreamed that?
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Question by:Ray
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Expert Comment

by:Qlemo
ID: 40342820
That's called a Natural Join, but I can't recall any popular DBMS supporting that. And it is always an Inner Join by definition.
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Author Comment

by:Ray
ID: 40342830
whats the specific syntax because it's not the typical ...
from t1 left join t2 on t2.mycol = t1.mycol
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Expert Comment

by:Qlemo
ID: 40342845
I don't get what you ask for.

Left Join is one kind of Outer Join, allowing one side (the right in this case) to be not matched and still return rows.

The Natural Join has theoretical syntax descriptions, but no implementation in DB2 and many more DBMS. It is "implemented" manually by enumerating the disjunct columns of at least one of the tables, so no change for you. And as said, Natural Join cannot be an Outer Join (left or right or full).
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Accepted Solution

by:
daveslash earned 500 total points
ID: 40342886
It works for me on DB2 for i :

select *              
  from deleteme 
  join deleteme2
 using (commonColumn)

Open in new window


HTH,
DaveSlash
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Author Closing Comment

by:Ray
ID: 40343891
"Using" is what I couldn't remember!  Thanks Dave :-)
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Author Comment

by:Ray
ID: 40343893
Thanks for the effort Qlemo.
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Expert Comment

by:daveslash
ID: 40344720
I realize this question is already answered, but here's another neat trick for shortening your queries that works beautifully on DB2 for i (v6.1) :

Instead of this:

SELECT someStuff
  FROM Table1 a
  left outer join Table2 b
    on b.column1 = a.column1
   and b.column2 = a.column2
  left outer join Table3 c 
    on c.column1 = b.column1
   and c.column2 = b.column2
   and c.column3 = b.column3
   and c.column4 = b.column4

Open in new window

You can do this:

SELECT someStuff
  FROM Table1 a
  left outer join Table2 b 
    on (b.column1,b.column2) = (a.column1,a.column2)
  left outer join Table3 c
    on  (c.column1,c.column2,c.column3,c.column4) =
        (b.column1,b.column2,b.column3,b.column4)

Open in new window


HTH,
DaveSlash
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Author Comment

by:Ray
ID: 40344965
VERY nice Dave!!!  I hope I remember this long enough to use it and turn it into a habit.  Thanks for the extra 'tip'!
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