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merge join

Posted on 2010-11-17
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Last Modified: 2013-11-10
merge join in distinctively SSIS, not TSQL, right? how does it differ than the other kinds of join (inner, left, self etc)

thanks
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Question by:anushahanna
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chapmandew earned 167 total points
ID: 34161464
Its in both....but they are essentially the same idea.

Inner/Outer/Full joins are LOGICAL joins in the database engine.  Nested Loop/Merge/Hash joins are the physical representations of the logical joins.  Nested loop joins are two outputs, usually a large input and a small input and the small input loops into the large input.  Merge has to sort the two inputs and then merge them together to join them.  Works really well if the inputs are already sorted.  Hash is for the looping scenario OR for two large inputs that are not indexed.  

No, the merge in SSIS is similar to the merge join in the db engine in that it has to physically sort (preceeded by a sort task) before you can merge the results.

Clear as mud?  :)
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by:chapmandew
ID: 34161469
Correction:
No, the merge in SSIS is similar to the merge join in the db engine in that it has to physically sort (preceeded by a sort task) before you can merge the results.

...missed the w on Now.
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Expert Comment

by:Reza Rad
ID: 34161893
as Champandew noted this is exactly like join types in t-sql.
there are these types there:
inner join --> like inner join in t-sql
full outer join--> like full outer join in t-sql
left outer join --> like let outer join in t-sql
there in no right outer join in merge join transform, but you can only change inputs and use left outer join to have right outer join .

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by:Reza Rad
Reza Rad earned 166 total points
ID: 34161907
And note that when you want to use Merge Join Transform in SSIS , your inputs should be SORTED.
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by:Racim BOUDJAKDJI
Racim BOUDJAKDJI earned 83 total points
ID: 34162309
As indicated, MERGE JOIN is the physical method that SQL Server's optimizer may select to implement some of its JOIN implementations.   SQL MERGER JOIN is the procedural implementation of the SQL JOINS, which are themselves the implementations of the relational JOIN.

In SSIS, though there are order prerequisites that do not exists in the TSQL engine.

Hope this helps...
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by:anushahanna
ID: 34162782
thanks-

merge join in SSIS- is it possible to say 'No thanks for physical sorting - just inner join the 2 OLEDB input and give it in any sort order"
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by:subhashpunia
subhashpunia earned 84 total points
ID: 34162825
Then use other type of joins: Either Nested Loop or HASH join.
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by:Reza Rad
Reza Rad earned 166 total points
ID: 34163727
NO,
if you want to use Merge Join Transformation , so you SHOULD sort inputs.
if you don't want to sort inputs, you can use Lookup transformation ( if your inputs are oledb, and you have only ONE joining field) or use t-sql joins in OLEDB Source command if your inputs are both from single server.
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by:chapmandew
chapmandew earned 167 total points
ID: 34163742
>>merge join in SSIS- is it possible to say 'No thanks for physical sorting - just inner join the 2 OLEDB input and give it in any sort order"

Nope, you can't do a "merge join" without sorted inputs.
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by:anushahanna
ID: 34165137
Thanks for the help on understanding the merge..
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