join depending on column value

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join depending on column value

Author

Topic

sqlclarify
Yak Posting Veteran

56 Posts

Posted - 2009-04-22 : 21:45:06

Hello,

I have a table A. I want to join table B (and get col b) if a column b in table B is not null, else join table C and get col c).

What is the best way to do this?

If I left join both table B and C won't I get a different number of rows than what I would get if I join only table B and table C depending on what needs to be joined?

nathans
Aged Yak Warrior

938 Posts

Posted - 2009-04-22 : 22:13:25

Ok, lets start with a test setup and see if we can zero in on what you are after. What would u like to return if no match in B or C (as reflected in 4A below)


declare @A table (id int, n varchar(10))
declare @B table (id int, n varchar(10))
declare @C table (id int, n varchar(10))

insert into @A
	select 1, '1A' union select 2, '2A' union
	select 3, '3A' union select 4, '4A'
	
insert into @B
	select 1, '1B' union select 3, '3B'

insert into @C
	select 2, '2C'
	
	
select	a.id, a.n,
	coalesce(b.n, c.n)
from	@A a 
left
join	@B b on a.id = b.id
left
join	@C c on a.id = c.id

Nathan Skerl

Transact Charlie
Master Smack Fu Yak Hacker

3451 Posts

Posted - 2009-04-23 : 04:30:24

You haven't said if there is a 1 to 1 relationship between the three tables.

If there is a 1 to many between A and B and a 1 to 1 between A and C then that makes this a little more complicated.

If it's 1 to 1 then you can do what you want by just LEFT JOINING b and c to a and then using a CASE statement in your SELECT list.

Charlie
===============================================================
Msg 3903, Level 16, State 1, Line 1736
The ROLLBACK TRANSACTION request has no corresponding BEGIN TRANSACTION

sqlclarify
Yak Posting Veteran

56 Posts

Posted - 2009-04-23 : 20:56:02

If there is no match in B or C I need to return null.

Yes, the problem is that the relationship between A and B is one to many and the relationship between A and C is one to many (from what I can understand). They have not explicitly stated whether the relationship between these tables is one-to-one or one-to-many. I assume it is one-to-many because if it was one-to-one these tables would still be one single table right?

sqlclarify
Yak Posting Veteran

56 Posts

Posted - 2009-04-24 : 23:21:45

Anybody have any advice?

khtan
In (Som, Ni, Yak)

17689 Posts

Posted - 2009-04-24 : 23:34:28

please provide some sample data and the expected result

KH
[spoiler]Time is always against us[/spoiler]

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