On 05/07/10 14:38, Richard Quadling wrote:
On 5 July 2010 14:02, Jason Pruim<li...@pruimphotography.com> wrote:
Hi everyone,
I'll admit right now that I'm still trying to wrestle with inner joins...
It is all about set theory. Imagine two circles, which overlap
(http://en.wikipedia.org/wiki/Venn_diagram#Example as an example).
For that example, simplistically, A contains me and my emu. B contains
my emu and the my deathwatch beetle.
SELECT * FROM A,B WHERE A.id = B.id (My emu)
SELECT * FROM A INNER JOIN B ON A.id = B.id (My emu)
SELECT * FROM A LEFT OUTER JOIN B ON A.id = B.id (Me and My emu)
SELECT * FROM A RIGHT OUTER JOIN B ON A.id = B.id (My emu and my
deathwatch beetle)
SELECT * FROM A FULL OUTER JOIN B ON A.id = B.id
returns in interesting set (essentially all things but 1 column for each table).
Me, null
My emu, my emu
null, My deathwatch beetle.
If you were using ISNULL ...
SELECT ISNULL(A.name, B.name) AS name FROM A FULL OUTER JOIN B ON A.id = B.id
would return all things
Me
My emu
My deathwatch beetle.
And, (I think), finally, an inversion of the inner join.
SELECT ISNULL(A.name, B.name) AS name FROM A FULL OUTER JOIN B ON A.id
= B.id WHERE A.id IS NULL OR B.id IS NULL
returns
Me
My deathwatch beetle.
All things except those 2 legged things that can fly.
I hope that helps.
Regards,
Richard.
P.S. I don't have an emu.
Clearly, or you'd know that they can't fly either...
:)
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