# Re: [PHP] Inner join woes... and sweet tea!

```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.

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