[SQL] No Not join

[David Hood]

> Can you explain me (just for interest) the logic of above your 2 linked
> tables and logic of above queries with <> ?
>

It's a distillation of a more complex query I was running, but it is 
basically trying to ask:
For a set of historical records (in this case electoral rolls) find me 
all of the individuals present in year A and year B whose occupational 
code number has changed between the years in question. I'm using two 
table to avoid the self relational flipping results thing.

For most database design, the real world objects and the relationships 
between them are known at the time of creation of the database. What I 
do is a little different. I analyse historical sources that have been 
transcribe or scanned onto the computer. By trying different ways of 
relating sources together I can find out things about the past not 
present in more formal histories.

In the work I do, the relationships between tables can, and should vary 
wildly as different facets of the past are explored and understanding 
of the nature of the sources increases. So I often use some vary odd 
linking criteria to see what light they shed.

Regards,
David Hood