[TransWarp] Topics for tomorrow, and beyond
alex at ank-sia.com
Thu Oct 16 03:53:21 EDT 2003
Phillip J. Eby wrote, at 16.10.2003 7:30:
> Note that the exists join can be emulated via an outer join, combined
> with an assertion that the columns on the target are not null.
sorry, i do not understand why do you need an outer join here. this is
exactly what a normal, inner join does.
'exists' differ from joins (both inner and outer) in that it does not
make a production of the "select" table and "exists" table. if you are
going to emulate 'exists' with a join, you will probably have to
aggregate rows in the join result, won't you?
> Plus, don't forget UNION... perhaps INTERSECT and DIFFERENCE as well.
> We don't use these right now because Sybase didn't do them years ago,
> but there were times that I really wanted them...
just curious: what intersect is needed for?
since outer joins appeared, i don't remember if i ever had a real need
in union, and i've never used intersect or difference, although i think
i can imagine how difference may be useful if we don't have outer joins.
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