tedd wrote:
Hi gang:
I just interviewed for a job teaching at the local college (imagine me
taking minds of mush and molding them to the world according to tedd --
frightening huh?)
In any event, the interviewer asked me how long I've been using MySQL
and I replied several years. After which she asked a single question,
which was "What does EXIST mean?"
There is no 'Exist' keyword in sql, it's 'EXISTS'. Everyone seems to
know the "drop table if exists" syntax but that's not the right one they
are after (and it's non-standard sql).
There is an 'Exists' keyword for subselects. Instead of doing an IN
query, you can do an 'exists' query - but it's use is very different.
It returns true if any row in the subquery exists or false if none
exist. The idea is instead of:
select * from accounts where company_id in (select company_id from
companies where name='X');
you do
select * from company where company_id exists (select company_id from
companies where name='X');
the subselect checks the company table for the id, if it exists, the
outer query runs. It does not matter what company_id is returned from
the subquery, just that at least 1 does return.
I've never used this anywhere myself because I usually care what id's
are returned from the subquery.
(Yes, that's from the top of my head :P but here's doc's to help).
http://dev.mysql.com/doc/refman/5.0/en/exists-and-not-exists-subqueries.html
http://www.postgresql.org/docs/8.3/static/functions-subquery.html
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