Friday, February 23, 2007

2min intro to NULL (sql)

http://www.oracle.com/technology/oramag/oracle/05-jul/o45sql.html is a more complete list of jolts. But for now ...

#1 jolt: "the NOT-trap" for any cond3 such as "amount > 10" or "id=order.customer_id" ... ,
if
select ... where cond3 --> 30% of the rows,
then
select ... where NOT cond3 --> ?


#2 jolt: "ignore"

where col3 = ...
where col3 > ...
where col3 LIKE ...
where col3 IN ...

These filters will ignore NULL entries. Yes, ignore their existence. Reason? null means "unknown" so such an entry is perhaps IN and perhaps not IN.

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