Sunday, June 17, 2007

null forkey ^ unmatched prikey

--simple illustration:
emplyee {employee_id, department_id, age, salary...}
department {department_id, size, average age...}

null-forkey: employee without department_id
unmatched prikey: department without employee

-- null forkey -- perhaps not a case for outer join

"null in forkey column". referential integrity violated.

http://sqlzoo.net/4.htm

Q 2a: One of the MSP (member of parliment) has no party (null forkey, violating referential integrity). How do you include him in a member-and-party listing?

A: select msp.name, party.name from msp, party where msp.party=party.code
union
select msp.name, null from msp where
msp.party is null

Neither makes sense:
wrong: msp.party=party.code(+) -- no effect. How can party.code (prikey) be a SUBSET of msp.party (forkey)? referential integrity violated


wrong: msp.party(+)=party.code -- showing empty parties


-- unmatched prikey -- #1 common scenario for outer join


"forkey = subset@prikey"

Heart of every outer join is a subset relationship. If the 2 join-columns are prikey-forkey, then the smaller set has to be the forkey -- referential integrity. An outer join will show ALL the rows from the prikey side, whether or not there's a matching row.


where employee.employee_id = bonus.employee_id(+) -- "+" on the forkey side




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