问题
I could not get a hint on how to write SQL queries for A and B for the following schema.
Programme (Pid:int, Department:string...)
Employee (Eid:int, Department:string..)
Participation (Pid:int, Eid:int, ..)
A. Names of programmes participated by all employees
B. Names of employees participating in all his department's programmes.
Any guidelines would be helpful.
回答1:
Haven't tried these, but this is what I would be thinking:
SELECT pg.Name
FROM Participation AS p INNER JOIN Programme AS pg ON p.Pid = pg.Pid
GROUP BY p.Pid
HAVING COUNT(*) = (SELECT COUNT(*) FROM Employeee)
SELECT e.Name
FROM Participation AS p INNER JOIN Employee AS e ON p.Eid = e.Eid
INNER JOIN Programme AS pg ON pg.Pid = p.Pid
WHERE pg.Department = e.Department
GROUP BY p.Eid, e.Department, e.Name
HAVING COUNT(*) = (SELECT COUNT(*)
FROM Programme AS pg2
WHERE pg2.Department = e.Department)
回答2:
The relational operator you require is division, popularly known as "the supplier who supplies all parts".
Things to consider are whether exact division or division with remainder and how to handle an empty divisor.
UPDATE: just to clarify, SQL lacks an explicit division operator or keyword**. However, relational division can be achieved in SQL using other operators. I'll refrain from posting a working example because of the 'homework' tag. But the one I usually use is similar to the "Division with Set Operators" example at this link.
Note that @Dylan Smith's is what is commonly known as "Celko's division" and @tobyodavies's answer uses a variation on what is commonly known as "Date's division" (Date wouldn't use an outer join but instead a second NOT EXISTS). But maybe they really did reinvent these well-established approaches themselves, who knows? ;)
** the same applies to many other relational operators e.g. SQL has no semi-difference operator but can be performed using other SQL operators e.g. NOT IN, NOT EXISTS, EXCEPT, etc.
回答3:
Use WHERE NOT EXISTS and Outer Joins
For all universal Programmes:
SELECT * FROM Programme
WHERE NOT EXISTS (SELECT * FROM
(Participation NATURAL JOIN Programme) LEFT JOIN Employee
USING (Eid,Department)
WHERE Employee.Eid IS NULL)
This should be fairly easy to explain - selects all programs where the is no employee not participating
For all enthusiatic employees:
SELECT * FROM Employee
WHERE NOT EXISTS (SELECT * FROM
Employee LEFT JOIN Participation
USING (Eid,department)
WHERE Participation.Eid IS NULL)
Again - selects all employyes where there are no programs in the same department in which that employee is not participating.
This may look familiar if you are familiar with formal logic at all - universal quantification is generally defined in terms of negated existential qualification
来源：https://stackoverflow.com/questions/7803775/sql-queries-involving-for-all