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详细说明： 一些习题，有助于加深SQL查询。 Relational algebra and SQL Query Consider the following schema of a company database: Employees(eid, ename, address, supereid) Departments(did, dname) Projects(pid, pname, did) Works_on(eid, pid, hours) (1)For each employee, find his/her name and the name of his/her supervisor. (2) Find the pid of projects of the department with dname = “Toys” for which at least two different Employees work. (3) Find the eids of employees who work on a project of every department, i.e. find the eids of employees who work for at least a project of every department. Answer: (SELECT E.eid FROM Employees E WHERE NOT EXISTS (SELECT * FROM Departments D WHERE NOT EXISTS (SELECT * FROM Projects P, Works_on W WHERE W.pid = P.pid AND P.did = D.did AND W.eid = E.eid))) ……………………………… ……………………………… ...展开收缩
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