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)))
………………………………
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2022-09-12 10:30:55
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高频电子线路课后习题答案-曾兴雯.doc
高频信号指的是适合天线发射、传播和接收的射频信号。 采用高频信号的原因主要是：
（1）频率越高，可利用的频带宽度就越宽，信道容量就越大，而且可以减小或避免频道间的干扰；
（2）高频信号更适合电线辐射和接收，因为只有天线尺寸大小可以与信号波长相比拟时，才有较高的辐射效率和接收效率，这样，可以采用较小的信号功率，传播较远的距离，也可获得较高的接收灵敏度。
2022-09-11 16:23:16
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信号与系统奥本海姆英文版第二版课后答案(非扫描.pdf 概率与统计习题册(川大版) (陈鸿建 著) 四川大学出版社 课后答案-
2022-09-10 23:19:27
1.29MB
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Thinking in Java第三版(中文+习题答案)+第四版(英文+习题答案)
2022-09-10 11:38:22
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自动控制原理习题解析(《自动控制原理(第五版)》同步辅导) 胡寿松主编 2007 科学出版社
2022-09-07 23:52:53
9.78MB
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