[英]Will I need to do a join of 4 tables to fulfill this SQL query or is there a simpler method?
我目前需要做以下事情：
找到所有课程的名称和在星期五下午注册的学生，并且至少有 2 名学生注册。
我在想我应该做的是将标题为 student、takes、course、section 和 time_slot 的表连接在一起，然后从那里执行 SQL 查询，但这对我来说似乎太过分了。
首先，我将通过加入 takes 和 students 表来获得所有课程和所有参加课程的学生姓名的列表。
然后，我会使用 time_slot_id 查找在星期五下午上课的所有课程，time_slot_id 必须是“D”或“F”或“G”，而日期将是“F”以表示星期五。 然后我会将时间段与该部分匹配。
在那之后我被卡住了。
这是我尝试过的：
SELECT name, course_ID FROM takes JOIN student ON takes.id = student.ID WHERE count(course_ID) > 1;
和
SELECT course_id
FROM takes JOIN student ON takes.id = student.ID
GROUP BY course_id
HAVING COUNT(*) > 1;
底部的查询有效，但没有学生的姓名，顶部的查询更接近我需要的，但返回错误。
这是我拥有的完整 mySQL 代码。
CREATE TABLE time_slot(
time_slot_id VARCHAR(100) NOT NULL,
day VARCHAR(100) NOT NULL,
start_hour TIME NOT NULL,
start_min TIME NOT NULL,
end_hour TIME NOT NULL,
end_min TIME NOT NULL,
PRIMARY KEY (time_slot_id, day, start_hour, start_min)
);
CREATE TABLE department(
dept_name VARCHAR(100) NOT NULL,
building VARCHAR(100) NOT NULL,
budget DECIMAL(10, 2) UNSIGNED,
PRIMARY KEY(dept_name)
);
CREATE TABLE student(
ID INT UNSIGNED NOT NULL AUTO_INCREMENT,
name VARCHAR(255) NOT NULL,
dept_name VARCHAR(100) NOT NULL,
tot_cred SMALLINT NOT NULL,
PRIMARY KEY (ID),
FOREIGN KEY (dept_name) REFERENCES department(dept_name)
);
CREATE TABLE instructor(
ID INT UNSIGNED NOT NULL AUTO_INCREMENT,
name VARCHAR(255) NOT NULL,
dept_name VARCHAR(100) NOT NULL,
salary DECIMAL(10, 2) UNSIGNED NOT NULL,
PRIMARY KEY (ID),
FOREIGN KEY (dept_name) REFERENCES department(dept_name)
);
CREATE TABLE course(
course_id VARCHAR(100) NOT NULL,
title VARCHAR(255) NOT NULL,
dept_name VARCHAR(100) NOT NULL,
cedits SMALLINT UNSIGNED NOT NULL,
PRIMARY KEY (course_id),
FOREIGN KEY (dept_name) REFERENCES department(dept_name)
);
CREATE TABLE prereq(
course_id VARCHAR(100) NOT NULL,
prereq_id VARCHAR(100) NOT NULL,
PRIMARY KEY(course_id, prereq_id),
FOREIGN KEY(course_id) REFERENCES course(course_id)
);
CREATE TABLE classroom(
building VARCHAR(100) NOT NULL,
room_no VARCHAR(100) NOT NULL,
capacity SMALLINT NOT NULL,
PRIMARY KEY (building, room_no)
);
CREATE TABLE section(
course_id VARCHAR(100) NOT NULL,
sec_id VARCHAR(100) NOT NULL,
semester VARCHAR(100) NOT NULL,
year SMALLINT UNSIGNED NOT NULL,
building VARCHAR(100) NOT NULL,
room_no VARCHAR(100) NOT NULL,
time_slot_id VARCHAR(100) NOT NULL,
PRIMARY KEY (course_id, sec_id, semester, year),
FOREIGN KEY (course_id) REFERENCES course(course_id),
FOREIGN KEY (building,room_no) REFERENCES classroom(building,room_no),
FOREIGN KEY (time_slot_id) REFERENCES time_slot(time_slot_id)
);
CREATE TABLE takes(
ID INT UNSIGNED,
course_id VARCHAR(100) NOT NULL,
sec_id VARCHAR(100) NOT NULL,
semester VARCHAR(100) NOT NULL,
year SMALLINT UNSIGNED NOT NULL,
grade VARCHAR(100),
PRIMARY KEY (ID, course_id, sec_id, semester, year),
FOREIGN KEY (ID) REFERENCES student(ID),
FOREIGN KEY (course_id,sec_id,semester,year) REFERENCES section(course_id,sec_id,semester,year)
);
CREATE TABLE teaches(
ID INT UNSIGNED,
course_id VARCHAR(100),
sec_id VARCHAR(100),
semester VARCHAR(100),
year SMALLINT UNSIGNED,
PRIMARY KEY(ID, course_id, sec_id, semester, year),
FOREIGN KEY(ID) REFERENCES instructor(ID),
FOREIGN KEY (course_id,sec_id,semester,year) REFERENCES section(course_id,sec_id,semester,year)
);
CREATE TABLE advisor(
s_id INT UNSIGNED NOT NULL,
i_id INT UNSIGNED NOT NULL,
PRIMARY KEY (s_id),
FOREIGN KEY (s_id) REFERENCES student(ID),
FOREIGN KEY (i_id) REFERENCES instructor(ID)
);
insert into classroom values ('Packard', '101', '500');
insert into classroom values ('Painter', '514', '10');
insert into classroom values ('Taylor', '3128', '70');
insert into classroom values ('Watson', '100', '30');
insert into classroom values ('Watson', '120', '50');
insert into department values ('Biology', 'Watson', '90000');
insert into department values ('Comp. Sci.', 'Taylor', '100000');
insert into department values ('Elec. Eng.', 'Taylor', '85000');
insert into department values ('Finance', 'Painter', '120000');
insert into department values ('History', 'Painter', '50000');
insert into department values ('Music', 'Packard', '80000');
insert into department values ('Physics', 'Watson', '70000');
insert into course values ('BIO-101', 'Intro. to Biology', 'Biology', '4');
insert into course values ('BIO-301', 'Genetics', 'Biology', '4');
insert into course values ('BIO-399', 'Computational Biology', 'Biology', '3');
insert into course values ('CS-101', 'Intro. to Computer Science', 'Comp. Sci.', '4');
insert into course values ('CS-190', 'Game Design', 'Comp. Sci.', '4');
insert into course values ('CS-315', 'Robotics', 'Comp. Sci.', '3');
insert into course values ('CS-319', 'Image Processing', 'Comp. Sci.', '3');
insert into course values ('CS-347', 'Database System Concepts', 'Comp. Sci.', '3');
insert into course values ('EE-181', 'Intro. to Digital Systems', 'Elec. Eng.', '3');
insert into course values ('FIN-201', 'Investment Banking', 'Finance', '3');
insert into course values ('HIS-351', 'World History', 'History', '3');
insert into course values ('MU-199', 'Music Video Production', 'Music', '3');
insert into course values ('PHY-101', 'Physical Principles', 'Physics', '4');
insert into instructor values ('10101', 'Srinivasan', 'Comp. Sci.', '65000');
insert into instructor values ('12121', 'Wu', 'Finance', '90000');
insert into instructor values ('15151', 'Mozart', 'Music', '40000');
insert into instructor values ('22222', 'Einstein', 'Physics', '95000');
insert into instructor values ('32343', 'El Said', 'History', '60000');
insert into instructor values ('33456', 'Gold', 'Physics', '87000');
insert into instructor values ('45565', 'Katz', 'Comp. Sci.', '75000');
insert into instructor values ('58583', 'Califieri', 'History', '62000');
insert into instructor values ('76543', 'Singh', 'Finance', '80000');
insert into instructor values ('76766', 'Crick', 'Biology', '72000');
insert into instructor values ('83821', 'Brandt', 'Comp. Sci.', '92000');
insert into instructor values ('98345', 'Kim', 'Elec. Eng.', '80000');
insert into time_slot values ('A', 'M', '8', '0', '8', '50');
insert into time_slot values ('A', 'W', '8', '0', '8', '50');
insert into time_slot values ('A', 'F', '8', '0', '8', '50');
insert into time_slot values ('B', 'M', '9', '0', '9', '50');
insert into time_slot values ('B', 'W', '9', '0', '9', '50');
insert into time_slot values ('B', 'F', '9', '0', '9', '50');
insert into time_slot values ('C', 'M', '11', '0', '11', '50');
insert into time_slot values ('C', 'W', '11', '0', '11', '50');
insert into time_slot values ('C', 'F', '11', '0', '11', '50');
insert into time_slot values ('D', 'M', '13', '0', '13', '50');
insert into time_slot values ('D', 'W', '13', '0', '13', '50');
insert into time_slot values ('D', 'F', '13', '0', '13', '50');
insert into time_slot values ('E', 'T', '10', '30', '11', '45');
insert into time_slot values ('E', 'R', '10', '30', '11', '45');
insert into time_slot values ('F', 'T', '14', '30', '15', '45');
insert into time_slot values ('F', 'R', '14', '30', '15', '45');
insert into time_slot values ('G', 'M', '16', '0', '16', '50');
insert into time_slot values ('G', 'W', '16', '0', '16', '50');
insert into time_slot values ('G', 'F', '16', '0', '16', '50');
insert into time_slot values ('H', 'W', '10', '0', '12', '30');
insert into section values ('BIO-101', '1', 'Summer', '2009', 'Painter', '514', 'B');
insert into section values ('BIO-301', '1', 'Summer', '2010', 'Painter', '514', 'A');
insert into section values ('CS-101', '1', 'Fall', '2009', 'Packard', '101', 'H');
insert into section values ('CS-101', '1', 'Spring', '2010', 'Packard', '101', 'F');
insert into section values ('CS-190', '1', 'Spring', '2009', 'Taylor', '3128', 'E');
insert into section values ('CS-190', '2', 'Spring', '2009', 'Taylor', '3128', 'A');
insert into section values ('CS-315', '1', 'Spring', '2010', 'Watson', '120', 'D');
insert into section values ('CS-319', '1', 'Spring', '2010', 'Watson', '100', 'B');
insert into section values ('CS-319', '2', 'Spring', '2010', 'Taylor', '3128', 'C');
insert into section values ('CS-347', '1', 'Fall', '2009', 'Taylor', '3128', 'A');
insert into section values ('EE-181', '1', 'Spring', '2009', 'Taylor', '3128', 'C');
insert into section values ('FIN-201', '1', 'Spring', '2010', 'Packard', '101', 'B');
insert into section values ('HIS-351', '1', 'Spring', '2010', 'Painter', '514', 'C');
insert into section values ('MU-199', '1', 'Spring', '2010', 'Packard', '101', 'D');
insert into section values ('PHY-101', '1', 'Fall', '2009', 'Watson', '100', 'A');
insert into teaches values ('10101', 'CS-101', '1', 'Fall', '2009');
insert into teaches values ('10101', 'CS-315', '1', 'Spring', '2010');
insert into teaches values ('10101', 'CS-347', '1', 'Fall', '2009');
insert into teaches values ('12121', 'FIN-201', '1', 'Spring', '2010');
insert into teaches values ('15151', 'MU-199', '1', 'Spring', '2010');
insert into teaches values ('22222', 'PHY-101', '1', 'Fall', '2009');
insert into teaches values ('32343', 'HIS-351', '1', 'Spring', '2010');
insert into teaches values ('45565', 'CS-101', '1', 'Spring', '2010');
insert into teaches values ('45565', 'CS-319', '1', 'Spring', '2010');
insert into teaches values ('76766', 'BIO-101', '1', 'Summer', '2009');
insert into teaches values ('76766', 'BIO-301', '1', 'Summer', '2010');
insert into teaches values ('83821', 'CS-190', '1', 'Spring', '2009');
insert into teaches values ('83821', 'CS-190', '2', 'Spring', '2009');
insert into teaches values ('83821', 'CS-319', '2', 'Spring', '2010');
insert into teaches values ('98345', 'EE-181', '1', 'Spring', '2009');
insert into student values ('00128', 'Zhang', 'Comp. Sci.', '102');
insert into student values ('12345', 'Shankar', 'Comp. Sci.', '32');
insert into student values ('19991', 'Brandt', 'History', '80');
insert into student values ('23121', 'Chavez', 'Finance', '110');
insert into student values ('44553', 'Peltier', 'Physics', '56');
insert into student values ('45678', 'Levy', 'Physics', '46');
insert into student values ('54321', 'Williams', 'Comp. Sci.', '54');
insert into student values ('55739', 'Sanchez', 'Music', '38');
insert into student values ('70557', 'Snow', 'Physics', '0');
insert into student values ('76543', 'Brown', 'Comp. Sci.', '58');
insert into student values ('76653', 'Aoi', 'Elec. Eng.', '60');
insert into student values ('98765', 'Bourikas', 'Elec. Eng.', '98');
insert into student values ('98988', 'Tanaka', 'Biology', '120');
insert into takes values ('00128', 'CS-101', '1', 'Fall', '2009', 'A');
insert into takes values ('00128', 'CS-347', '1', 'Fall', '2009', 'A-');
insert into takes values ('12345', 'CS-101', '1', 'Fall', '2009', 'C');
insert into takes values ('12345', 'CS-190', '2', 'Spring', '2009', 'A');
insert into takes values ('12345', 'CS-315', '1', 'Spring', '2010', 'A');
insert into takes values ('12345', 'CS-347', '1', 'Fall', '2009', 'A');
insert into takes values ('19991', 'HIS-351', '1', 'Spring', '2010', 'B');
insert into takes values ('23121', 'FIN-201', '1', 'Spring', '2010', 'C+');
insert into takes values ('44553', 'PHY-101', '1', 'Fall', '2009', 'B-');
insert into takes values ('45678', 'CS-101', '1', 'Fall', '2009', 'F');
insert into takes values ('45678', 'CS-101', '1', 'Spring', '2010', 'B+');
insert into takes values ('45678', 'CS-319', '1', 'Spring', '2010', 'B');
insert into takes values ('54321', 'CS-101', '1', 'Fall', '2009', 'A-');
insert into takes values ('54321', 'CS-190', '2', 'Spring', '2009', 'B+');
insert into takes values ('55739', 'MU-199', '1', 'Spring', '2010', 'A-');
insert into takes values ('76543', 'CS-101', '1', 'Fall', '2009', 'A');
insert into takes values ('76543', 'CS-319', '2', 'Spring', '2010', 'A');
insert into takes values ('76653', 'EE-181', '1', 'Spring', '2009', 'C');
insert into takes values ('98765', 'CS-101', '1', 'Fall', '2009', 'C-');
insert into takes values ('98765', 'CS-315', '1', 'Spring', '2010', 'B');
insert into takes values ('98988', 'BIO-101', '1', 'Summer', '2009', 'A');
insert into takes values ('98988', 'BIO-301', '1', 'Summer', '2010', null);
insert into advisor values ('00128', '45565');
insert into advisor values ('12345', '10101');
insert into advisor values ('23121', '76543');
insert into advisor values ('44553', '22222');
insert into advisor values ('45678', '22222');
insert into advisor values ('76543', '45565');
insert into advisor values ('76653', '98345');
insert into advisor values ('98765', '98345');
insert into advisor values ('98988', '76766');
insert into prereq values ('BIO-301', 'BIO-101');
insert into prereq values ('BIO-399', 'BIO-101');
insert into prereq values ('CS-190', 'CS-101');
insert into prereq values ('CS-315', 'CS-101');
insert into prereq values ('CS-319', 'CS-101');
insert into prereq values ('CS-347', 'CS-101');
insert into prereq values ('EE-181', 'PHY-101');
好的...让我们使用您的策略来构建它。
首先，我将通过加入 takes 和 students 表来获得所有课程和所有参加课程的学生姓名的列表。
您在下面的查询是正确的开始 - 它会找到所有超过一名学生的课程。
SELECT course_id
FROM takes JOIN student ON takes.id = student.ID
GROUP BY course_id
HAVING COUNT(*) > 1;
对上述内容的一个重要警告是，学生在哪个学期/等等做这件事并不重要——只是该课程在任何时间、任何时间段等都有不止一个人注册。
要获取其他信息（课程和学生的姓名），您只需将其链接到课程表并获取和学生表，例如，
SELECT c.title AS Course_Title, s.name AS Student_Name
FROM course AS c
INNER JOIN takes AS t ON c.course_id = t.course_id
INNER JOIN student AS s ON t.ID = s.ID
INNER JOIN
(SELECT course_id
FROM takes JOIN student ON takes.id = student.ID
GROUP BY course_id
HAVING COUNT(*) > 1
) AS courses_2plus_students ON c.course_id = courses_2plus_students.course_id;
当然，您可以更改以上内容，使子查询位于 WHERE 子句或 CTE 中。
请注意，有一种更有效的方法可以使用 Windowed 函数执行上述操作，但我猜目前这是一座太过分的桥梁。
但是，正如您在策略中所说的那样——这不是全部答案——我们需要过滤掉课程。
然后，我会使用 time_slot_id 查找在星期五下午上课的所有课程，time_slot_id 必须是“D”或“F”或“G”，而日期将是“F”以表示星期五。 然后我会将时间段与该部分匹配。
我假设您希望计算时间段而不是硬编码（以防您添加新的时间段，例如，在星期一、星期二、星期四早上运行的时间段）。 因此，我将“下午”时段定义为从中午 12:00 或之后但在下午 6:00 之前开始的时段。
我们可以通过以下方式获取相关的时隙
SELECT DISTINCT ts.time_slot_id
FROM time_slot ts
WHERE ts.`day` = 'F'
AND ts.start_hour >= 12 and ts.start_hour < 18;
以上结果应为“D”和“G”（请注意，根据您的数据，F 不算数）。 我在上面使用了 DISTINCT 来删除重复项，以防给定时间段有双倍的 class 例如，给定下午有 2 节课。
然后我们需要相关的 course_ids——我们从上面的部分中得到它（并删除对 time_slot_id 的引用，因为我们不再需要它）。
SELECT DISTINCT sec.course_id
FROM section sec
INNER JOIN time_slot ts ON sec.time_slot_id = ts.time_slot_id
WHERE ts.`day` = 'F'
AND ts.start_hour >= 12 and ts.start_hour < 18;
现在，我们需要做的就是使用上面的部分过滤掉上面的课程。 所以得到（初始）答案的 SQL 是......
SELECT c.title AS Course_Title, s.name AS Student_Name
FROM course AS c
INNER JOIN takes AS t ON c.course_id = t.course_id
INNER JOIN student AS s ON t.ID = s.ID
INNER JOIN
(SELECT course_id
FROM takes JOIN student ON takes.id = student.ID
GROUP BY course_id
HAVING COUNT(*) > 1
) AS courses_2plus_students ON c.course_id = courses_2plus_students.course_id
INNER JOIN
(SELECT DISTINCT sec.course_id
FROM section sec
INNER JOIN time_slot ts ON sec.time_slot_id = ts.time_slot_id
WHERE ts.`day` = 'F'
AND ts.start_hour >= 12 and ts.start_hour < 18
) AS Friday_afternoon_courses on c.course_id = Friday_afternoon_courses.course_id;
再一次，您可以使用 WHERE 或 CTE 等代替子查询。
这是一个 db<>fiddle ，其中包含上面的每个步骤。 请注意，第一部分是隐藏的 - 它包含您的数据结构和数据（但课程时间设置为int而不是time ）。
注意——这是一个重要的注意事项——你需要非常小心这代表什么。
CS-101在 time_slot H 的 2009 年秋季交付； 它也在 Spring 2010 的 time_slot F 中交付。虽然我的回答可能给出了问题的最初字面答案，但更有意义的问题解释（“周五下午哪些班级同时有超过 1 名学生？”）可能是他们要问的。
由于这看起来像是家庭作业 - 我建议修改以上内容应该是您的下一步。 我在上面的答案中选择子查询进行过滤的原因是，我发现修改然后进行不同的过滤更容易。
PS 这是关于如何修改/修改什么的提示：上面的答案围绕“课程”表进行所有工作，例如，它按课程查找学生，按课程查找时间段等。相反，您可能希望关注“部分” ' 表，因为它提供有关特定时间课程的数据。
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