Question

我有以下MySQL表。他们代表学校的CS课程，申请人是特定课程的助教（助教）。

我想创建一个查询，打印每个课程的“最佳”申请人。最佳申请人的限制是：

首先匹配Applicants.level = 7的申请人。
选择ApplicantsToCourses.returning = true的申请人为第二名。
所有其他申请人均未经进一步歧视。

表定义是：

CREATE TABLE Courses (
 course_number SMALLINT(3) UNSIGNED NOT NULL,
 course_section SMALLINT(1) UNSIGNED NOT NULL,
 name CHAR(30) NOT NULL,
 instructor CHAR(30),
 lab_time CHAR(30),
 PRIMARY KEY(course_number, section),
 FOREIGN KEY(course_number, section) REFERENCES ApplicantsToCourses(course_number, course_section)
)

CREATE TABLE Applicants (
 student_id CHAR(10) NOT NULL,
 name CHAR(30),
 email CHAR(30),
 gpa DECIMAL(4,3) UNSIGNED,
 level CHAR(2),
 timestamp TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
 PRIMARY KEY(student_id),
 FOREIGN KEY(student_id) REFERENCES ApplicantsToCourses(student_id),
 CHECK(gpa <= 4.000)
)

CREATE TABLE ApplicantsToCourses (
 student_id CHAR(10) NOT NULL,
 returning BOOLEAN DEFAULT FALSE NOT NULL,
 course_number SMALLINT(3) UNSIGNED NOT NULL,
 course_section SMALLINT(1) UNSIGNED NOT NULL,
 PRIMARY KEY(student_id, course_number, course_section),
 FOREIGN KEY(student_id) REFERENCES Applicants(student_id),
 FOREIGN KEY(course_number, course_section) REFERENCES Courses(course_number, course_section)
)

我对查询的尝试是。。

select a.student_id, ac.course_number, ac.course_section
from Applicants a, ApplicantsToCourses ac, Courses c
where a.student_id = ac.student_id and ac.course_number = c.course_number and ac.course_section = c.course_section
order by a.level, ac.returning desc

。。。但那肯定没有正确的逻辑。

Answer 1

您可以使用以下伪代码创建一些临时表，以帮助您找到最终解决方案。

SELECT * 
FROM  Applicants APP
JOIN  ApplicantsToCourses ATC ON ATC.student_id = APP.student_id
JOIN  Courses COU ON COU.number = ATC.course_number AND COU.section = ATC.course_section
WHERE APP.level = 7

SELECT * 
FROM  Applicants APP
JOIN  ApplicantsToCourses ATC ON ATC.student_id = APP.student_id
JOIN  Courses COU ON COU.number = ATC.course_number AND COU.section = ATC.course_section
WHERE ATC.returning = true

通过MySQL查询选择职位申请人

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