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mysql - mysql子查询COUNT与WHERE子句混淆

问题描述

我整天都在捣碎按钮，但无法让这个查询工作。我有 3 个表学生、课程和注册表，显示学生注册了哪些课程

查询需要检索至少有 2 名学生注册的所有课程，按学生人数最多的课程排序

我想出了如何检索每个班级的注册人数，但无法将注册人数过滤为 >= 2 名学生

-- coursetable -----------------------------

CREATE TABLE StudentTable(
studentID VARCHAR(255) NOT NULL,
firstName VARCHAR(255) NOT NULL,
LastName VARCHAR(255) NOT NULL,
DOB DATE NULL,
CONSTRAINT pk_studentTable PRIMARY KEY(studentID)
);

-- coursetable -----------------------

 CREATE TABLE CourseTable(
 courseID VARCHAR(255) NOT NULL,
 courseName VARCHAR(255) NOT NULL,
 hoursPerWeek int(11) NULL,
 startDate DATE NULL,
 CONSTRAINT pk_courseTable PRIMARY KEY(courseID)
 );

-- enrolment table --

CREATE TABLE EnrolmentTable(
    studentID VARCHAR(255) NOT NULL,
    CourseID VARCHAR(255) NOT NULL,
CONSTRAINT pk_enrolmentTable PRIMARY KEY(studentID, CourseID)
);

这是我可以做的查询，显示所有班级的注册情况，但它显示一个班级只有 1 名学生注册。我需要它只显示 => 2 个注册的课程

    SELECT ct.CourseName AS Course_Name, COUNT(st.studentID) AS Students_Enrolled
FROM EnrolmentTable et
INNER JOIN courseTable ct ON ct.courseID = et.courseID
INNER JOIN studentTable st ON st.studentID = et.studentID
GROUP BY et.courseID;

我需要使用子查询对吗？但不确定如何

标签： mysqlsubquerymariadbaggregate-functions

解决方案

您可以使用HAVING过滤结果

SELECT ct.CourseName AS Course_Name, COUNT(st.studentID) AS Students_Enrolled
FROM EnrolmentTable et
INNER JOIN courseTable ct ON ct.courseID = et.courseID
INNER JOIN studentTable st ON st.studentID = et.studentID
GROUP BY et.courseID
HAVING Students_Enrolled> 1
ORDER BY Students_Enrolled DESC

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