SELECT von einer Tabelle ein Element, das mehrmals in einer anderen Tabelle auftritt

Question

Ich muss die Namen der Schüler herausfinden, die mindestens zwei Kurse eingeschrieben haben.

erstellen die beiden Tabellen:

CREATE TABLE Student (
    StudentID varchar(20) PRIMARY KEY, 
    FirstName varchar(15), 
    LastName varchar(30), 
    Sex varchar(6), 
    DOB date, 
    Email varchar(40) 
); 

CREATE TABLE Enrols (
    StudentID varchar(20), 
    CourseID varchar(20), 
    CONSTRAINT Student_Course PRIMARY KEY (StudentID,CourseID), 
    FOREIGN KEY (StudentID) REFERENCES Student(StudentID), 
    FOREIGN KEY (CourseID) REFERENCES Course(CourseID) 
); 

My (erfolglos) versucht:

SELECT DISTINCT Student.FirstName, 
    Student.LastName, 
    Enrols.CourseID,     
    COUNT (Student.FirstName) AS NumberOfNames 
    FROM Student 
    INNER JOIN Enrols 
    ON Student.StudentID = Enrols.StudentID 
    WHERE COUNT(Student.Firstname) >= 2; 

Answer 1

SELECT Student.FirstName AS "First Name", 
    Student.LastName AS "Last Name", 
    Enrols.CourseID AS "Course ID", 
    COUNT(Student.FirstName) AS "Number of Names" 
    FROM Student 
    INNER JOIN Enrols 
    ON Student.StudentID = Enrols.StudentID 
    GROUP BY Student.FirstName 
    HAVING COUNT(Student.FirstName) >= 2; 

Wird arbeiten, aber Sie werden nur ein CourseId sehen. Sie brauchen DISTINCT nicht.

Answer 2

sollten Sie verwenden HAVING und GROUP BY (aber nicht die curseID, wenn Sie verwenden Gruppe von)

SELECT Student.FirstName AS "First Name", 
    Student.LastName AS "Last Name",   
    COUNT (*) AS "Number of Names" 
    FROM Student 
    INNER JOIN Enrols 
    ON Student.StudentID = Enrols.StudentID 
    HAVING COUNT(Student.Firstname) >= 2 
    GROUP BY Student.FirstName, Student.LastName; 

Answer 3

Anstelle von where können Sie die Kombinationverwendenund having Klausel, wie unten -

SELECT Student.FirstName, 
     Student.LastName, 
     Student.StudentID, 
     COUNT(CourseID) AS NumberOfCourceEnroll 
    FROM Student 
INNER JOIN Enrols 
    ON Student.StudentID = Enrols.StudentID 
group by Student.FirstName, Student.LastName, Student.StudentID 
having COUNT(CourseID) >= 2; 

Sie StudentID werden Gruppierung sollte, da dies in der Tabelle Student Master-Primärschlüssel ist.

Answer 4

SELECT DISTINCT s.* 
      FROM student s 
      JOIN enrols x 
      ON x.studentid = s.studentid 
      JOIN enrols y 
      ON y.studentid = x.studentid 
      AND y.courseid < x.courseid;