从表中选择一个字段具有相同值的行

Question

I have a MySQL database with these two tables:

Tutor(tutorId, initials, lastName, email, phone, office)
Student(studentId, initials, lastName, email, tutorId)

What is the query to return the initials and last names of any student who share the same tutor?

I tried SELECT intials, lastName FROM Student WHERE tutorId = tutorId but that just returns the names of all students.

Answer 1

You'll have to join students against itself:

SELECT s1.initials, s1.lastName
FROM Student s1, Student s2
WHERE s1.studentId <> s2.studentID /* Every student has the same tutor as himself */
AND s1.tutorId = s2.tutorid

If you want to output the pairs:

SELECT s1.initials, s1.lastName, s2.initials, s2.lastName
FROM Student s1, Student s2
WHERE s1.studentId <> s2.studentID /* Every student has the same tutor as himself */
AND s1.tutorId = s2.tutorid

To get a list of Tutor - Students:

SELECT tutorId, GROUP_CONCAT( initials, lastName SEPARATOR ', ') 
FROM `Student` 
GROUP BY tutorId
/* to only show tutors that have more than 1 student: */
/* HAVING COUNT(studentid) > 1 */

Answer 2

SELECT Tutor.tutorId, Student.initials, Student.lastName FROM Student INNER JOIN Tutor ON Tutor.tutorId = Student.tutorId GROUP BY tutorId

This will return (not tested, but it should) a list of student initials and last names grouped by tutorId. Is that what you want?

Answer 3

Join Student table to itself

SELECT S1.intials, S1.lastName
FROM Student S1, Student S2 
WHERE S1.tutorId = S2.tutorId 
AND S1.studentId <> S2.studentId

Answer 4

这是SQL Server中的查询，我确定这个想法与mySql非常接近：

 select s1.initials,s1.lastname,s2.initials,s2.lastname from students s1 inner join students s2 on s1.tutorid= s2.tutorid and s1.studentid <> s2.studentid

Answer 5

You will have to make a query for every single tutorId. Pseudo-Code:

for id in tutorIds
    query('SELECT intials, lastName FROM Student WHERE tutorId = '+id )

If you wanna have a list containing all Tutors who actually have students, do a

SELECT tutorId FROM Student GROUP BY tutorId