Question

如何让所有学生的名字都由老师教授，名字叫John？

学生

id
name

student_has_teacher

teacher_id
student_id

教师

id
name

Answer 1

您必须使用Inner Joins。内部联接通过基于连接谓词组合两个表的列值来创建新的结果表。您的查询应该看起来像这样：

SELECT      student.name
FROM        student
INNER JOIN  student_has_teacher ON (student_has_teacher.student_id = student.id)
INNER JOIN  teacher ON (teacher.id = student_has_teacher.teacher_id)
WHERE       teacher.name = 'John';

Answer 2

SELECT s.name
FROM student s 
    INNER JOIN student_has_teacher st ON s.id = st.student_id
    INNER JOIN teacher t ON st.teacher_id = t.id
WHERE t.name = 'John'

Answer 3

下面的第一个查询假设student_has_teacher中可能存在重复记录，并且即使是这种情况也会避免导致重复结果：

select s.id, s.name
from student s
where s.id in (
    select sht.student_id 
    from student_has_teacher sht
    inner join teacher on sht.teacher_id = t.id
    where t.name = 'John'
)

如果student_has_teacher中没有重复项，那么您可以使用联接，如下所示：

select s.id, s.name
from student s
inner join student_has_teacher sht on s.id = sht.student_id
inner join teacher on sht.teacher_id = t.id
where t.name = 'John'

Answer 4

以下查询肯定会为您提供所需的结果。

SELECT st.name
FROM student AS st
LEFT JOIN student_has_teacher AS stte ON st.id = stte.student_id
LEFT JOIN teacher AS te ON stte.teacher_id = te.id
WHERE te.name = "John"

MySQL - 我怎样才能得到所有学生的名字？

4 个答案: