Mysql Join返回重复值

Question

我有两个表时间表和主题，我需要来自主题表的id，subject_name，而不是在特定部分的时间表中使用。

时间表 主题表

但它会返回多个值，如下图所示。

我正在使用此查询

SELECT subject.id, subject.subject_name 
FROM timetable 
LEFT JOIN subject 
ON timetable.subject_id != subject.id AND timetable.class_id = subject.class_id 
WHERE timetable.class_id = 1 and timetable.section_id =1

Answer 1

我仍然认为这是一个重复的问题。虽然你的问题有点复杂。所以试试这个：

SELECT subject.id, subject.subject_name 
FROM subject
LEFT JOIN timetable 
  ON  timetable.subject_id = subject.id
  AND timetable.class_id   = subject.class_id 
  AND timetable.section_id = 1
WHERE subject.class_id = 1
  AND timetable.id IS NULL

Answer 2

这个简单的查询有效。

SELECT id, subject_name FROM subject WHERE class_id = 1 
AND id NOT IN (SELECT subject_id from timetable where section_id=1)