mysql - 如何从多个表中选择数据？

Question

我有 4 个表，我很难选择数据。

我的 4 个表中有这些字段

t1 -> student_id
  given_name
  user_name

t2 -> school_year
  sem
  class
  student_id
  main_id

t3 -> subject_id
  main_id

t4 -> subject_id
  subject_name

我需要选择 (given_name, subject_name, class & main_id)。怎么办？

我这样试试

SELECT t2.school_year, t2.rp_main_sem, t2.rp_main_class,  t4.name, t4.subject_id, t1.given_name, t1.user_name FROM t1, t2, t3, t4 WHERE t2.school_year = 2011 AND t2.sem = 1 AND t2.class = 'ES3A' AND t3.subject_id = t4.subject_id AND t1.student_id = t2.rp_main_student_id

Answer 1

我猜你需要这样的查询:

SELECT t1.given_name, t4.subject_name, t2.main_id
FROM t1
LEFT JOIN t2 ON t1.student_id = t2.student_id
LEFT JOIN t3 ON t2.main_id = t3.main_id
LEFT JOIN t4 ON t3.subject_id = t4.subject_id