如何从多个表中选择数据？答案

【问题标题】：how to select data from multiple tables?如何从多个表中选择数据？
【发布时间】：2011-09-15 08:13:55
【问题描述】：

我有 4 个表，我很困惑选择数据。

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）。怎么样？

我试试这样

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

【问题讨论】：

SELECT t1.given_name, t4.subject_name, t2.class, t2.main_id FROM t1, t2, t4
在 t3 中主键是 main_id（或在 t2 中）？
school_year = 2011 和 sem = 1（在 where 条件下）
@apssiva：我在查询中添加了WHERE 子句，但是如果您需要在问题中添加一些内容，则必须单击问题下方的edit 按钮以添加更多详细信息.

标签： mysql select multiple-tables

【解决方案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

【讨论】：

【解决方案2】：

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

应该这样做，但请告诉我们您需要数据的形式。你想得到每个学生的课程吗？还是每个班级的学生？

【讨论】：

【解决方案3】：

您需要加入表格。每个表都应该有一个 PRIMARY（或 UNIQUE）键，以及对其他表的引用（所谓的 FOREIGN KEY）。

SELECT given_name , t4.subject_name, t2.class, t3.main_id
FROM t1 
JOIN t2 
ON t1.student_id = t2.student_id
JOIN t3
ON t2.main_id = t3.main_id
JOIN t4
ON t2.subject_id = t3.subject_id
WHERE t2.school_year = "2011""  and t2.sem = 1 --added where because of your comment

所以我们假设

学生桌t1有PKstudent_id
科目表t4有PKsubject_id
t2 是链接学生的关联表（使用他们的 student_id) 到具有main_id 的类。
t3 是一个关联表，将科目与班级联系起来。

【讨论】：

【解决方案4】：

SELECT table_name1.given_name,table_name4.subject_name,
table_name2.class,table_name2.main_id
FROM table_name1
INNER JOIN table_name2
ON table_name1.studen_id=table_name2.studen_id
INNER JOIN table_name3
ON table_name2.main_id=table_name3.main_id
INNER JOIN table_name4
ON table_name3.subject_id=table_name4.subject_id

【讨论】：

【解决方案5】：

试试：

   SELECT t1.given_name, t4.subject_name, t2.class, 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
    WHERE t2.school_year = 2011 
      AND t2.sem = 1

【讨论】：

【解决方案6】：

select t1.given_name, t4.subject_name, t2.class, t2.main_id
  from t1 
  join t2 on t1.student_id = t2.student_id
  join t2.main_id = t3.main_id
  join t3.subject_id = t4.subject_id

【讨论】：