与MySQL

Question

水平JOIN我想查询2个表，一个学生和包含与学生的意见另一个表。

学生表：

| ID | Name | 
------------- 
| 1 | John | 
| 2 | Alex | 

评语表：

| ID | Student ID | Comment | 
----------------------------- 
| 1 | 1   | phoned | 
| 2 | 1   | emailed | 
| 3 | 2   | phoned | 
| 4 | 2   | emailed | 
| 5 | 2   | mailed | 

我所需的输出会是这样的：

| Student | Comment 3 | Comment 2 | Comment 1 | 
----------------------------------------------- 
| John | null  | phoned | emailed | 
| Alex | mailed | emailed | called | 

任何帮助，将不胜感激。这样做将是如下的

Answer 1

方式一：

SELECT students.name, c1.comment, c2.comment, c3.comment FROM students 
LEFT JOIN (SELECT * FROM comments WHERE comment = 'phoned') as c1 
ON c1.student_id = students.id 
LEFT JOIN (SELECT * FROM comments WHERE comment = 'emailed') as c2 
ON c2.student_id = students.id 
LEFT JOIN (SELECT * FROM comments WHERE comment = 'mailed') as c3 
ON c3.student_id = students.id 

你必须增加的连接数为每个评论的类型。