與MySQL

Question

水平JOIN我想查詢2個表，一個學生和包含與學生的意見另一個表。

學生表：

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

評語表：

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

我所需的輸出會是這樣的：

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

任何幫助，將不勝感激。這樣做將是如下的

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 

你必須增加的連接數爲每個評論的類型。