MySQL的加入了特定的行只

Question

我三個表

意見表

--------------------------------------------------------------------------- 
id | commented_by  | comment   | commented_by 
--------------------------------------------------------------------------- 
1  | asdf156df5s4fd  | student comment | student 
2  | klsdfjklsdfno  | student comment | student 
3  | jafkadfjlkdnvwa | teacher comment | teacher 
4  | adsjflandsfinndfs | student comment | student 
5  | dsf5614dsf8wef4 | teacher comment | teacher 
--------------------------------------------------------------------------- 

老師表

---

id | teacher_id  | teacher_name | teaches 
--------------------------------------------------------------------------- 
1  | jafkadfjlkdnvwa | abc cde  | maths 
2  | dsf5614dsf8wef4 | egc kjl  | science 
. 
. 
. 
--------------------------------------------------------------------------- 

學生表

---

id | student_id  | student_name | 
--------------------------------------------------------------------------- 
1  | asdf156df5s4fd | student_1 | 
2  | klsdfjklsdfno  | student_2 | 
3  | adsjflandsfinndfs | student_3 | 
. 
. 
--------------------------------------------------------------------------- 

現在，我們希望是mysql join可能返回這樣的事情

---

id | comment_id   | comment_by | name | teaches 
--------------------------------------------------------------------------- 
1  | asdf156df5s4fd  | student | student_1 | null 
2  | klsdfjklsdfno  | student | student_2 | null 
3  | jafkadfjlkdnvwa | teacher | abc cde  | maths 
4  | adsjflandsfinndfs | student | student_3 | null 
5  | dsf5614dsf8wef4 | teacher | egc kjl  | science 
--------------------------------------------------------------------------- 

無法找到，我們可以用笨的活動記錄實現這一目標的精確查詢

Answer 1

這是真正的髒表計劃，但這是你想要的查詢：

SELECT 
    c.id, 
    commented_by as comment_id, 
    commented_by, 
    IF(commented_by = 'student', student_name, teacher_name) as name, 
    teaches 
FROM comments c 
LEFT JOIN teacher t 
    ON t.teacher_id = c.commented_by 
AND c.commented_by = 'teacher' 
LEFT JOIN student s 
    ON s.student_id = c.commented_by 
AND c.commented_by = 'student' 

更好的表方案：

CREATE TABLE comments (
    comment_id INT NOT NULL AUTO_INCREMENT, 
    student_id INT, 
    teacher_id INT, 
    PRIMARY KEY (comment_id) 
); 

CREATE TABLE teachers (
    teacher_id INT NOT NULL AUTO_INCREMENT, 
    teacher_name VARCHAR(200) NOT NULL, 
    teacher_subject VARCHAR(50), 
    PRIMARY KEY (teacher_id) 
); 

CREATE TABLE students (
    student_id INT NOT NULL AUTO_INCREMENT, 
    student_name VARCHAR(200) NOT NULL, 
    PRIMARY KEY (student_id) 
); 

SELECT 
    comments.*, 
    IF(student_id IS NOT NULL, student_name, teacher_name) as name, 
    teacher_subject 
FROM comments 
LEFT JOIN teachers USING(teacher_id) 
LEFT JOIN students USING(student_id) 

Answer 2

你需要加入學生和教師表：

select id, comment_id as comment_by , commented_by, 
(case when commented_by = 'teacher' THEN teacher_name else student_name) AS name, teaches 
from comments c left join teacher t ON t.teacher_id = c.commented_by left 
join student s ON s.student_id = c.commented_by