在MySQL中使用自我连接进行多次连接，并按行值拆分列中的行

Question

I have three tables "Users" , "Subjects" and "Marks" like

Users Table

id  name
1   A
2   B
3   C
4   D
5   E
6   A
7   B

Subjects Table

id  name
1   Chemistry
2   Physics
3   English
4   Maths
5   History

Marks Table

u_id is the foreign key of Users (id) and s_id is foreign key of Subjects(id)

id  u_id    s_id    marks
 1     1       1     60
 2     1       2     70
 3     1       3     80
 4     2       2     80
 5     2       3     44
 6     3       1     50
 7     5       4     50
 8     4       5     50
 9     5       4    100
10     2       5    100

and I wish for the result to be like

id  Name    Chemistry Physics English 
1   A       60        70      80
2   B       NULL      80      44
3   3       50        NULL    NULL

Using Join

So far I have only been able to get

name    name        marks
A       English     80
A       Physics     70
A       Chemistry   60
B       English     44
B       Physics     80
C       Chemistry   50

Using the following query

SELECT u.name, s.name , m.marks
FROM Users as u 
RIGHT JOIN Marks as m ON m.u_id = u.id 
LEFT JOIN Subjects as s ON m.s_id = s.id  
WHERE s.name = "English" 
  OR s.name = "Physics" 
  OR s.name = "Chemistry"
ORDER BY u.name; "

Answer 1

Well, after reading the answers, I wanted to post my own one:

SELECT 
  u.id
, u.name

, MAX(IF(s.id = 1, COALESCE(m.mark), 0)) as 'Chem'
, MAX(IF(s.id = 2, COALESCE(m.mark), 0)) as 'Phys'
, MAX(IF(s.id = 3, COALESCE(m.mark), 0)) as 'Eng'

FROM marks m
INNER JOIN subjects s
ON s.id = m.subjects_id
INNER JOIN users u 
ON u.id = m.users_id
GROUP BY u.id

You can check that makes all you want in SqlFiddle: http://sqlfiddle.com/#!9/f567b/1

The important part is the grouping of all the elements according to the user id, and the way of writing the results from rows in a table to columns in another table. As written in @TheShalit answer, the way of achieving that is just assigning the value as a column. Problem is that when grouping by user, you'll have a lot of values there from where you have to select the important one (the one that is not 0 neither NULL , XD). COALESCE function makes sure that you always return a integer, just in case a NULL is given.

It's also important to notice that you'll have to build the SQL with the names of the subjects and the ids from database, as SQL can't retrieve the name of the elements to write them directly as names of the columns. That's why I wrote 'Chem', 'Phys' and 'Eng' instead of the right names. In fact, would be easier if you just wrote the id of the subject instead of a name, just to retrieve the elements later when you'll fetch the rows.

Take into account that is VERY IMPORTANT that you'll table will have the right indexes there. Make sure you have an UNIQUE id on the table marks with users and subjects to avoid having more than one value there stored

Answer 2

Use select like this(with joins and group by student):

MAX(If(subjects.name="Chemistry",marks.marks,'')) Chemistry,
MAX(If(subjects.name="Physics",marks.marks,'')) Physics,
.....

Answer 3

You will need to do something like:

SELECT u.NAME AS NAME,
       m_e.marks AS english,          
       m_p.marks AS physics,
       m_c.marks AS chemistry
FROM users AS u 
JOIN marks AS m_e ON m_e.u_id = u.id 
JOIN marks AS m_p ON m_p.u_id = u.id 
JOIN marks AS m_c ON m_c.u_id = u.id 
WHERE m_e.s_id = 3 AND m_c.s_id = 1 AND m_p.s_id = 2

You are getting 3 different values from a single table but different rows so you need to join the marks table with itself to be able to get the values from 3 different records into 1 result row

I used the values that you defined as primary id's for your 3 subjects in your question in the where clause to make sure you are getting the correct result for each subject