MySQL数据透视表计数

Question

I've three tables:

users
------
user_id
name

subjects
--------
subject_id
name

pivot_user_subject
------------------
user_id
subject_id

I've this query to show subjects and usernames:

SELECT a.name User, c.name Subject 
FROM users a, pivot_user_subject b, subjects c 
WHERE a.user_id = b.user_id AND c.subject_id = b.subject_id;

and works great. but now I need to show how many subjects an user has.

Example output:

User      Subjects                      Subjects_count
Juan      Math, Biology, Spanish               3
Rosa      NULL                                 0
Rodrigo   Math                                 1

Any idea about how can I accomplish that output only with SQL queries? Thanks.

Answer 1

Try this

SELECT u.name, GROUP_CONCAT(s.name), COUNT(s.subject_id) as Subject_count
FROM users u
LEFT JOIN  pivot_user_subject us ON u.user_id = us.user_id
LEFT JOIN subjects s ON s.subject_id = us.subject_id
GROUP BY u.user_id, u.name

SQL Fiddle

Answer 2

If c.name is separated by , or it is just one subject or it is just null , you may try following;)

SELECT
    a.name AS User,
    c.name AS Subjects,
    IF(c.name IS NULL, 0, LENGTH(c.name) - LENGTH(REPLACE(c.name, ',', '')) + 1) AS Subjects_count
FROM users a, pivot_user_subject b, subjects c
WHERE a.user_id = b.user_id AND c.subject_id = b.subject_id;