MySQL的數據透視表計數

Question

我有三個表：

users 
------ 
user_id 
name 

subjects 
-------- 
subject_id 
name 

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

我有此查詢展示主題和用戶名：

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; 

偉大工程。但現在我需要顯示用戶有多少個主題。

輸出示例：

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

我如何能夠做到這僅輸出與SQL查詢的任何想法？謝謝。

Answer 1

試試這個

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

如果c.name由,分開或者它僅僅是一個主題或者它僅僅是null，您可以嘗試以下;）

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;