需要從MySQL返回各部門的名稱和數量

Question

表：

departments: id, title 

comments: id, year_code 

department_comments: comment_id, department_id 

我想列出每個部門的標題，用的有多少意見具有計數。

東西沿着

select 
d.title, count(c.id) as total_comments 
from 
departments d, comments c 
left join 
department_comments dc on c.id = dc.comment_id 
where 
c.year_code = 1011 

上述查詢的行不行，但應該給你什麼，我試圖完成一個想法。

我要顯示給用戶的是部門標題和它旁邊，像下面的註釋總＃：

d.title (count(c.id)) 
--------------------- 
Maintenance (10) 
Finance (5) 
Security (13) 

Answer 1

你接近。您的JOIN語法有點偏離，並且您缺少GROUP BY。

SELECT d.title, COUNT(dc.comment_id) AS total_comments 
    FROM departments d 
     LEFT JOIN department_commments dc 
      INNER JOIN comments c 
       ON dc.commment_id = c.id 
        AND c.year_code = 1011 
      ON d.id = dc.department_id 
    GROUP BY d.title