Mysql多重连接并计数

Question

我试图创建一个连接两个表并返回一个计数的SQL语句。即使没有找到计数项目，我也需要返回所有记录。

如果我只是这样做：

SELECT s.subject,s2.subject from subjects s 
left join subjects s2 on s.parent_id = s2.id 

这是罚款，但没有计数。我想它加入到另一个表，我需要做的这样计数：

SELECT s.subject,s2.subject,count(q.id) from subjects s 
left join subjects s2 on s.parent_id = s2.id 
join questions q on q.subject_id = s.id 

，但我只是得到不具有任何意义了一个结果。

Answer 1

试试这个：

SELECT s.subject, s2.subject, COUNT(q.id) 
FROM subjects s 
LEFT JOIN subjects s2 ON s.parent_id = s2.id 
LEFT JOIN questions q ON q.subject_id = s.id 
GROUP BY s.subject_id, s2.subject_id, q.subject_id 

Answer 2

如果你可以有零的问题，你需要的COUNT添加到一个子查询，那么你可以LEFT JOIN的结果是这样的：

SELECT s.subject, s2.subject, NVL(q.q_count,0) as question_count 
FROM subjects s 
LEFT JOIN subjects s2 ON s.parent_id = s2.id 
LEFT JOIN (SELECT q.subject_id, count(1) as q_count 
    FROM questions q 
    GROUP BY q.subject_id) q ON q.subject_id = s.id