Mysql多重連接並計數

Question

我試圖創建一個連接兩個表並返回一個計數的SQL語句。即使沒有找到計數項目，我也需要返回所有記錄。

如果我只是這樣做：

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

這是罰款，但沒有計數。我想它加入到另一個表，我需要做的這樣計數：

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義了一個結果。

Answer 1

試試這個：

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

如果你可以有零的問題，你需要的COUNT添加到一個子查詢，那麼你可以LEFT JOIN的結果是這樣的：

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