mysql - 查询还返回值为 0 的列

Question

我有两个表:

tb_question
**id_quest**    | **desc_quest**
    1           | How do you do...?
    2           | How are you...?

tb_answer
**id_quest** | **date_answer**
    1        |    2013/11/25
    1        |    2013/11/26
    1        |    2013/11/27

我的查询:

SELECT 
q.id_quest,
q.desc_quest,
COUNT(a.id_quest) as total_answer -- count total answer by question

FROM tb_question q
INNER JOIN tb_answer a
ON q.id_quest = a.id_quest;

结果:

ID_QUEST  | DESC_QUEST              | TOTAL_ANSWER
   1      | How do you do...?       |    3

如何才能同时返回问题 id = 2 且计数为 0 的问题？

预期结果:

ID_QUEST  | DESC_QUEST              | TOTAL_ANSWER
   1      | How do you do...?       |    3
   2      | How are you...?         |    0

http://sqlfiddle.com/#!2/3bfe7/1

Answer 1

两个问题:

1. 您必须使用外连接来获取没有答案的问题。

2. 您遗漏了 GROUP BY 子句。

SELECT 
    q.id_quest,
    q.desc_quest,
    COUNT(a.id_quest) as total_answer
FROM tb_question q
LEFT OUTER JOIN tb_answer a ON q.id_quest = a.id_quest
GROUP BY q.id_quest

DEMO