查询还会返回值为0的列

Question

I have two tables:

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

And my query:

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;

Result:

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

How could it do to return also question id = 2 with 0 count ?

Expected result:

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

Two problems:

1. You have to use an outer join to get questions with no answers.

2. You left out the GROUP BY clause.

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

Answer 2

不要使用内部联接来选择所有idquest，使用内部联接来产生表的子集

SELECT tbq.id_quest,tbq.desc_quest,COUNT(tba.id_quest) FROM tb_question tbq LEFT OUTER JOIN tb_answer tba ON tba.id_uest=tbq.id_quest GROUP BY tbq.id_quest ORDER BY tbq.id_quest