如何使用联接从三个表中选择数据

Question

我想从以下三个表中选择survey_id ， question_id ， question_text和answer_id 。

在SurveyTable中，我有：

Survey{survey_id,survey_title}

在QuestionTable中，我有：

Question{survey_id,question_id,question_text}

在AnswerTable中：

Answer{question_id,answer_id,answer_text}

我想使用联接从这些表中选择。 当survey_id等于QuestionTable和SurveyTable中的值时。

Answer 1

好吧，您可以从类似

SELECT  s.survey_id ,  
        q.question_id, 
        q.question_text, 
        a.answer_id,
        a.answer_text
FROM    Survey s INNER JOIN
        Question q  ON  s.survey_id = q.survey_id INNER JOIN
        Answer a    ON  q.question_id = a.question_id

INNER JOIN s将确保您仅在有问题和答案的地方进行调查。

如果您希望返回所有调查，无论它们是否有问题或答案，或者甚至所有有问题的调查而无论答案如何，都可以使用LEFT JOINS

SELECT  s.survey_id ,  
        q.question_id, 
        q.question_text, 
        a.answer_id,
        a.answer_text
FROM    Survey s LEFT JOIN
        Question q  ON  s.survey_id = q.survey_id LEFT JOIN
        Answer a    ON  q.question_id = a.question_id

您必须尝试并记住，LEFT JOUN指出

返回左侧表中的所有数据，仅返回右侧中与左侧匹配的数据。

看看这篇文章，它做了一个很好的图形解释。

SQL SERVER – JOIN的介绍– JOIN的基础

Answer 2

select  
    survey.survey_id , 
    question.question_id,
    question.question_text,
    answer.answer_id
from survey 
left join question on question.survey_id = survey.survey_id 
left join answer on answer.question_id = question.question_id

Answer 3

我正在考虑一个问题可以有多个答案，所以我给出了答案，这样您就可以得到答案表的所有行

 $select = "SELECT a.answer_id,a.answer_text,q.question_id, q.question_text,s.survey_id,s.survey_title FROM Answer a "
        . "LEFT JOIN Question q ON (a.question_id = q.question_id) "
        . "LEFT JOIN Survey s ON (q.survey_id = s.survey_id)";