如何使用连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我：

Survey{survey_id,survey_title} 

在QuestionTable我：

Question{survey_id,question_id,question_text} 

在AnswerTable：

Answer{question_id,answer_id,answer_text} 

我想用连接这些表来选择。当survey_id等于QuestionTable和SurveyTable中的值时。

Answer 1

好了，你可以用的东西开始像

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 

的INNER JOIN旨意确保你只有在有可用的问题和答案的调查。

如果您想返回所有的调查，无论他们是否有问题或答案，或不管答案的问题，甚至所有的调查，你可以使用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 

你必须尝试并且记住LEFT JOUN状态

返回左侧表格中的所有数据，仅返回右侧与左侧相匹配的数据。

看看这篇文章，它做了一个很好的图形解释。

SQL SERVER – Introduction to JOINs – Basic of JOINs

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

我考虑到，所以我给的答案，所以你得到的答案表中的所有行

$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)";