mysql从两个表中连接查询结果

Question

I have Two Table One to store Questions and Other to store Replies to Questions as Below

I have Shown the Table Structure and Column in table as Below

Question Table

Question_Id(PK)   |   Question   |   Name   |  EmailAddress

Answer Table

Answer_Id  |  Question_Id  |   Question  |   Name  |  EmailAddress

What ever question is posted it will be added to Question table and What ever Replies people post will be added to answer table

Now when ever Some one post a Reply to Question I Should Send mail to one who Posted Question and to those who posted Replies to the Question

Please Suggest a mysql Query for the above

Thank you

Answer 1

In theory you should know in the application the id of the question (qid) someone is replying to. Based on this id you can issue the following query:

Select EmailAddress from Question where Question_Id=qid
Union
Select EmailAddress from answer where Question_id=qid

Depending on the logic of your application this might also select the address of the current user. If you want to avoid this you should include in both select statments a condition to exclude the current replier. Something like:

Select EmailAddress from Question where Question_Id=qid and EmailAddress!=curentUserAddress
Union
Select EmailAddress from answer where Question_id=qid and EmailAddress!=curentUserAddress

Answer 2

Select * 
from questions 
left join answers 
on questions.id = answers.question_id 
where question_id = 1

Answer 3

select * from Question q, Answer a 
where q.Question_Id = a.Question_id and
      q.Question_Id = question_id