Question

I have these two tables

people
============
id, name

and

answer_sheets
============
id, person_id, answer, date_answered

• person_id is a foreign key from people.id

Now, what I wanted to do is to sort people in such in order basing it on the latest answer_sheets.date_answered (we can derive that one people row can have many answer_sheets rows)

Say for example we have the tables

people  
============
id  name  
1   Person1  
2   Person2  
3   Person3  
4   Person4  
5   Person5  


answer_sheets  
=============
id  person_id   answer  date_answered  
1   1           string  JUN 13  
2   2           string  JUN 15  
3   3           string  JUN 17  
4   2           string  JUN 18  
5   1           string  JUN 19  
6   3           string  JUN 20  
7   2           string  JUN 25  

and I wanted to order people in ASC order based on a people row’s answer_sheets.date_answered

the output must be

=============
id  name      last_date_answered
4   Person4   NIL
5   Person5   NIL
1   Person1   JUN 19
3   Person3   JUN 20
2   Person2   JUN 25

You can observe that people with ids 4 and 5 does not have an answer_sheet and yet they should be included in the list.
Question: What must be the appropriate SQL query for this? Thanks.

Answer 1

To get records to display even if there is no match, you can use a LEFT JOIN.

SELECT p.id, p.name, MAX(a.date_answered)
FROM people p
LEFT JOIN answer_sheets a on p.id = a.personID
GROUP BY p.id, p.name
ORDER BY MAX(date_answered) ASC

And, if you want to try it out, or play with it more, I made a SQLFiddle…