Select max value in subquery - mysql

Question

I have these two tables:
Student:
| name | email |
|---------------------|-------------------------|
| Arturo Vidal | arturo.vidal#usm.cl |
| Bastian Quezada | bastian#usm.cl |
| Javier Jeria | javier#usm.cl |
| Sebastian Piñera | sebastian#presidente.cl |
| Sebastian Gallardo | sebastian#usm.cl |
Class:
| classId | email | signUpDate |
|---------|-------------------------|-------------|
| 1 | sebastian#usm.cl | 2018-01-01 |
| 1 | javier#usm.cl | 2019-10-01 |
| 1 | bastian#usm.cl | 2018-07-01 |
| 2 | sebastian#usm.cl | 2018-05-04 |
| 2 | bastian#usm.cl | 2018-01-01 |
| 3 | bastian#usm.cl | 2018-12-05 |
| 3 | sebastian#usm.cl | 2018-02-01 |
| 4 | arturo.vidal#usm.cl | 2018-03-01 |
| 5 | sebastian#presidente.cl | 2018-03-01 |
I want to show the name the last student that signed up for each classId. That means, I should get a name for classId 1, one for classId 2, etc. My solution for firstly getting the mails (to know the student's name after) is this:
select classId, email, max(signUpDate)
from Class
group by classId
it prints the max date, which is ok, but it also prints the wrong mails for each date:
| ClassId | email | max(signUpDate) |
|---------|-------------------------|-----------------|
| 1 | sebastian#usm.cl | 2019-10-01 |
| 2 | sebastian#usm.cl | 2018-05-04 |
| 3 | bastian#usm.cl | 2018-12-05 |
| 4 | arturo.vidal#usm.cl | 2018-03-01 |
| 5 | sebastian#presidente.cl | 2018-03-01 |
which is completely wrong (). Therefore, when I try to join the the values for getting the names, I get incorrect values.
In other words, I don't understand why are the rows mixing up. Is there any solution for getting correct emails for the max(signUpDate) for each ClassId?
Thanks for your time

Answer 1

i have created the test data fiddle and made an easy and understandable query to fetch the required data, i.e:
SELECT DISTINCT classId,
std.name,
Class.email,
signUpDate
FROM CLASS
INNER JOIN Student std ON std.email = Class.email
WHERE signUpDate IN
(SELECT max(signUpDate)
FROM CLASS
GROUP BY classId)
Sql Fiddle here

Answer 2

This is an instance of a very common class of questions: find the whole row FOR EACH GROUP of the field that maximizes some value (in the group). In your case, you want to GROUP BY the ClassId, and FOR EACH ONE OF THESE GROUPS, you want the whole row of the field with the maximum signupDate.
SHORT ANSWER: You can use this query:
SELECT
C.ClassId,
S.name
FROM
(
SELECT A.*
FROM Class AS A
LEFT JOIN Class AS B
ON A.email = B.email AND A.signupDate < B.signupDate
WHERE B.email IS NULL
) AS C
LEFT JOIN Student AS S ON S.email=C.email
LONG ANSWER:
Here you can find a very clear explanation of what I have just said.
Assuming that we can use the e-mail at your tables as unique identifier, you can do FIRST a join (on the e-mail field) of the table "Class" with itself, to select the "maximum date" for each class id. After that, you join (on the e-mail field) with the table "Student". After that, you will have a table with all the fields of the "Class" table and all the fields of the "Student" table. You can select the fields that you need. In the following example, I will select "Class.classId" and "Student.name"
If you run this query:
SELECT A.*
FROM Class AS A
LEFT JOIN Class AS B
ON A.email = B.email AND A.signupDate < B.signupDate
WHERE B.email IS NULL
You obtain this table:
+---------+-------------------------+------------+
| ClassId | email | signupDate |
+---------+-------------------------+------------+
| 1 | javier#usm.cl | 2019-10-01 |
| 2 | sebastian#usm.cl | 2018-05-04 |
| 3 | bastian#usm.cl | 2018-12-05 |
| 4 | arturo.vidal#usm.cl | 2018-03-01 |
| 5 | sebastian#presidente.cl | 2018-03-01 |
+---------+-------------------------+------------+
Now you can join this with the table "Student", and select the fields that you want. If you run the query provided in the "short answer" part of this post, you get the following result:
+---------+--------------------+
| ClassId | name |
+---------+--------------------+
| 4 | Arturo Vidal |
| 3 | Bastian Quezada |
| 1 | Javier Jeria |
| 5 | Sebastian Piñera |
| 2 | Sebastian Gallardo |
+---------+--------------------+

Answer 3

Try this:
SELECT A.classId, C.name, C.email, B.signUpDate
FROM
(SELECT classId, max(signUpDate) maxSignUpDate
FROM Class
GROUP BY classId) A JOIN Class B
ON A.classId=B.classId AND A.maxSignUpDate=B.signUpDate
JOIN Student C ON C.email=B.email;
I assume the email to be the ID field of the Student table. See MySQL Join Made Easy and MySQL GROUP BY for insights.
See it run on SQL Fiddle.