php mysql select by month between records - php

Question

I've this MySQL table my_table:
+-------+------------+-----------+
|Student| Date | Classroom |
+-------+------------+-----------+
| 1 | 2018-01-01 | 101 |
| 2 | 2018-01-01 | 102 |
| 3 | 2018-01-01 | 103 |
| 1 | 2018-03-01 | 104 |
| 2 | 2018-06-01 | 103 |
| 3 | 2018-09-01 | 104 |
| 1 | 2018-11-01 | 106 |
| 2 | 2018-12-01 | 101 |
+-------+------------+-----------+
The students stay in the assigned classroom till changed.
I'm trying to get which classroom they were in for a certain month.
For example in October(10), student 1 was in 104, 2 was in 103, and 3 was in 104.
I'm really unsure on how to proceed with this one so any help is appreciated.
Currently using this query based on Strawberry answer
SELECT x.*
FROM my_table x
LEFT OUTER JOIN my_table y
ON y.student = x.student
AND y.date < x.date
WHERE x.date <= LAST_DAY('2018-10-01')
GROUP BY student

Answer 1

DROP TABLE IF EXISTS my_table;
CREATE TABLE my_table
(Student INT NOT NULL, Date DATE NOT NULL, Classroom INT NOT NULL,PRIMARY KEY(student,classroom));
INSERT INTO my_table VALUES
(1,'2018-01-01',101),
(2,'2018-01-01',102),
(3,'2018-01-01',103),
(1,'2018-03-01',104),
(2,'2018-06-01',103),
(3,'2018-09-01',104),
(1,'2018-11-01',106),
(2,'2018-12-01',101);
SELECT x.*
FROM my_table x
JOIN
( SELECT student
, MAX(date) date
FROM my_table
WHERE date <= LAST_DAY('2018-10-01')
GROUP
BY student
) y
ON y.student = x.student
AND y.date = x.date;
+---------+------------+-----------+
| Student | Date | Classroom |
+---------+------------+-----------+
| 1 | 2018-03-01 | 104 |
| 2 | 2018-06-01 | 103 |
| 3 | 2018-09-01 | 104 |
+---------+------------+-----------+

Answer 2

Here's a go at it (snippet to go in a stored procedure; assumes table called example & output to table months). It produces a row per student for each month of the range.
drop table months;
create table months (month date, student integer, classroom integer);
set #month = (select min(date) from example);
start_loop: LOOP
insert into months select #month, s1.student, classroom from
(select student, max(date) as maxdate from example where date <= #month group by student) s1
join example s2 on s1.student = s2.student and maxdate = date;
if #month = (select max(date) from example) then
leave start_loop;
end if;
set #month = #month + interval 1 month;
END LOOP start_loop;

Answer 3

Let's break the problem into two parts. Firstly, find all the rooms which have been allocated to student A so far and sort them using the date. Next, find the record which is just before or equal to the required month.
For example:
Consider student 1. We get
+-------+------------+-----------+
|Student| Date | Classroom |
+-------+------------+-----------+
| 1 | 2018-01-01 | 101 |
| 1 | 2018-03-01 | 104 |
| 1 | 2018-11-01 | 106 |
+-------+------------+-----------+
Now, let's say for month June we try to find month just less than or equal to 2018-06-01 to get the required room number. I hope this will help.