Return value for MySQL NULL result - mysql

Question

I am working in MySQL. I have the following query:
SELECT tar.ID, COUNT(tsr.StudentID) AS Students
FROM Teacher_ApplicationRecord tar
LEFT JOIN Teacher_StudentRecord tsr ON sar.ID = tsr.TeacherID AND tsr.Session = 1 AND tsr.Year = 2017
WHERE tar.ApplicationYear = 2017
AND tar.Session1 = 1
This query returns no results. However if I take the COUNT(tsr.StudentID) out of the SELECT statement, it returns all the teacher ID's with NULL for the tsr table.
What I want is a query that returns all the teacher ID's and a count of the students assigned to that teacher, with 0 if the result is NULL.
I have tried COALESCE(COUNT(tsr.StudentID), 0) AND IFNULL(COUNT(tsr.StudentID), 0) with no success so far. Any other thoughts?
UPDATE:
The tsr table has 4 columns: TeacherID, StudentID, Year, Session. It has no records yet. It will be populated next year when students are assigned to teachers.
The tar table has a list of TeacherID's in it with some other data, such as year and faculty.
I want my results to look like below:
+-----------+-----------------+
| TeacherID | COUNT(StudentID)|
+-----------+-----------------+
| 1 | 0 |
+-----------+-----------------+
| 2 | 0 |
+-----------+-----------------+
etc.
As students are assigned to teachers, the COUNT(StudentID) numbers will go up. Hope this helps.
UPDATE 2
The tar table looks like this:
+---------+---------------+
|TeacherID|ApplicationYear|
+---------+---------------+
| 1 | 2017 |
+---------+---------------+
| 2 | 2017 |
+---------+---------------+
| 3 | 2017 |
+---------+---------------+
It has other columns but they are not relevant to the question.
The tsr table looks like this:
+---------+---------+----+-------+
|TeacherID|StudentID|Year|Session|
+---------+---------+----+-------+
| 1 | 10 |2017| 1 |
+---------+---------+----+-------+
| 1 | 11 |2017| 1 |
+---------+---------+----+-------+
| 2 | 12 |2017| 1 |

Answer 1

You can try joining the teacher table to the student table (in that order), and then using GROUP BY to count the number of students per teacher:
SELECT tsr.ID,
COUNT(sar.ID) AS numOfStudents -- count no matching students as zero
FROM Teacher_StudentRecord tsr
LEFT JOIN Student_ApplicationRecord sar
ON tsr.TeacherID = sar.ID AND
tsr.Session = 1 AND
tsr.Year = 2017
GROUP BY tsr.ID
The usefulness of a LEFT JOIN here is the edge case where a teacher has no matching students. In this case, the result set, before aggregation happens, would have a single record for that teacher, and the student ID value would be NULL, which be ignored by COUNT, resulting in a correct zero count.
Note that I removed the WHERE clause, whose intended logic is already contained in the ON clause. This WHERE clause was throwing off your results by removing teacher records before they could even be aggregated.
Update:
Please try the following query to see if you get results:
SELECT tsr.ID,
COUNT(sar.ID) AS numOfStudents
FROM Teacher_StudentRecord tsr
LEFT JOIN Student_ApplicationRecord sar
ON tsr.TeacherID = sar.ID
GROUP BY tsr.ID
If this gives you no results, then no teachers are actually connected to any students, and your data has a problem.
Here is a demo using your sample data. It works as expected:
SQLFiddle