How to count rows of a column by depending on same values of another column?

Question

I have five columns in a table called tbl_enrolments(id, student_id, semester_id, unit_id, date) . Here I just need to count the number of same unit_id according to same semester_id from this table. This is how I've tried so far..

SELECT e.unit_id, COUNT(e.unit_id) as count, 
       u.unit_title, 
       u.unit_name, 
       s.sem_name 
FROM tbl_enrolments e
INNER JOIN tbl_unit_of_study u
    ON e.unit_id = u.id
INNER JOIN tbl_semesters s
    ON e.semester_id = s.id
GROUP BY e.unit_id

Here this query is counting all same unit_id from all different semester_id . But I need to count all same unit_id with only same semester_id . How can I do that?

My table is like this:

Answer 1

Check if this is what you are looking for and if it helps you-

mysql> select semester_id, count(unit_id) from tbl_enrolments group by semester_id;
+-------------+----------------+
| semester_id | count(unit_id) |
+-------------+----------------+
|           2 |              5 |
|           1 |              8 |
|          12 |              2 |
+-------------+----------------+
3 rows in set (0.00 sec)

mysql> select semester_id, unit_id, count(unit_id) from tbl_enrolments group by semester_id, unit_id;
+-------------+---------+----------------+
| semester_id | unit_id | count(unit_id) |
+-------------+---------+----------------+
|           2 |       3 |              3 |
|           2 |       1 |              2 |
|           1 |       2 |              2 |
|           1 |       6 |              2 |
|           1 |       4 |              4 |
|          12 |       1 |              1 |
|          12 |       6 |              1 |
+-------------+---------+----------------+
7 rows in set (0.00 sec)

mysql> select semester_id, unit_id, count(unit_id) from tbl_enrolments group by semester_id, unit_id having semester_id = 2;
+-------------+---------+----------------+
| semester_id | unit_id | count(unit_id) |
+-------------+---------+----------------+
|           2 |       3 |              3 |
|           2 |       1 |              2 |
+-------------+---------+----------------+
2 rows in set (0.00 sec)

mysql> select semester_id, unit_id, count(unit_id) from tbl_enrolments group by semester_id, unit_id having semester_id = 2 and unit_id = 3;
+-------------+---------+----------------+
| semester_id | unit_id | count(unit_id) |
+-------------+---------+----------------+
|           2 |       3 |              3 |
+-------------+---------+----------------+
1 row in set (0.00 sec)