如何根据另一列的相同值来计算一列的行数？

Question

我在名为tbl_enrolments(id, student_id, semester_id, unit_id, date)的表中有五列。在这里，我只需要根据此表中的相同unit_id计算相同semester_id的数量。到目前为止，这就是我尝试过的方法。

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

此处，此查询正在计算来自所有不同unit_id的所有相同semester_id。但是我需要用相同的unit_id来计算所有相同的semester_id。我该怎么办？

我的桌子是这样的：

Answer 1

检查这是否在寻找您的内容，是否对您有帮助-

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)