mysql计数查询

Question

在此查询中，我需要删除重复导师ID并对其进行计数..

SELECT COUNT(t.tutor_id) FROM tutors AS t
    INNER JOIN tutor_category_subject as tcs ON t.tutor_id = tcs.tutor_id   
    INNER JOIN subject AS s ON tcs.subject_id = s.subject_id
WHERE s.subjects LIKE '%business%' 

这是查询输出：

+-------------------+
| COUNT(t.tutor_id) |
+-------------------+
|                 6 |
+-------------------+

但应该是4 ..为什么导师ID是3,6,15,15,16,16

Answer 1

使用distinct

SELECT COUNT(DISTINCT t.tutor_id) FROM tutors AS t
    INNER JOIN tutor_category_subject as tcs ON t.tutor_id = tcs.tutor_id   
    INNER JOIN subject AS s ON tcs.subject_id = s.subject_id
WHERE s.subjects LIKE '%business%' 

Answer 2

例如，将名为“total”的别名放入带有计数的列，然后在查询结尾处添加GROUP BY总计。