MySQL的计数查询

Question

在此查询我需要删除重复导师ID和计数他们..

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 | 
+-------------------+ 

，但它应该是4 ..为什么家教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中，然后在查询结尾处添加GROUP BY。