MySQL的計數查詢

Question

在此查詢我需要刪除重複導師ID和計數他們..

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%' 

這是查詢輸出：

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

，但它應該是4 ..爲什麼家教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，將名爲'total'的別名放入帶有計數的列中，然後在查詢結尾處添加GROUP BY。