我有三个mysql表,类别,学生和学生类别。对于每个学生,将有一个或多个类别,并将其存储在student_category中,如下所示。
1) Categgory
----------------------------
id | category_name
---------------------------
1 | A
2 | B
3 | C
4 | D
2) Students
--------------------------
id | name
--------------------------
1 | John
2 | Kumar
3 | Ashok
4 | Jorge
5 | Suku
-------------------------
2) student_category
-----------------------------------------
id | student_id | category_id
-----------------------------------------
1 | 1 | 2
2 | 1 | 4
3 | 2 | 3
4 | 2 | 1
5 | 3 | 2
------------------------------------------
我需要选择包含category_id 2和4的学生
我使用了如下查询,但它返回的学生要么包含类别2,要么包含类别4。
select A.name from students A, student_category B where A.id=B.student_id
and B.category_id IN (2,4)
试试这个查询:
SELECT t1.id,
t3.name
FROM students t1
INNER JOIN student_category t2
ON t1.id = t2.student_id
INNER JOIN students t3
ON t1.id = t3.id
WHERE t2.category_id IN (2, 4)
GROUP BY t1.id
HAVING COUNT(DISTINCT t2.category_id) = 2
解释:
这个查询将students
和student_category
表连接在一起,然后删除所有不是类别2或4的记录。这意味着每个学生只有第2类和第4类记录与他相关联。然后HAVING
子句进一步限制,要求学生有两个不同的类别,如果为真,则必须意味着该学生同时具有类别2和类别4。
演示:
SQLFiddle
try this:
select name from Students where id in (select student_id from student_category where category_id in (2,4))
试试这个:
select
s.name
from
Students s,
Categgory c,
student_category sc
where
sc.student_id = s.id
and sc.category_id = c.id
and c.id = 2
and c.id = 4
你可以在SQL Fiddle上查看。必须取distinct
学生名,因为如果一个学生属于多个类别,它会重复。