Question

我有以下问题。有两个表 - groups和students以及第三个数据透视表group_student。

查询来自特定群体（身份：1,8）的学生是明确的......

   SELECT DISTINCT s.*
   FROM students AS s
   Inner Join group_student AS gs ON gs.student_id = s.id
   Inner Join groups AS g ON g.id = gs.group_id
   WHERE g.id IN ("1","8")

有效。但是，如果我想只选择组ID为1的学生段，如何查询。例如s.name = "john"。所以结果应该是：all students from group id 8 + all students with name "john" from group id 1。

Thanx帖子： - ）

Answer 1

试试这个：

SELECT DISTINCT s.*
   FROM students AS s
   Inner Join group_student AS gs ON gs.student_id = s.id
   Inner Join groups AS g ON g.id = gs.group_id
   WHERE g.id ="8" or (g.id="1" and s.name = "john")

Answer 2

你也可以使用UNION

SELECT DISTINCT s.*
   FROM students AS s
   Inner Join group_student AS gs ON gs.student_id = s.id
   Inner Join groups AS g ON g.id = gs.group_id
   WHERE g.id = 8
UNION
SELECT DISTINCT s.*
   FROM students AS s
   Inner Join group_student AS gs ON gs.student_id = s.id
   Inner Join groups AS g ON g.id = gs.group_id
   WHERE gp.id="1" and s.name = "john"

Answer 3

还有一个选项，即避免DISTINCT和（可能不需要）加入groups：

SELECT s.*
FROM students AS s
WHERE EXISTS
        ( SELECT *
          FROM group_student AS gs 
          WHERE gs.student_id = s.id
            AND (  gs.group_id = 8
                OR (gs.group_id, s.name) = (1, 'john')
                )
        )

使用过滤器进行多对多查询

3 个答案: