mysql - 如何忽略表 1 中的记录(其 fk 引用必须存在于表 2 中)但条件不满足

Question

我的查询对你们大多数人来说可能听起来很简单，但它确实让我着迷了很长一段时间!

table **user**
+----+-------+
| id | name  |
+----+-------+
| 1  | Sanam |
+----+-------+
| 2  | Raj   |
+----+-------+
| 3  | bipul |
+----+-------+

table **msg**
+----+-----------+-----------+
| id | msg       | createdBY |
+----+-----------+-----------+
| 1  | Hi Raj    | 1         |
+----+-----------+-----------+
| 2  | Hi bipul  | 1         |
+----+-----------+-----------+
| 3  | Hi public | 1         |
+----+-----------+-----------+

table **msgToSpecificPeople**
+----+-------+-----------------+
| id | msgId | receiverId      |
+----+-------+-----------------+
| 1  | 1     | 2    --raj id   |
+----+-------+-----------------+
| 1  | 2     | 3    --bipul id |
+----+-------+-----------------+

现在，我想接收输出，就好像 raj 是接收者一样，然后他获得公共(public)消息和 ​​1 他收到的消息，即“hi raj”。我想忽略“hi bipul msg”。

左外连接给了我所有的记录。

SELECT m.msgContent AS MSG 
FROM msg AS m
left outer JOIN msgToSpecificPeople AS p ON m.id = p.msgId and p.receiverId = 2

有人可以帮我解决这个问题吗？

编辑:还让我知道如何在 linq 中实现此目的

Answer 1

我想你想要:

select m.*
from msg m
where exists (select 1
              from msgToSpecificPeople tsp
              where m.id = p.msgId and tsp.receiverId = 2
             ) or
      not exists (select 1
                  from msgToSpecificPeople tsp
                  where m.id = p.msgId 
                 );