MySQL存储过程循环遍历两个表，选择值，并存储在第三个表中

Question

我有三个桌子-- users，memos和memos_read。

users

+---+----------+
|ID | NAME     |
+---+----------+
| 1 | Mike     |
| 4 | Susan    |
| 5 | Peter    |
| 6 | Mary     |
+---+----------+

memos

+---+----------+
|ID | CONTENT  |
+---+----------+
| 15| Hello    |
| 16| World    |
| 17| Foo      |
| 23| Bar      |
+---+----------+

我想要创建一个带有嵌套循环的存储过程，为已读取的备忘录创建一个"read map“，创建如下内容：

memos_read

+---------+----------+
| MEMO_ID | USER_ID  |
+---------+----------+
|    15   |    1     |
|    15   |    4     |
|    15   |    5     |
|    15   |    6     |
|    16   |    1     |
|    16   |    4     |
|    16   |    5     |
|    16   |    6     |
|    17   |    1     |
|    17   |    4     |
|    17   |    5     |
|    17   |    6     |
|    23   |    1     |
|    23   |    4     |
|    23   |    5     |
|    23   |    6     |
+---------+----------+

这个是可能的吗？谢谢你的进阶！

Answer 1

@Nick的回答应该适用于您；但是，如果您对创建一个新表不感兴趣，而是在查询中动态使用"read“；您可以使用一个导出表。

一般查询结构如下：

SELECT
  read_map.MEMO_ID,
  read_map.USER_ID,
  ...
FROM 
(
  SELECT 
    m.ID AS MEMO_ID, 
    u.ID AS USER_ID
  FROM users AS u 
  CROSS JOIN memos AS m 
) AS read_map 

...  -- here you can join with other tables as required
WHERE ...

Answer 2

可以使用CREATE TABLE ... SELECT语法创建memos_read表：

CREATE TABLE memos_read AS
SELECT m.ID AS MEMO_ID, u.ID AS USER_ID
FROM memos m 
CROSS JOIN users u

输出是您所要求的。dbfiddle演示