Question

我对SQL有疑问。

我有3张表格，消息，付款和报告。

我想从一个查询中的每个表中获取用户的所有事件，所以这是我想要获得的：

SELECT * FROM messages WHERE receiver_id = 10
SELECT * FROM payments WHERE client_id = 10
SELECT * FROM reports WHERE receiver_id = 10

但是在1个查询中。

这可能吗？

Table structures:
- messages
id
sender_id
receiver_id
type
text
created_at
updates_at

- payments
id
user_id
client_id
method
cost
status
text
created_at
updated_at

- reports
id
sender_id
receiver_id
text
created_at
updates_at

Answer 1

它非常直接，提供的receiver_id和client_id保持相同的值：

SELECT * FROM `messages` LEFT JOIN `payments` ON 
`messages`.`receiver_id` = `payments`.`client_id` LEFT JOIN `reports`
ON `messages`.`receiver_id` = `reports`.`receiver_id`

Answer 2

我真的不明白你想要什么，你想在1个查询中获取所有数据，还是需要加入它们？

如果您想要一张表中的所有数据，我可以这样做，我想：

SELECT * FROM messages as M, payments as P, reports as R
WHERE M.receiver_id, P.client_id, R.receiver_id = 10

否则，您需要向我们展示关于这种关系的更多细节

Answer 3

如果3个表之间没有关系，那么您可以使用UNION。它可以这么简单：

SELECT receiver_id as event_id, event_name, event_description, event_timestamp FROM messages WHERE receiver_id = 10
UNION
SELECT client_id as event_id, event_name, event_description, event_timestamp FROM payments WHERE client_id = 10
UNION
SELECT receiver_id as event_id, event_name, event_description, event_timestamp FROM reports WHERE receiver_id = 10

这假设您感兴趣的所有表中都有一组通用的列。但是，如果列具有不同的名称但包含相同的数据（和数据类型），则可以使用AS重命名查询中的列实现UNION

SELECT receiver_id as event_id, col1 as event_name, col2 as event_description, col3 event_timestamp FROM messages WHERE receiver_id = 10
UNION
SELECT client_id as event_id, col_a as event_name, col_b as event_description, col_c event_timestamp FROM payments WHERE client_id = 10
UNION
SELECT receiver_id as event_id, col_x as event_name, col_y as event_description, col_z event_timestamp FROM reports WHERE receiver_id = 10

请注意，UNION将消除数据集中的重复行。如果这不是您想要的行为，则使用UNION ALL代替（它不会消除重复）。

另请注意，如果表中的列数据类型不同，则必须将它们转换为通用类型。转换高度依赖于您的DBMS，因此请确保使用正确的方法来执行此操作。

Answer 4

您可以使用Union或Union All来连接3个查询的结果集。但是，所有3个表必须具有相同数量的列才能工作。发布表结构以获得更精确的解决方案。

SELECT * FROM messages WHERE receiver_id = 10
Union All
SELECT * FROM payments WHERE client_id = 10
Union All
SELECT * FROM reports WHERE receiver_id = 10

SQL选择多个表

4 个答案: