sql - UNION 或 JOIN 用于从多个表中进行 SELECT

Question

我的问题

我试图根据参数从多个表中选择一行，但我对 SQL 连接的有限知识阻碍了我。有人能指出我正确的方向吗？

考虑这些表结构:

+-----------------------+     +---------------------+
| Customers             |     | Sellers             |
+-------------+---------+     +-----------+---------+
| Customer_ID | Warning |     | Seller_ID | Warning |
+-------------+---------+     +-----------+---------+
| 00001       | Test 1  |     | 00008     | Testing |
| 00002       | Test 2  |     | 00010     | Testing |
+-------------+---------+     +-----------+---------+

我想要做的是一个 SELECT 来仅检索一行，并且在这一行中将是基于 X_ID 字段的每个表的“警告”字段。

期望的结果

因此，如果我提交以下信息，我将收到以下结果:

示例1:

Customer_ID = 00001
Seller_ID = 00008

Results:
+-----------------------------------+
| Customer_Warning | Seller_Warning |
+------------------+----------------+
| Test 1           | Testing        |
+------------------+----------------+

示例2:

Customer_ID = 00001
Seller_ID = 00200

Results:
+-----------------------------------+
| Customer_Warning | Seller_Warning |
+------------------+----------------+
| Test 1           | NULL           |
+------------------+----------------+

我尝试过的

这是我当前的代码(我收到大量行):

SELECT c.Warning 'Customer_Warning', s.Warning AS 'Seller_Warning'
FROM Customers c,Sellers s
WHERE c.Customer_ID = @Customer_ID
    OR s.Seller_ID = @Seller_ID

但我也尝试过 UNION、UNION ALL 和 JOIN。我应该采用哪种方法？

Answer 1

由于您并未真正将表连接在一起，只是从每个表中选择一行，因此您可以这样做:

SELECT 
    (SELECT Warning 
     FROM Customers 
     WHERE Customer_ID = @Customer_ID) AS Customer_Warning,
    (SELECT Warning 
     FROM Sellers 
     WHERE Seller_ID = @Seller_ID) AS Seller_Warning