UNION 语句相同的选定值 - 奇怪的行为 [重复]

Posted 2023-02-24

【中文标题】UNION 语句相同的选定值 - 奇怪的行为 [重复]

【英文标题】：UNION Statement Same Selected Values - Strange Behavior [duplicate]

【发布时间】：2014-10-14 16:55:55

【问题描述】：

我在具有相同值的 UNIONed select 语句中发现了一个非常奇怪的行为。

试图在网络上查找发生了什么，但不知道如何表达对这个问题的搜索，需要解释，如下：

测试 1：

SELECT 
        ID, SUM(AMT) AS AMT
    FROM
    (
        SELECT 1 AS ID, 100 AS AMT FROM DUAL UNION
        SELECT 1 AS ID, 100 AS AMT FROM DUAL UNION
        SELECT 1 AS ID, 100 AS AMT FROM DUAL UNION
        SELECT 1 AS ID, 100 AS AMT FROM DUAL
    )
    GROUP BY ID; 

测试 2：

SELECT 1 AS ID, 100 AS AMT FROM DUAL UNION
SELECT 1 AS ID, 100 AS AMT FROM DUAL UNION
SELECT 1 AS ID, 100 AS AMT FROM DUAL UNION
SELECT 1 AS ID, 100 AS AMT FROM DUAL;

所有这些选择语句都给出相同的结果：

所以，假设我正在编写一个动态代码，在运行时在我的一个应用程序中生成和执行，而代码恰好如上所示！如何使这个 select 语句起作用？

【问题讨论】：

你追求的是 UNION ALL 吗？

谢谢，现在对我有意义

【参考方案1】：

请尝试以下一个

SELECT 1 AS ID, 100 AS AMT FROM DUAL UNION all

SELECT 1 AS ID, 100 AS AMT FROM DUAL UNION all

SELECT 1 AS ID, 100 AS AMT FROM DUAL UNION all

SELECT 1 AS ID, 100 AS AMT FROM DUAL;

【参考方案2】：

您可以尝试使用 UNION ALL

SELECT 
        ID, SUM(AMT) AS AMT
    FROM
    (
        SELECT 1 AS ID, 100 AS AMT FROM DUAL UNION ALL
        SELECT 1 AS ID, 100 AS AMT FROM DUAL UNION ALL
        SELECT 1 AS ID, 100 AS AMT FROM DUAL UNION ALL
        SELECT 1 AS ID, 100 AS AMT FROM DUAL
    )
    GROUP BY ID;