SQL Oracle IN运算符行为

Question

我使用Oracle而我无法解释这种行为：

SELECT DUMMY 
FROM   DUAL 
WHERE  DUMMY IN (SELECT DUMMY FROM z_sites);

返回

DUMMY
-----
X

而且：

SELECT DUMMY FROM z_sites;

返回

  

ORA-00904：＆＃34; DUMMY＆＃34; ：识别非valide   00904. 00000 - ＆＃34;％s：无效的标识符＆＃34;

所以在我的第一个查询中，子查询是错误的，但查询仍然可以自行解决？！

Answer 1

关于范围：

SELECT DUMMY 
FROM   DUAL 
WHERE  DUMMY IN (SELECT DUMMY FROM z_sites);

与：

相同

SELECT DUMMY 
FROM   DUAL 
WHERE  DUMMY IN (SELECT DUAL.DUMMY FROM z_sites);

鉴于：

SELECT DUMMY FROM z_sites;
-- there is no dummy column in z_sites table

为了让事情变得更有趣，你可以检查一下如果z_sites包含虚拟列会发生什么：

CREATE TABLE z_sites(dummy VARCHAR2(100));
INSERT INTO z_sites(dummy) VALUES('Y');

SELECT DUMMY 
FROM   DUAL 
WHERE  DUMMY IN (SELECT DUMMY FROM z_sites);
-- 0 rows selected
<=>
SELECT DUMMY 
FROM   DUAL 
WHERE  DUMMY IN (SELECT z_sites.DUMMY FROM z_sites);
-- 0 rows selected

SELECT DUMMY 
FROM   DUAL 
WHERE  DUMMY IN (SELECT DUAL.DUMMY FROM z_sites);

DUMMY
-----
X

<强> DBFiddle Demo

总结：始终在子查询中添加前缀以避免混淆。