转自:https://www.cnblogs.com/lurenjia1994/p/7111124.html
Oracle语句优先级
SQL> SELECT SAL SALARY FROM EMP WHERE SALARY<2500;
Warning: connection was lost and re-established
SELECT SAL SALARY FROM EMP WHERE SALARY<2500
ORA-00904: "SALARY": 标识符无效
在Oracle中,在WHERE子句中引用列别名会出错,这是为什么呢?
请看SELECT 语句的执行顺序:
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from语句
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where语句(结合条件)
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start with语句
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connect by语句
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where语句
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group by语句
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having语句
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model语句
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select语句
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union、minus、intersect等集合演算演算
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order by语句
SELECT 语句在WHERE语句后面才执行,而列的别名是在SELECT 时才生成的,故在WHERE子句中根本看不到列的别名,自然无法引用列的别名了。
如果想用列的别名就要这样:
SELECT SALARY FROM (SELECT SAL SALARY FROM EMP) WHERE SALARY<2500;