了解 Oracle 别名 - 为什么除非包含在第二个查询中，否则查询中无法识别别名?

2021-12-24 00:00:00 sql alias oracle plsql

我有一个问题


SELECT COUNT(*) AS "CNT",
       imei
FROM   devices

执行得很好.我想用 WHERE 语句进一步限制查询.(人为)合乎逻辑的下一步是按以下方式修改查询:

which executes just fine. I want to further restrict the query with a WHERE statement. The (humanly) logical next step is to modify the query followingly:


SELECT COUNT(*) AS "CNT",
       imei
FROM   devices
WHERE  CNT > 1

然而，这会导致错误消息ORA-00904:CNT":标识符无效.出于某种原因，将查询包装在另一个查询中会产生所需的结果:

However, this results in a error message ORA-00904: "CNT": invalid identifier. For some reason, wrapping the query in another query produces the desired result:


SELECT *
FROM   (SELECT COUNT(*) AS "CNT",
               imei
        FROM   devices
        GROUP  BY imei)
WHERE  CNT > 1

为什么 Oracle 在第二个查询中无法识别别名CNT"?

Why does Oracle not recognize the alias "CNT" in the second query?

推荐答案

简单的答案是 AS 子句定义了结果中将调用的列，与查询本身.

The simple answer is that the AS clause defines what the column will be called in the result, which is a different scope than the query itself.

在您的示例中，使用 HAVING 子句效果最佳:

In your example, using the HAVING clause would work best:

SELECT COUNT(*) AS "CNT",
       imei
FROM   devices
GROUP  BY imei
HAVING COUNT(*) > 1

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