Question

我正在使用Oracle 11g R2。有没有办法在同一表达式中的SQL查询之前为从表中选择的单个值赋予名称（别名）？这是一个单独的SQL命令，我的意思是，没有PL / SQL。

我最接近的是：

WITH
Approved AS (SELECT c.value FROM configuration c WHERE c.code = 'Approved'),
Arrived AS (SELECT c.value FROM configuration c WHERE c.code = 'Arrived'),
Scheduled AS (SELECT c.value FROM configuration c WHERE c.code = 'Scheduled')

SELECT *
FROM list l WHERE l.status_key > (SELECT value FROM Approved);

我正在寻找类似的东西，比如说：

WITH
Approved AS CONSTANT (SELECT c.value FROM configuration c WHERE c.code = 'Approved'),
Arrived AS CONSTANT (SELECT c.value FROM configuration c WHERE c.code = 'Arrived'),
Scheduled AS CONSTANT (SELECT c.value FROM configuration c WHERE c.code = 'Scheduled')

SELECT *
FROM list l WHERE l.status_key > Approved;

我不想内联值的select语句的原因是我的查询很复杂，如果可能的话我宁愿采取一些复杂性。

Answer 1

我有时会使用这样的结构：

WITH const as
      (select max(case when c.code = 'Approved' then c.value end) as Approved,
              max(case when c.code = 'Approved' then c.value end) as Approved,
              max(case when c.code = 'Scheduled' then c.value end) as Scheduled
       from configuration c
      ),
      . . .
SELECT
FROM const cross join
     list l
WHERE status_key > Approved;

有时如果我需要查询中不同位置的常量，那么我必须不止一次地引入const CTE。

Answer 2

简短的回答是否定的 - 你不能这样做。

你可以创建一个视图来隐藏一些初始的复杂性。

...或者如果你真的想要你可以创建一个函数，并将其用作常量......类似于（不包括任何错误处理）：

CREATE OR REPLACE FUNCTION config_code (code IN VARCHAR2) 
RETURN configuration.value%TYPE AS

value       configuration.value%TYPE;

BEGIN
    SELECT c.value INTO value FROM configuration c WHERE c.code = code;
    RETURN value;
END;

然后您可以将其用作：

SELECT * FROM list l WHERE l.status_key > config_code('Approved');

如何在SQL查询中声明常量？

2 个答案: