指向另一個虛擬列的虛擬列定義

Question

是否有可能擁有一個虛擬列，其列表達式定義引用另一個虛擬列，該虛擬列之前已在表中定義過？ The documentation for column expressions說：

the defining column_expression must refer only to columns of the subject table that have already been defined, in the current statement or in a prior statement

這是我想要做的，與第三虛擬列指的是其他虛擬列一個簡單的例子已經定義：

create table numberManipulation 
(
    value1 VARCHAR2 (10), 
    Value2 VARCHAR2 (10), 
    Addition AS (DECODE (value1, 'late', 1, 0) + DECODE (value2, 'late', 1, 0)), 
    subtraction AS(DECODE (value1, 'present', 1, 0) + DECODE (value2, 'present', 1, 0)), 
    Compile AS (ADDITION + SUBTRACTION) 
); 

但這會得到錯誤：

ORA-54012: virtual column is referenced in a column expression 

是我想要做的可能嗎？

Answer 1

不，雖然the documentation for column expressions沒有明確說你不能。這一限制在My Oracle Support的文檔466916.1表示，雖然，在create table documentation：

The column_expression in the AS clause has the following restrictions:

• It cannot refer to another virtual column by name.
  ...

你將不得不從先前的每個虛擬列的重複計算在第三個：

create table numberManipulation 
(
    value1 VARCHAR2 (10), 
    Value2 VARCHAR2 (10), 
    Addition AS (DECODE (value1, 'late', 1, 0) + DECODE (value2, 'late', 1, 0)), 
    subtraction AS(DECODE (value1, 'present', 1, 0) + DECODE (value2, 'present', 1, 0)), 
    Compile AS (DECODE (value1, 'late', 1, 0) + DECODE (value2, 'late', 1, 0) 
    + DECODE (value1, 'present', 1, 0) + DECODE (value2, 'present', 1, 0)) 
); 

Table NUMBERMANIPULATION created. 

或者稍微更簡單：

Compile AS (DECODE (value1, 'late', 1, 'present', 1, 0) 
    + DECODE (value2, 'late', 1, 'present', 1, 0)) 

...儘管你可能寧願保持兩個較長表達式匹配的一致性和米早期專欄aintenance。

有一個虛擬列引用另一個虛擬列將決定他們必須被評估的順序，並可能導致循環依賴關係。

另一種方法是有計算基表從兩個虛擬列最後列的視圖：

create table numberManipulation 
(
    value1 VARCHAR2 (10), 
    Value2 VARCHAR2 (10), 
    Addition AS (DECODE (value1, 'late', 1, 0) + DECODE (value2, 'late', 1, 0)), 
    subtraction AS(DECODE (value1, 'present', 1, 0) + DECODE (value2, 'present', 1, 0)) 
); 

Table NUMBERMANIPULATION created. 

create view vNumberManipulation as 
select numberManipulation.*, addition + subtraction as compile 
from numberManipulation; 

View VNUMBERMANIPULATION created. 

insert into numberManipulation (value1, value2) values ('late', 'late'); 
insert into numberManipulation (value1, value2) values ('late', 'present'); 
insert into numberManipulation (value1, value2) values ('present', null); 
select * from vNumberManipulation; 

VALUE1  VALUE2  ADDITION SUBTRACTION  COMPILE 
---------- ---------- ----------- ------------ ------------ 
late  late     2   0   2 
late  present    1   1   2 
present       0   1   1 

的視圖也不能引用自己的列，所以你還需要增加和在基表中減法。您也可以在視圖上使用instead-of trigger，以便嘗試修改視圖實際上會更改基表，這會使其更友好。