oracle – 如何在视图中创建非空列

CREATE TABLE "MyTable" 
(
  "MyColumn" NUMBER NOT NULL
);

我想创建一个视图,如：

CREATE VIEW "MyView" AS
SELECT
    CAST("MyColumn" AS BINARY_DOUBLE) AS "MyColumn"
FROM "MyTable";

仅在“MyColumn”列为“NOT NULL”的位置.

在SQL Server中,这非常简单：

CREATE VIEW [MyView] AS
SELECT
    ISNULL(CAST([MyColumn] AS Float),0.0) AS [MyColumn]
FROM [MyTable];

但是,Oracle等效项会产生“NULL”列：

CREATE VIEW "MyView" AS
SELECT
    NVL(CAST("MyColumn" AS BINARY_DOUBLE),0.0) AS "MyColumn"
FROM "MyTable";

反正是否强制Oracle将视图的列标记为元数据中的“NOT NULL”？

我想到这个效果的唯一方法就是,如果你在11g上,将转换值作为虚拟列添加到表中,并且(如果仍然需要)创建视图：

ALTER TABLE "MyTable" ADD "MyBDColumn" AS
    (CAST("MyColumn" AS BINARY_DOUBLE)) NOT NULL;

CREATE OR REPLACE VIEW "MyView" AS
SELECT
    "MyBDColumn" AS "MyColumn"
FROM "MyTable";

desc "MyView"

 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 MyColumn                                  NOT NULL BINARY_DOUBLE

既然你在对dba.se的评论中说这是用来模拟某些东西,你可以使用普通列和触发器来模拟虚拟列：

CREATE TABLE "MyTable" 
(
  "MyColumn" NUMBER NOT NULL,"MyBDColumn" BINARY_DOUBLE NOT NULL
);

CREATE TRIGGER "MyTrigger" before update or insert on "MyTable"
FOR EACH ROW
BEGIN
    :new."MyBDColumn" := :new."MyColumn";
END;
/

CREATE VIEW "MyView" AS
SELECT
    "MyBDColumn" AS "MyColumn"
FROM "MyTable";

INSERT INTO "MyTable" ("MyColumn") values (2);

SELECT * FROM "MyView";

  MyColumn
----------
  2.0E+000

并且desc“MyView”仍然给出：

Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 MyColumn                                  NOT NULL BINARY_DOUBLE

正如Leigh所提到的(同样在dba.se上),如果你确实想要插入/更新视图,你可以使用VC或假版本而不是触发器.

（编辑：李大同）

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