如何在视图中创建非空列

Question

给出如下表:

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"？

Answer 1

您不能向视图添加非null或检查约束; 看到这个和同一页面上的"NOT NULL约束条件限制"和"检查约束条件限制".您可以with check option向视图添加(针对冗余where子句),但不会not null在数据字典中将其标记为.

我认为获得此效果的唯一方法是,如果您使用11g,则将转换值添加为表中的虚拟列,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

而且desc "MyView"还给出了:

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

正如Leigh所提到的(也在dba.se上),如果你确实想插入/更新视图,你可以使用instead ofVC或假版本的触发器.