是否可以在继承的ORACLE DBMS的表中插入一行？

Question

I just had a quick question, The below three tables have inheritance using UNDER,

CREATE TYPE MenuType AS OBJECT(
 MenuCode VARCHAR2(5),
 Description VARCHAR2(20)) NOT FINAL;
/

CREATE TABLE Menu of MenuType (
MenuCode PRIMARY KEY);

CREATE TYPE VegetarianType UNDER MenuType (
 ProteinLevel VARCHAR2(20));
/

CREATE TABLE Vegetarian of VegetarianType;

CREATE TYPE LowFatType UNDER MenuType (
 KCal VARCHAR2(20));
/

CREATE TABLE LowFat of LowFatType;

Is it possible to insert a row in the Menu table which represents a Vegetarian diet? or would it be just inserting a row in the Vegetarian Table?

Thanks in advance,

Ahsan

Answer 1

You do not need separate tables for the sub-types and can just insert them into the MENU table and then use the TREAT function to convert to the sub-type as needed:

CREATE TYPE MenuType AS OBJECT(
 MenuCode VARCHAR2(5),
 Description VARCHAR2(20)
) NOT FINAL;
/

CREATE TYPE VegetarianType UNDER MenuType (
 ProteinLevel VARCHAR2(20)
);
/

CREATE TYPE LowFatType UNDER MenuType (
 KCal VARCHAR2(20)
);
/

CREATE TABLE Menu of MenuType (
  MenuCode PRIMARY KEY
);

Then you can INSERT :

INSERT INTO Menu
VALUES ( VegetarianType( 'VEGE1', 'Vegetarian Meal 1', 'Lots of Protein' ) );

INSERT INTO Menu VALUES ( LowFatType( 'LFAT1', 'Low Fat Meal 1', '500kCal' ) );

Then you can use TREAT to get the values from the sub-type's fields:

SELECT m.*,
       TREAT( VALUE(m) AS VegetarianType ).ProteinLevel AS ProteinLevel,
       TREAT( VALUE(m) AS LowFatType ).KCal AS KCal
FROM   Menu m;

Which outputs:

 MENUCODE | DESCRIPTION | PROTEINLEVEL | KCAL:------- |:---------------- |:-------------- |:------ VEGE1 | Vegetarian Meal 1 | Lots of Protein | null LFAT1 |  Low Fat Meal 1 | null |  500kCal 

db<>fiddle here