如何通过给定的超级表在SQL主键识别子类型表

Question

I have 3 tables product, CD, OtherProduct

product is super table and CD, OtherProduct are sub types of product and has primary key of product as foreign key in it.

ie

Product Table
prod_num 
1
2
3


CD Table
prod_num                                  cd_title    cd_length
1(fk taken by Product table)            rock n roll       30


OtherProduct  
 prod_num                                  size           brand
 3 (fk taken by product table)             large          nikie

i want to extract information by sql query product numer, product type and description(if product is cd then title, cd_length and if product is belongs to other product then size, brand)

I tried in this way, but it is not correct, confuse how to come to know product type and then hoe fetch detail of that product.

select product.prod_num from product, cd, otherProduct
where product.prod_num = cd.prod_num 

AND 

product.prod_num = other_product.product_num

Answer 1

i want to extract information by sql query product numer, product type and description(if product is cd then title, cd_length and if product is belongs to other product then size, brand)

The question is about determination of the Subtype { CD | OtherProduct }, { CD | OtherProduct }, without the presence of a ProductType column in the Basetype. That is a valid assignment question.

The answer requires an understanding of what a Subtype is.

• There are two types of Subtype: Exclusive and Non-exclusive.

• This one is an Exclusive Subtype. They require a Discriminator column in the Basetype, such as ProductType.

In this instance, the problem given does not have a Discriminator, you have to figure out the Subtypes without that. There are integrity issues, but again, for a classroom problem, the absence of such is not "incorrect".

Answer

What is a database ?

• The database is a collection of Facts. Generally, each row is a Fact, collectively we say, each table is a collection of the instances of a Fact.

What is an Exclusive Subtype ?

• An Exclusive subtype is always a pair: the Basetype plus one Subtype, it is never either Basetpe xor Subtype alone.

• Thus while in the normal (not-a-subtype) case, a Fact is stored in one table, the Fact of a Subtype, its existence, is stored in two tables, the Basetype plus one Subtype.

• An existence check is simply a join (straight join, inner join, not outer, not left, not right).

You can determine CD Facts as follows. Once we determine them, we can assert the missing ProducteType:

    SELECT  CD.prod_num,
            prod_type = "C",
            cd_title,
            cd_length
        FROM Product P
            JOIN CD ON P.prod_num = CD.prod_num

To determine OtherProduct Facts:

    SELECT  OP.prod_num,
            prod_type = "O",
            size,
            brand
        FROM Product P
            JOIN OtherProduct OP ON P.prod_num = OP.prod_num

• No nulls.

• Scales beautifully.

If you need them in Views, stick CREATE VIEW AS in front of them

If you need both Subtypes in a single View ¹ :

    SELECT  CD.prod_num,
            prod_type = "C",
            cd_title,
            cd_length,
            size = "",
            brand = ""
        FROM Product P
            JOIN CD ON P.prod_num = CD.prod_num
    UNION
    SELECT  OP.prod_num,
            prod_type = "O",
            "",
            "",
            size,
            brand
        FROM Product P
            JOIN OtherProduct OP ON P.prod_num = OP.prod_num

1. Note that a view is a derived relation, not a base relation (table), so we don't care that it has Nulls, whereas we would not implement Nulls in a base relation.

2. Nevertheless, NULL is a pretty stupid value to place in a report because it confuses the user, and then you have to spend time de-confusing them. Thus it is good practice to substitute NULL with something that is not confusing.

3. I have given the column titles in the first SELECT, in order to have them displayed. Column titles in the second and subsequent SELECT are ignored.

Answer 2

Looks like you need a left join and sum IFNULL() trickery!

SELECT
  IF(cd.prod_num IS NULL,'other','cd') AS product_type,
  CONCAT (
    IFNULL(otherProduct.size, cd.cd_title),
    ', ',
    IFNULL(otherProduct.brand, CONCAT(cd.cd_length, ' mins'))
  ) AS description 
FROM
  product
  LEFT JOIN cd ON product.prod_num=cd.prod_num
  LEFT JOIN otherProduct ON product.prod_num=otherProduct.prod_num
;

Answer 3

You could create a view that unifies the type and the subtypes, more or less like the following

create view all_products as
select *
    from product
    inner join CD on (CD.prod_num=product.prod_num)
union all
select *
    from product
    inner join otherproduct (on otherproduct.prod_num = product.prod_num);

Then, you could query the view as follows:

select * from all_product
 where prod_num in (1, 2, 3)

This has not been tested, so I may have a few typos, but I hope you get the idea.

This solution won't scale up well. With a few hundred products,it should run ok. with a few million products, it will be too slow. It will also result in lots of NULLs in cases where a data item is inapplicable, because it pertains to the wrong subtype.

Answer 4

You have not clearly explained what you want. Maybe it's this:

select prod_num,cd_title,cd_length,null as size,null as brand
from CD
union
select prod_num,null as cd_title,null as cd_length,size,brand
from OtherProduct

(If we want rows per condition1 AND rows per condition2 then we want rows per condition1 -OR- condition2 . These kinds of AND and OR mean a UNION of rows.)