Inventory Database Design

问题

I got 2 tables User and Company. These two have a inventory.

DataTables

Item

ItemId | Name
================
   1   | Glass
   4   | Wood

User

UId | Name
============
 1  | Max

Company

 CId  | Name
==================
  1   | EvilCorp

Inventory

 RowId  |   UId  |   CId  |  ItemId  |  amount 
=================================================
   1    |   2    |   Null |    4     |   10 
   2    |   23   |   Null |    4     |   5 
   3    |   Null |   1    |    1     |   7
   4    |   Null |   1    |    4     |   70

Let say I have 500 users and 300 companys and every one has 20 inventory slots, I will have 16000 null values in my Inventory table (6000 UId nulls + 10000 CId nulls).

I want a SQL query that will say this information.

Result

Owner     | Item  | Amount
===========================
 MAX      | Wood  | 10
 EvilCorp | Glass | 7
 EvilCorp | Wood  | 40

My problem is that my Inventory table is bad due to all the nulls that will appears against CId when the record is for a User, and vice versa.

Do you know how to create a good table, without huge or/and complex SQL queries?

回答1:

You have Users, Companies and Owners. Users and Companies are Owners.

This is a common situation that can be put into tables various ways.

Every table holds the rows that make some statement true:

// "user [OId] has ..."
User(OId,...)
    fk OId references Owner -- a user is an owner

// "company [OId] has ..."
Company(OId,....)
    fk OId references Owner -- a company is an owner

// "Owner [OId] has ..."
Owner(OId,...)

// "[OId] owns [n] of item [itemId]"
Inventory(Oid,ItemId,amount)
    fk OId references Owner
    fk ItemId references Item

This comes from choosing tables that make plain statements about parts of your application. For variations read about subtables and supertables for subtypes and supertypes. See this answer or this one.

来源：https://stackoverflow.com/questions/24418273/inventory-database-design

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sql

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database-design