Modelling Hierarchical attributes - sql

Question

I am trying to figure out the data model for a retailer.
The retailer has several stores across the country and they are modeled using the following hierarchy:
Channel -> Zone -> City -> Store
Each store contains several articles. And each article has attributes like
Activation flag (this indicates the presence of article)
Price
Supplier
Warehouse
Now, the retailer can set these attributes at any level in the hierarchy. Consider the following cases:
Setting price for an article at channel level will apply it to all stores.
The price set at a higher level can be overridden at any other level. For e.g., at a city level for just the stores in a city or for a particular store.
This applies to all attributes listed above.
As of now, they have modeled it using RDBMS by defining global rules at the top of hierarchy and calling out exceptions separately as individual rows. Say, price table, will have price set for an article at Channel level and any changes at any level will be specified separately. Obviously, this is not very efficient when fetching the attributes at the store level.
Sample Data
Assume Channel, Zone, City and Store are collectively called entities. Channel will have ids ranging >= 4000, Zone >= 3000, City >= 2000 and stores range from 0 to 1000.
A subset of the hierarchy relationship data is given below:
Channel | Zone | City | Store |
----------+----------+------------------
4001 | 3001 | 2001 | 13 |
4001 | 3001 | 2001 | 14 |
4001 | 3001 | 2002 | 15 |
4001 | 3002 | 2003 | 16 |
4001 | 3003 | 2006 | 74 |
Price Table
ArticleID | EntityID | Price
----------+----------+----------
12345 | 4001 | 2.5
12345 | 2003 | 2.9
12345 | 74 | 3.0
Here, the price 2.5 for channel 4001 will be set for the article in all stores. The next two rows set the price exceptions in some stores. The second price 2.9 set for city 2003 will be applicable only for the article in store 16 as per the hierarchy relationship mentioned above. And the third row directly sets price 3.0 for the article in store 74.
Hope this gives an idea of current model. So, can you please suggest a better way to store this?

Answer 1

There are few ways to represent trees and hierarchies in relational model -- a search on SO will return quite a few answers.
The main idea in this model is to represent hierarchy with a closure table and a hierarchy-level.
So, the Level table has (1, Channel) , (2, Zone) , (3, City) , (4, Store).
The closure table exposes each node and all descendants of the node. It is important to note that each node is its own descendant too.
The first CTE query (q_00) selects a node's price and assigns it to all descendants. The LevelDiff column calculates how many levels from the descendant node is the price-specifying node.
Because the price may be specified on more than one level, the final query selects store prices specified on the level with minimum LevelDiff.
The syntax is PostgeSQL, but should be easy to convert to others too.
with q_00 as (
select
a.ProductID
, c.LocationID
, c.LocationLevel
, a.Price
, t.DescendantLocationID
, t.DescendantLevel
, (t.DescendantLevel - c.LocationLevel ) as LevelDiff
from ProductPrice as a
join Product as b on b.ProductId = a.ProductID
join Location as c on c.LocationID = a.LocationID
join TreeClosure as t on t.LocationID = c.LocationID
)
select
a.ProductID
, DescendantLocationID
, Price
from q_00 as a
join Level as w on w.LevelNo = a.DescendantLevel
where w.LevelName = 'Store'
and a.Leveldiff = (select min(LevelDiff)
from q_00 as x
where x.DescendantLocationID = a.DescendantLocationID
and x.ProductID = a.ProductID ) ;
So to summarize, here is a test result, pricing was defined as:
Channel=1, Product=1, Price = 11.0
Channel=1, City=111, Product=1, Price = 11.5
Channel=1, City=111, Store =1112, Product=1, Price = 12.0
The query returns (see test data below)
ProductID | DescendantLocationID | PriceID
-----------------------------------------
1 1231 11.00
1 1232 11.00
1 1111 11.50
1 1112 12.00
Here is the DDL (PosgreSQL)
CREATE TABLE Level (
LevelNo integer NOT NULL ,
LevelName varchar(20) NOT NULL
);
ALTER TABLE Level ADD CONSTRAINT XPKLevel PRIMARY KEY (LevelNo) ;
CREATE TABLE Location (
LocationID integer NOT NULL ,
LocationLevel integer NOT NULL
);
ALTER TABLE Location ADD CONSTRAINT XPKLocation PRIMARY KEY (LocationID);
ALTER TABLE Location ADD CONSTRAINT XAK1Location UNIQUE (LocationID, LocationLevel) ;
CREATE TABLE Product (
ProductID integer NOT NULL
);
ALTER TABLE Product ADD CONSTRAINT XPKProduct PRIMARY KEY (ProductID);
CREATE TABLE ProductPrice (
ProductID integer NOT NULL ,
LocationID integer NOT NULL ,
Price decimal(19,2) NOT NULL
);
ALTER TABLE ProductPrice ADD CONSTRAINT XPKProductPrice PRIMARY KEY (ProductID, LocationID);
CREATE TABLE ProductSupplier (
ProductID integer NOT NULL ,
LocationID integer NOT NULL ,
SupplierID integer NOT NULL
);
ALTER TABLE ProductSupplier ADD CONSTRAINT XPKProductSupplier PRIMARY KEY (ProductID, LocationID);
CREATE TABLE Supplier (
SupplierID integer NOT NULL
);
ALTER TABLE Supplier ADD CONSTRAINT XPKSupplier PRIMARY KEY (SupplierID) ;
CREATE TABLE TreeClosure (
LocationID integer NOT NULL ,
DescendantLocationID integer NOT NULL ,
DescendantLevel integer NOT NULL
);
ALTER TABLE TreeClosure ADD CONSTRAINT XPKTreeClosure PRIMARY KEY (LocationID, DescendantLocationID);
ALTER TABLE Location
ADD CONSTRAINT FK1_Location FOREIGN KEY (LocationLevel) REFERENCES Level(LevelNo);
ALTER TABLE ProductPrice
ADD CONSTRAINT FK1_ProductPrice FOREIGN KEY (ProductID) REFERENCES Product(ProductID);
ALTER TABLE ProductPrice
ADD CONSTRAINT FK2_ProductPrice FOREIGN KEY (LocationID) REFERENCES Location(LocationID);
ALTER TABLE ProductSupplier
ADD CONSTRAINT FK1_PrdSup FOREIGN KEY (ProductID) REFERENCES Product(ProductID);
ALTER TABLE ProductSupplier
ADD CONSTRAINT FK2_PrdSup FOREIGN KEY (SupplierID) REFERENCES Supplier(SupplierID);
ALTER TABLE ProductSupplier
ADD CONSTRAINT FK3_PrdSup FOREIGN KEY (LocationID) REFERENCES Location(LocationID);
ALTER TABLE TreeClosure
ADD CONSTRAINT FK1_TC FOREIGN KEY (LocationID) REFERENCES Location(LocationID);
ALTER TABLE TreeClosure
ADD CONSTRAINT FK2_TC FOREIGN KEY (DescendantLocationID,DescendantLevel) REFERENCES Location(LocationID,LocationLevel);
And some data to test with
insert into Level (LevelNo, LevelName)
values
(1, 'Channel')
, (2, 'Zone')
, (3, 'City')
, (4, 'Store')
;
insert into Product (ProductID)
values (1) , (2) , (3)
;
-- Locations
insert into Location (LocationID, LocationLevel)
values
(1, 1)
, (11, 2)
, (111, 3)
, (1111, 4)
, (1112, 4)
, (12, 2)
, (123, 3)
, (1231, 4)
, (1232, 4)
;
-- Tree closure (hierarchy)
insert into TreeClosure (LocationID, DescendantLocationID, DescendantLevel)
values
(1 , 1 , 1)
, (1 , 11 , 2)
, (1 , 111 , 3)
, (1 , 1111, 4)
, (1 , 1112, 4)
, (11 , 11 , 2)
, (11 , 111 , 3)
, (11 , 1111, 4)
, (11 , 1112, 4)
, (111 , 111 , 3)
, (111 , 1111, 4)
, (111 , 1112, 4)
, (1111, 1111, 4)
, (1112, 1112, 4)
--
, (1 , 12 , 2)
, (1 , 123 , 3)
, (1 , 1231, 4)
, (1 , 1232, 4)
, (12 , 12 , 2)
, (12 , 123 , 3)
, (12 , 1231, 4)
, (12 , 1232, 4)
, (123 , 123, 3)
, (123 , 1231, 4)
, (123 , 1232, 4)
, (1231, 1231, 4)
, (1232, 1232, 4)
;
-- pricing
insert into ProductPrice (ProductID, LocationID, Price) values (1, 1 , 11.0);
insert into ProductPrice (ProductID, LocationID, Price) values (1, 111 , 11.5);
insert into ProductPrice (ProductID, LocationID, Price) values (1, 1112, 12.0);