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Slowly Changing Dimension Designs - Date Range with Parent
Published: Nov 29, 2020
In a previous post I went through what a Slowly Changing Dimension is and how it can be a pain to all and sundry when designing a database. Before I move on to implementation in regard to code, I just wanted to touch on a few design thoughts in regard to the tables themselves as there is more than a couple of ways to store a slowly changing dimension. There’s no right or wrong, therefore these are just a few ideas to consider depending on what you feel suits you best or you’re most comfortable with.

Let's get started...

Basic Table

This is the basic setup I’ll be using (single line formatting for compactness only):

drop table if exists dbo.FactSalesByMonth, dbo.DimIndustry, dbo.DimCompany
go

create table DimIndustry(industryID tinyint identity, industryName varchar(100) not null)
create table DimCompany(companyID tinyint identity, companyName varchar(100) not null)
create table FactSalesByMonth(salesID int identity, industryID tinyint, companyID tinyint, dateID int, salesAmt money)
go

alter table DimIndustry add constraint pk_industry primary key clustered(industryID)
alter table DimCompany add constraint pk_company primary key clustered(companyID)
alter table FactSalesByMonth add constraint pk_sales primary key clustered(salesID)
go

alter table DimIndustry add constraint fk_salesIndustry foreign key(industryID) references DimIndustry(industryID)
alter table DimCompany add constraint fk_salesCompany foreign key(companyID) references DimCompany(companyID)
go

insert into DimIndustry select 'Technology'
insert into DimCompany select 'Pear Computers'
insert into FactSalesByMonth select 1, 1, 201801, 1000000
insert into FactSalesByMonth select 1, 1, 201802, 1000000
insert into FactSalesByMonth select 1, 1, 201803, 1000000
insert into FactSalesByMonth select 1, 1, 201804, 1200000
insert into FactSalesByMonth select 1, 1, 201805, 1200000
insert into FactSalesByMonth select 1, 1, 201806, 1200000
go

select * from DimIndustry
select * from DimCompany
select * from FactSalesByMonth
go



We’re going to deal with the fact that Pear Computers has changed its name over the years and we need to know what it was on a Point In Time basis.

Date Based with Parent

In this method we add To and From dates to our table in order to show when a value was active, but instead of adding an arbitrary surrogate key into our table we add a ParentCompanyID. For example, our company table changes as follows:

drop table if exists dbo.FactSalesByMonth, dbo.DimCompany
go

create table DimCompany
(
    
companyID tinyint identity not null,
    
fromDate datetime not null,
    
toDate datetime not null,
    
parentCompanyID tinyint not null,
    
companyName varchar(100) not null
)

create table FactSalesByMonth
(
    
salesID int identity,
    
industryID tinyint,
    
companyID tinyint,
    
parentCompanyID tinyint,
    
dateID int,
    
salesAmt money
)
go



Summary (main points, not exhaustive)

Pros

  • You can easily maintain a Primary Key / Foreign Key relationship


  • Both CompanyID and ParentCompanyID


  • Easy to find latest record via toDate


  • Aggregation at companyID and parentCompanyID is simple


I’ve actually left Cons empty here, not because there aren’t any as I’m sure some people will have found several, but because there aren’t any that I consider glaring and hugely cumbersome. Therefore this tends to be the method I’ve leant towards in the past. You just need to ensure that you heavily index the Dimension tables in order to accommodate the several likely access routes.

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SQL  World  CEO
Kevin  Urquhart

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I am a SQL Server DBA, Architect, Developer, Trainer, and CEO of SQL World. This is my blog in which I’m simply trying to share my SQL knowledge and experiences with the world.

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