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Sayed Ahmed

Logical Design of a Data Warehouse

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TOPICS - KEYWORDS

Design a Data Warehouse Star Schema Snow Flake Schema Dimension Tables Fact Tables Auditing Surrogate Keys Type 1, Type 2, Type 3, and Mixed solutions for

slowly changing dimension data ( SCD management) Pivoting for Analysis

To help with SSAS on data warehouse

TOPICS - KEYWORDS

Design a Data Warehouse Additive measures

Semi additive measures Hierarchies for dimensions

Attributes in dimensions Attributes in lookup tables

Long term data warehouse design Usually Star Schema

Short term data warehouse design POC Usually snowflake schema

TOPICS - KEYWORDS

Fact Tables  measures foreign keys and possibly an additional primary key and lineage columns  granularity of fact tables auditing and lineage needs

Measures can be additive non-additive semi-additive

TOPICS - KEYWORDS

dimension keys names attributes member properties translations and lineage

TOPICS - KEYWORDS

attributes natural hierarchies

many-to-many fact table relationships you can introduce an additional

intermediate dimension

CONCLUSION

Not much – right However, if you understand all the terms and

can implement all these concepts in your data warehouse That will be great Not necessarily you will need to use all of these

concepts; however, you may need to justify based on the situation, will all or any of these will help?

What will help and what will not help

Check our sub sequent videos and tutorials

THANK YOU

Any Concerns? http://ask.justetc.net Or comment below...

TOOLS AND SOFTWARE REQUIREMENTS Download the Adventure Works databases

OLTP database (LOB database) Data warehouse Database From

http://msftdbprodsamples.codeplex.com/releases/view/55330

For this tutorial, you can just check our slides Though the following tools will help

And probably check the details in the downloaded databases esp. The AdventureWorksDW2012

You will need help from SQL Server and SQL Server MGMT Studio Tools

REQUIRED TOOLS

Useful/Required SQL Server Components Database Engine Services Documentation Components Management Tools - Basic Management Tools – Complete SQL Server Data Tools

DATA WAREHOUSE DESIGN – THE DETAILS Data Warehouse Logical Design

Topics: Design and Implement a Data Warehouse Design and implement dimensions. Design and implement fact tables Design Auditing

track the source and time for data coming into a DW through auditing i.e lineage information

Why a Data Warehouse? It is hard to

generate reports from OLTP/LOB/Transactional database To do Analysis on OLTP database data (some times) Get useful information/useful summarized and details data to

be used to take business decisions

DATA WAREHOUSE DESIGN – THE DETAILS Why a Data Warehouse?

Data in OLTP are heavily normalized. The goal was to keep one data only in one single place to reduce redundancy and consistency of data

You may end up with many tables 100s, 1000s To generate reports you may need to join many

tables – will be slow Historical data may not be there Data quality is also an issue For reporting or analyzing, you may need data

from multiple databases across many departments

WHY A DATA WAREHOUSE?

So you can create a Data Warehouse By cleaning data With historical data Combining data from multiple sources Denormalizing data Using specific design geared towards Data

Warehouse design Some or many consider DW design is less complex

than relational database design Though it also has some complex areas to address... (by

those some or many)

SO WHAT DOES A DATA WAREHOUSE CONTAIN?

Usually two schemas are used for a DW Star Schema-> looks like a star Snow Flake Schema

Another one called Dimensional Model Includes both Star and Snow Flake in the

same Data Warehouse Both Schemas has tables of two types

Dimension Tables Fact Tables

SO WHAT DOES A DATA WAREHOUSE CONTAIN?

Fact Tables are in the center A Fact table joins/combines all the data required for this

reporting or for the business aspect of this reporting Usually combines the primary keys of different tables that

contain data for this report/business aspect

Dimension tables are all the other tables that contain actual data Dimension tables are the tables that contain data

these can be the actual tables in the OLTP database without any modification (Snow Flake)

Or Dimension tables can be newly created by denormalizing the existing OLTP databases (Star)

SO WHAT DOES A DATA WAREHOUSE CONTAIN?

So, you know now what are dimension tables and what are fact tables Fact tables contain primary keys of all related

tables (here they are foreign keys) Dimension tables contain data

Usually, it’s better that you keep your data warehouse separate from your OLTP database So bring all the tables (dimension) here Or denormalize them and bring them here in the

new database

SIMPLIFIED: WHAT ARE STAR AND SNOWFLAKE SCHEMAS

If you just create Fact tables and take all the related tables from your OLTP/LOB databases You get a Snow Flake Schema Here all Dimension tables are still normalized (as

you just took them from the actual database) This is easy –

so good for short-term, quick, and experimental Data Warehouse

One note, your reporting and analysis services queries (MDX, DMV) will be slow with Snow Flake Schemas

SO WHAT DOES A DATA WAREHOUSE CONTAIN?

Now, when you denormalize the dimension tables You get the start schema The Fact tables remain the same for

example Star Schema is kind of standard and

used a lot Originally was developed in 1980’s

EXAMPLES: WHY REPORTING IN OLTP DATABASE IS NOT A GREAT IDEA

Sales amount for internet sales by different countries and historical years

WHY REPORTING IN OLTP DATABASE IS NOT A GREAT IDEA

issues that I did not mention before If your OLTP database was well designed

(?) It may be hard to find the tables related to the

reporting The table names and the column names can be

tricky – do not follow any conventions – do not have meaning

So it can be hard to find data for the reporting

WHY REPORTING IN OLTP DATABASE IS NOT A GREAT IDEA

Note: Reality: The OLTP may not even be well designed (that makes

reporting hard sometimes) even the relationships as well as normalization

– here we assumed that OLTP is perfect In a long back project

I had to re-write/verify/check/change/optimize/had to deal with (whatever you say) 100s (not really 100s, can be close to 100) of queries for a reporting system

Had to change the interface from one button for one report (easy to get lost)

Into a drop down list of reports The relations among data were arbitrary – actually had only in the

mind of the designer – did not follow any standards – No ER – no standard concepts---

So it was a hard job.. Anyway..

WHY REPORTING IN OLTP DATABASE IS NOT A GREAT IDEA

In such cases Tools such as SQL Profiler might help you could create a test environment,

try to insert some data through an LOB application have SQL Profiler identify where the data was inserted

Another, issue with this particular example No lookup for dates and years

You need to extract The tables may not contain even historical data

No date field So no historical data

WHY REPORTING IN OLTP DATABASE IS NOT A GREAT IDEA

If sales data reside in multiple databases even by multiple departments How do you merge Identify and match Customer data can be in different database with no

common identification Data quality can be low

Data missing Partial data Inconsistent data in multiple databases Data can be represented differenlt in different database

M or F for gender 1 or 0 for gender

STAR SCHEMA/FACT/DIMENSION/CUBE

TOTAL DW: MULTIPLE STAR SCHEMAS You saw one Star Schema for Internet Sales You can see another for Offline Sales Another for Accounting Your DW has many such Star Schemas And these start schemas need to be connected/related They will be connected when you use the same dimensions

for them i.e. If two star schemas have the same dimension they can share

that dimension Called: shared or conformed dimensions

For SSAS, you can use shared dimensions only There is a concept of private dimension

Not a great idea in practical and real life applications You cannot connect/compare/verify the data over the shared dimension

SHARED/CONFORMED DIMENSIONS

DENORMALIZED DIMDATE TABLE

SNOW FLAKES WILL BE MORE AND MORE NORMALIZED

Everything can be normalized Or the first level can be normalized

others are not

NORMALIZED PRODUCT DIMENSION

In the Star Schema, you could use these normalized product table to get snow flake schema (partially.) Could use all normalized dimensions to get full snow flake

SNOW FLAKE

In Snow flake, you may see partial than full snow flakes in reality

Though, in reality, better to go for star schema Queries will be faster

PARTIAL SNOW FLAKE

GRANULARITY

The number of Dimension Tables connected to a fact table Dimension of a star schema Cube = 3 dimension SSAS operates/analyzes on Cube

AUDITING AND LINEAGE

I will be very short on this In data warehouse, you may want some

auditing tables For every update, you should audit

who made the update, when it was made, and how many rows were transferred

to each dimension and fact table

in your DW

AUDITING AND LINEAGE

You will need additional fields/columns in your dimension and fact tables to track When, and who, and from where the row

data was/were updated Your ETL process needs to be updated If you used SSIS for the ETL

Modify SSIS packages so that you can record these information

THANK YOU

Any Concerns? http://ask.justetc.net Or comment below...