The Simplest Way to Understand SCD in Data Warehouse

What is SCD?

SCD stands for Slowly Changing Dimensions.

It is very important in Data Warehouse.

As we know, ETL (Extract, Transform, Load) is between data sources and data warehouse.

When ETL runs, it will pick up all records and update them in the Dimension tables.

Why we need SCD?

Because we have some problems in updating data in Data Warehouse when data in data sources are changing.

In the dimension tables, if we want to keep some old records, how we can do this?

Using SCD.

Types of SCD:

Note:The types of SCD are defined on Column level, not on the table level.

There are two popular types in SCD.

Type 1: Overwrite

An old record is updated by the new record. It means covering the old records.

Type 2: Store history by creating another row

Type 2 is to add new records rather than covering the old records.

As long as we have a type 2 in tables, we must have two extra values: ‘StartDate’ and ‘EndDate’. The ‘EndDate’ is when the changes happen, as the end date of historical data.

We can have a third one, ‘IsCurrent’, to identify or mark the current record.

We don’t need to update the records, and we just need to update ‘StartDate’ and ‘EndDate’.

If you are interested in or have any problems with fact tables and dimension tables, feel free to contact me .

Rules of Creating Dimension Tables and Fact Tables

What is a dimension table? What is a fact table?

Why we need both of them in Data Warehouse?

Because Data Warehouse is used to make reports for business decisions.

Every report is made of two parts: Fact and Dimension.

Here is a picture of fact tables and dimension tables in a star schema in data warehouse.

So, this blog we will talk about what are the rules of creating dimension tables and fact tables.

First of all, we need to illustrate a definition.

What is surrogate key?

It is the primary key in demension tables.

Rules of creating dimension tables:

  1. Primary key (surrogate key, auto-increase number, only unique number in data warehouse)
  2. Business key (the key can be linked back to data source, with business meaning)
  3. Attributes (descriptive information from data source)

There are two kinds of data: Master data and transactional data.

Master data refers to the entity (e.g. employee) whereas transactional data refers to all the transactions that are carried out using that entity.

Master data is limited whereas transactional data can be billions.

In dimension tables, most data is master data.

Rules of creating fact tables:

  1. Primary key (surrogate key/alternate key, auto-increase number)
  2. Foreign key (primary key/surrogate key/alternate key from dimension tables)
  3. Measure (addictive number/semi-addictive number)

Tips: No descriptive data in Fact tables.

If you are interested in or have any problems with fact tables and dimension tables, feel free to contact me .

Or you can connect with me through my LinkedIn.

4 Reasons Why We Need Data Warehouse

Here is a basic process in Business Intelligence.

Maybe some people will be confused, why we need data warehouse?

Without data warehouse, we can also analyze the data.

We can get the data and create the report directly.

So, what the benefits of data warehouse in an organization?

Here we list 4 reasons why we need data warehouse.

Integrate data from various data sources and centralize the data into one place.

Have data loaded into data warehouse so that reporting won’t impact live system or database.

That is why we have a seperate data warehouse and the data is stored in the data warehouse.

We can make a scheduled job running at night to centralize the data from operational databases to data warehouse.

Easy access (one place of data and single source of truth).

It is easy for people to go to data warehouse to get the data, and they don’t need to worry other problems, e.g., we have so many data sources and where can I get the data?

We can trust the data warehouse where we can get the data.

Build model: choose the best design model to get the best flexibility and performance, especially for those large datasets.

We usually use kimball methodology – star schema/snowflake schema (de-normalization).

For example, we use the star schema to improve the query performance.

It is a methodology developed by Ralph Kimball which includes a set of methods, techniques and concepts for use in data warehouse design.

There are also some other methodologies we can use in data warehouse, e.g., inmon methodology, datavault methodology.

If you are interested in or have any problems with data warehouse, feel free to contact me.

Or you can connect with me through my LinkedIn.

Troubleshooting SSMS Error: 15517

If you meet with the same error as me in SSMS 2016.

Error:

“Cannot execute as the database principal because the principal “dbo” does not exist, this type of principal cannot be impersonated, or you do not have permission (Microsoft SQL Server, Error: 15517)”

Solution:

use [databasename]
GO
EXEC sp_changedbowner 'sa'
GO

Hope this solution can help you.

If you are interested in or have any problems with SQL and SSMS, feel free to contact me .

Or you can connect with me through my LinkedIn.

Stored Procedure in SQL

This article I will give you some basic syntax about stored procedure in SQL.

Stored Procedure:

Stored Procedure is the prepared SQL codes that we can save, so the code can be reused over and over again.

It is suitable for some SQL queries we need to use frequently.

There are three kinds of stored procedure:

  1. No parameter
  2. One parameter
  3. multiple parameters

Stored Procedure (No parameter) Syntax

CREATE PROCEDURE procedure_name
AS
sql_statement
GO
EXEC procedure_name

Stored Procedure (One parameter) Syntax

CREATE PROCEDURE [dbo].[oneparameter]
@ProductCatergoryID int
AS
SELECT *FROM Production.ProductCategoryID
GO
EXEC oneparameter @ProductCatergoryID = '4'

Stored Procedure (multiparameter) Syntax

CREATE PROCEDURE [dbo].[multiparameter]
@ProductCategoryID int, @Name varchar(50)
AS
SELECT *FROM Production.ProductCategory pc
WHERE pc.ProductCategoryID = @ProductCategoryID and pc.Name = @Name
GO
EXEC multiparameter @ProductCategoryID = '4', @Name = 'Accessories'

I will record all knowledge I touch in my Business Intelligence journey.

Next blog will be around Data Warehouse.

If you are interested in or have any problems with Business Intelligence, feel free to contact me .

Or you can connect with me through my LinkedIn.

Some Cheat Sheets about Business Intelligence

There are several parts of my Business Intelligence journey.

So I write this blog for recording some cheat sheets.

Installation:

Firstly, some tools need to be installed on the laptop.

All of them are Microsoft softwares, so I prepare a surface laptop 2.

  1. SQL Server 2016 Developer Edition: Seclect the features we need to install, such as SQL Server Integration Services(SSIS), SQL Server Analysis Services(SSAS) and SQL Server Reporting Services(SSRS).
  2. SQL Server Management Studio(SSMS)
  3. SQL Server Data Tools(SSDT)
  4. Report Builder

What is Business Intelligence(BI)?

I have mentioned it in the previous blog. It is to transform raw data into meaningful information for analysis.

BI Process:

  1. Import data from different data sources
  2. Produce reports or graghs through SSIS, SSAS and SSRS
  3. Make business decisions

Some terms:

ETL: Extraction, Transformation and Loading

OLTP: Online Transaction Processing

Cube: Multi-dimensional data structure built using dimensions and facts, using MDX (Multi dimensional expression) language

SQL and Relational Database: which I have mentioned them in the previous blog

Primary Key: A single attribute or an unique id in a Relational Database

Foreign Key: A set of attributes that references a candidate key in a Relational Database

Some differences:

Traditional Normalized Database VS Data Warehouse: Formal one has no duplicate data but latter one has duplicate data for efficiency.

Null VS Blank(where clause): For null, the clause is where xxxxx is null; For blank, the clause is where xxxxx is ”

Delete VS truncate: truncate table xxxxx ->no rollback&faster; delete from xxxxx -> rollback

SQL Language:

Learning sources:

  1. w3schools.com
  2. SQL Server tutorial for beginner

Some common commands:

One table:

SELECT…FROM

WHERE CLAUSE

ORDER BY CLAUSE

Two tables:

Inner Join

Other Joins, e.g., LEFT OUTER JOIN

GROUP BY

If you are interested in or have any problems with Business Intellgence, feel free to contact me .

Or you can connect with me through my LinkedIn.

The Simplest Way to Install SQL Server 2017 on macOS

This article we will show how to install Microsoft SQL Server 2017 on macOS, as we talk it in previous blog.

Which method we use?

Prior to SQL Server 2017, if we want to install it on macOS, a virtual machine (like Parallels Desktop) is essential. Then we can install a Windows system in it and then we install and run SQL Server.

Luckily, from SQL Server 2017, we can choose to install SQL Server on Docker containers.

How to install SQL Server on Docker?

1 What is Docker?

Docker is a platform which can make softwares to run in it. It is called a container, which is an isolated environment.

2 Download and install Docker

If you haven’t installed Docker on your Mac, the next step is to install Docker.

Go to Docker page to download the .dmg file and then double click and install it according to the instructions.

3 Run Docker and increase the memory

Run Docker as you used to run other softwares and the next step is to increase the memory. This is because the default value of Docker memory is 2 GB but SQL needs at least 3.25 GB.

I recommend we set the memory value to 4 GB.

  1. Click Docker icon on top menu of your Mac
  2. Click Preferences
  3. Set Memory under Advanced to 4 GB.

Click Apply&Restart

4 Download Microsoft SQL Server 2017

The next step is to download SQL Server 2017 from Terminal, which is an easy way.

Copy and paste the command in Terminal of your Mac:

docker pull microsoft/mssql-server-linux

Through it, the latest version of MS Server SQL can be downloaded.

5 Run a Docker image

Copy, change and paste the command in Terminal of your Mac:

docker run -d --name xxxxxxx -e 'ACCEPT_EULA=Y' -e 'SA_PASSWORD=xxxx\xxxx' -p 1433:1433 microsoft/mssql-server-linux

We need to change the name and password as your own here.

This step is to run an instance of Docker image. A Docker image is a file, which is used to execute codes in a Docker container.

If you want to check, copy and paste the following command to see whether the Docker container is running:

docker ps

If it works, it will show like this:

6 Install the sql-cli command line tool

The next step is to install the sql-cli command line tool. It can allow you to run commands against your SQL Server instance.

Copy and paste the command in Terminal of your Mac:

npm install -g sql-cli

If an error happened and shows you do not have the permissions to access this file as the current user, just try add sudo in your command:

sudo npm install -g sql-cli

7 Connect to SQL Server

After sql-cli is installed, now we can connect to SQL Server using the mssql command.

mssql -u xxxxxxx -p xxxxxxx

Here xxxxxxx and xxxxxxx means your name and password.

Then you will see:


Now you’ve successfully connected to your instance of SQL Server.

Next blog we will continue our journey with SQL Server and Azure Data Studio. Maybe you will want to read more about Azure and Microsoft Learn in the previous blog.

If you are interested in or have any problems with Business Intelligence, feel free to contact me .

Or you can connect with me through my LinkedIn.

An Introduction To Microsoft Learn

In the beginning, I want to state some reasons why I want to keep writing as a blogger.

Because:

  1. I want to become a better writer.
  2. I feel like I have some professional things to talk about.
  3. I want a new challenge.

I want to improve my communication skills by making professional knowledge easier for readers to understand.

Because the most important thing in communication is not speaking, it is how other people can understand your words.

To achieve this goal, sometimes maybe I will need to sacrifice the accuracy of some information.

However, if the audience can accept some basic terms in the first place, I can express more precise concepts to them.

As a nearly graduate IT student, I feel that there is still a gap between what is learned in universities and the practical skills used in real world

Luckily, the Internet is convenient enough for us to find what we want to learn nowadays.

Practitioners in the IT industry are supposed to maintain the ability to life-long learning.

I have accumulated some self-learning experience, and I want to share some of the learning resources with you.

The official documents are highly recommended: They will provide the latest tutorials, then we can write a few demos to get started. 

Consequently, this blog I want to talk about is Microsoft Learn, a learning platform which provides me with the best experience. 


Microsoft Learn
 is a new learning platform created by Microsoft and it focuses on Azure.

Azure is not free.

Although we can sign up for a free trial account, we have to re-apply it after expiring, which is more troublesome. 

But Azure is widely used in New Zealand.

If you want to find an IT job in New Zealand, understanding and mastering Azure skills should be considered as the basic requirements in most cases.

However, an Azure subscription is indispensable. That’s why I highly recommend Microsoft Learn.

Because it provides free Azure subscriptions and an online Azure Lab in addition to general teaching tutorials.

It means that we can use Azure subscriptions for free, create resources, and use them during the course.

The subscription will be automatically released after a while, probably several hours.

If you haven’t finished, you can continue to learn by recreating another one, which is very convenient.

Once the sandbox is activated, we can use the Azure subscription, and the Azure Cloud Shell interface will appear on the right side of the browser. We can enter commands directly into the browser to operate Azure.

Every time I complete a course, I will get the corresponding badges and scores. I have already reached the 8th level !!

This learning experience seems to play games, and I can’t stop it!

If you are interested in or have any problems about Microsoft Azure, feel free to contact me .

Or you can connect with me through my LinkedIn.