What is a slowly changing dimension (SCD)?

It is a dimension where the attribute values are getting changed slowly over a certain period of time.

Ways of handling slowly changing dimension:

There are 3 major ways are available to handle the data load process for an SCD type dimension when any modification happens in the source system.

1. SCD Type 1

-Modifications will be done on the same record

-Here no history of changes will be maintained

2. SCD Type 2

-An existing record will be marked as expired with is_active flag or Expired_date column

-This type allows tracking the history of changes

3. SCD Type 3

-A new value will be tracked as a column

-Here no history of changes will be maintained

Example scenario:

The below employee table stores the city information about where the employee is currently working/living. The employee location will change in slow pace only.

SCD-Slowly changing dimension SCD-example scenario

Employee 101 is moving Bangalore to Chennai.

Update:

After a certain period of time John is moving to Delhi. To track this change in the dimension table, we have below options.

Type 1 – Update the record same

SCD-Slowly changing dimension SCD-types-1

Type 2 – New record will be inserted with is_active=1, old record is_active=0

SCD-Slowly changing dimension SCD-types-2

Type 2 – New record will be inserted with Expired_date=NULL, old record Expired_date= SYSDATE or SYSDATE-1

Type 3 – New column will be added and tracked the both old and new value

SCD-Slowly changing dimension SCD-types-3

Delete scenario:

After a certain period of time, John is resigned from the company. To track this change in the dimension table, we have below options.

Type 1 – Record will be deleted

Type 2 – The record will be expired by setting is_active=0

Type 2 – The record will be expired by setting expired_date= SYSDATE or SYSDATE-1

Type 3 – All the values will be set to NULL

Advantages and Disadvantages of each SCD Type:

SCD Type1

Advantages -> No additional memory is required

Disadvantage -> We cannot trace back to the history of modifications

SCD Type2

Advantages -> You can trace back to the history

Disadvantages -> The memory is getting consumed since keeping old records

SCD Type3

Advantages -> You can trace back to the history

Disadvantages -> The memory is getting consumed since adding new columns. We cannot add columns to keep every time changes, there is a limitation in number of changes keeping

Key Factors to choose SCD type

Factor	SCD Type
I don’t want to keep the old records	Type1
I want to keep old records but not ready of adding additional columns	Type2
I want to keep old records irrespective of number of changes	Type3
I am more concern about memory size	Type1
I am not worrying about memory size	Type2/Type3