## The Three Key Ideas about Warehouse

The purpose of the data warehouse should be clear. In most cases, it is for the analysis of data, not for data production.1

1. Subject-oriented: since data warehouses are for decision-makers, arrange them into subjects makes it much easier to access.
2. Integrated: many sources are integrated for easy analysis
3. Time-variant: observation time should be recorded since the data is also used to analyze the time evolution
4. Nonvolatile: simply for analysis

## OLTP and OLAP

1. OLTP: online transaction processing
2. OLAP: online analytical processing
OLTP OLAP
user customer data scientist, managers
purpose production analysis
content everything cleaner data
database entity relation model, application-oriented star/snowflake model, subject-oriented
history usually no need to record the history history is crucial
query short and frequent read and write read-only and but complicated analysis

## Scope of Data Warehouse

1. Enterprise warehouse: targeting the whole organization
2. Data mart: for a specific group of people
3. Virtual warehouse: views not tables

## Fact and Dimension

Fact is the value of something specified by the dimension. For example, we have a table that tells us the revenue of the year 2018 for keyboards is 3 million euros.

Year Category Revenue
2017 Keyboard 2,500,000
2018 Keyboard 3,000,000

The columns Year and Category serve as dimensions and Revenue serves as the fact.

The dimension tells us what we are talking about and the fact tells us the values. For the previous example, we could set up a coordinate system, with two axes, Year and Category. On the Year axis, we have two values, 2017 and 2018. On the Category axis, we have two values Keyboard and Headset. If we are looking for the revenue of keyboards in 2018, then we are looking for 2018 and Headset. It is clearly a 2 dimensional data cube.

In principle, we could have N-dimensional data cube. For example, we would like to look at the revenue of

1. keyboards (Category),
2. in 2018 (Year),
3. in Cologne (City),
4. sold offline (Channel).

Then we have a 4-dimensional dataset. Revenue is the fact and Category, Year, City and Channel are the dimensions.

## Concept Hierarchies

Concept hierarchies are crucial to OLAP operations.

We consider a data cube about revenue with the dimension Date with values such as 2018-11-26. We would like to know the monthly revenue, the quarterly revenue, and the annual revenue. It’s straightforward to calculate those. The idea is that monthly, quarterly, and annual revenue form a hierarchy: day $\in$ month $\in$ quarter $\in$ year. This is a convention used by everyone on this planet.

In some other cases, concept hierarchies are very much designed instead of being a convention. Suppose we record the prices of houses. It would be nice to categorize the prices into different ranges of prices. Some would say, we divide them into five ranges:

1. below 25 percentile
2. 25 to 50 percentile
3. 50 to 75 percentile
4. above 75 percentile

Others would say, we divide them into 100 euros segments

1. 0 to 100 euros
2. 100 to 200 euros

1. In fact, this is not always the case. Sometimes, data products can be delivered using some assist of data warehouse if data product doesn’t rely on the performance of the data source. ↩︎

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L Ma (2018). 'Some Concepts about Data Warehouse', Datumorphism, 11 April. Available at: https://datumorphism.leima.is/wiki/data-warehouse/data-warehouse-concepts/.