Monday, June 17, 2019

FINAL INTERVIEW QUESTIONS ( Data warehouse) Part-2


What are the types of datawarehousing?

EDW (Enterprise datawarehousing)

1-  It provides a central database for decision support throughout the enterprise
2-   It is a collection of DATAMARTS


1-   It is a subset of Datawarehousing
2-   It is a subject oriented database which supports the needs of individuals depts. in an organizations
3-  It is called high performance query structure
4-   It supports particular line of business like sales, marketing etc..

ODS (Operational data store)
5-  It is defined as an integrated view of operational database designed to support operational monitoring
6-   It is a collection of operational data sources designed to support Transaction processing
7-  Data is refreshed near real-time and used for business activity
8-   It is an intermediate between the OLTP and OLAP which helps to create an instance reports


 What are the types of Approach in DWH?

Bottom up approach: first we need to develop data mart then we integrate these data mart into EDW

Top down approach: first we need to develop EDW then form that EDW we develop data mart
Bottom up

OLTP             ETL                Data mart                   DWH        OLAP

Top down

OLTP             ETL                DWH                Data mart             OLAP

Top down

1-  Cost of initial planning & design is high
2-  Takes longer duration of more than an year

Bottom up

1-  Planning & Designing the Data Marts without waiting for the Global warehouse design
2-  Immediate results from the data marts
3-  Tends to take less time to implement
4-   Errors in critical modules are detected earlier.
5-   Benefits are realized in the early phases.
6-  It is a Best Approach

Data Modeling Types:

1   Conceptual Data Modeling
2   Logical Data Modeling
3   Physical Data Modeling
4   Dimensional Data Modeling

1.    Conceptual Data Modeling

1-   Conceptual data model includes all major entities and relationships and does not contain much detailed level of information about attributes and is often used in the INITIAL PLANNING PHASE
2-  Conceptual data model is created by gathering business requirements from various sources like business documents, discussion with functional teams, business analysts, smart management experts and end users who do the reporting on the database. Data modelers create conceptual data model and forward that model to functional team for their review.
3-  Conceptual data modeling gives an idea to the functional and technical team about how business requirements would be projected in the logical data model.

2.    Logical Data Modeling

1- This is the actual implementation and extension of a conceptual data model. Logical data model includes all required entities, attributes, key groups, and relationships that represent business information and define business rules.

3.    Physical Data Modeling

1-    Physical data model includes all required tables, columns, relationships, database properties for the physical implementation of databases. Database performance, indexing strategy, physical storage and demoralization are important parameters of a physical model.

Logical vs. Physical Data Modeling

Logical Data Model
Physical Data Model
Represents business information and defines business rules
Represents the physical implementation of the model in a database.
Primary Key
Primary Key Constraint
Alternate Key
Unique Constraint or Unique Index
Inversion Key Entry
Non Unique Index
Check Constraint, Default Value
Foreign Key

Dimensional Data Modeling

1-  Dimension model consists of fact and dimension tables
2-    It is an approach to develop the schema DB designs

Types of Dimensional modeling

1-  Star schema
ü   Snow flake schema
ü   Star flake schema (or) Hybrid schema
ü   Multi star schema

What is Star Schema?

ü   The Star Schema Logical database design which contains a centrally located fact table surrounded by at least one or more dimension tables

ü   Since the database design looks like a star, hence it is called star schema db

ü   The Dimension table contains Primary keys and the textual descriptions

ü   It contain de-normalized business information

ü   A Fact table contains a composite key and measures

ü   The measure are of types of key performance indicators which are used to evaluate the enterprise performance in the form of success and failure

ü   Eg: Total revenue , Product sale , Discount given, no of customers

ü   To generate meaningful report the report should contain at least one dimension and one fact table

The advantage of star schema

ü   Less number of joins
ü   Improve query performance
ü   Slicing down
ü   Easy understanding of data.
ü    Require more storage space

Example of Star Schema:

Snowflake Schema

ü   In star schema, If the dimension tables are spitted into one or more dimension tables

ü   The de-normalized dimension tables are spitted into a normalized dimension table

Example of Snowflake Schema:

ü   In Snowflake schema, the example diagram shown below has 4 dimension tables, 4 lookup tables and 1 fact table. The reason is that hierarchies (category, branch, state, and month) are being broken out of the dimension tables (PRODUCT, ORGANIZATION, LOCATION, and TIME) respectively and separately.

ü   It increases the number of joins and poor performance in retrieval of data.

ü   In few organizations, they try to normalize the dimension tables to save space.

ü   Since dimension tables hold less space snow flake schema approach may be avoided.

ü   Bit map indexes cannot be effectively utilized

Important aspects of Star Schema & Snow Flake Schema

ü   In a star schema every dimension will have a primary key.

ü   In a star schema, a dimension table will not have any parent table.

ü   Whereas in a snow flake schema, a dimension table will have one or more parent tables.

ü   Hierarchies for the dimensions are stored in the dimensional table itself in star schema.

ü   Whereas hierarchies are broken into separate tables in snow flake schema. These hierarchies help to drill down the data from topmost hierarchies to the lowermost hierarchies.

Star flake schema (or) Hybrid Schema

ü   Hybrid schema is a combination of Star and Snowflake schema

Multi Star schema

ü   Multiple fact tables sharing a set of dimension tables

ü   Confirmed Dimensions are nothing but Reusable Dimensions.

ü   The dimensions which u r using multiple times or in multiple data marts.

ü   Those are common in different data marts
Measure Types (or) Types of Facts

  • Additive - Measures that can be summed up across all dimensions.
    • Ex: Sales Revenue
  • Semi Additive - Measures that can be summed up across few dimensions and not with others
    • Ex: Current Balance
  • Non Additive - Measures that cannot be summed up across any of the dimensions.
    • Ex: Student attendance

Surrogate Key

ü   Joins between fact and dimension tables should be based on surrogate keys

ü   Users should not obtain any information by looking at these keys

ü   These keys should be simple integers

A sample data warehouse schema

Why need staging area for DWH?

ü   Staging area needs to clean operational data before loading into data warehouse.

ü   Cleaning in the sense your merging data which comes from different source.

ü   It’s the area where most of the ETL is done

Data Cleansing

ü   It is used  to remove duplications

ü   It is used to correct wrong email addresses

ü   It is used to identify missing data

ü   It used to convert the data types

ü   It is used to capitalize name & addresses.

Types of Dimensions:

There are three types of Dimensions

ü   Confirmed Dimensions

ü   Junk Dimensions Garbage Dimension

ü   Degenerative Dimensions

  Slowly changing Dimensions

Garbage Dimension or Junk Dimension

ü   Confirmed is something which can be shared by multiple Fact Tables or multiple Data Marts.

ü   Junk Dimensions is grouping flagged values

ü   Degenerative Dimension is something dimensional in nature but exist fact table.(Invoice No)

  Which is neither fact nor strictly dimension attributes.

These are useful for some kind of analysis. These are kept as attributes in fact table called degenerated dimension

Degenerate dimension: A column of the key section of the fact table that does not have the associated dimension table but used for reporting and analysis, such column is called degenerate dimension or line item dimension.

For ex,

 we have a fact table with customer_id, product_id, branch_id, employee_id, bill_no, and date in key section and price, quantity, amount in measure section. In this fact table, bill_no from key section is a single value; it has no associated dimension table. Instead of creating a
Separate dimension table for that single value, we can Include it in fact table to improve performance. SO here the column, bill_no is a degenerate dimension or line item dimension.


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