Friday, December 7, 2007

Data Warehouse Interview-7

What Snow Flake Schema? Snowflake Schema, each dimension has a primary dimension table, to which one or more additional dimensions can join. The primary dimension table is the only table that can join to the fact table.

What is real time data-warehousing? Real-time data warehousing is a combination of two things: 1) real-time activity and 2) data warehousing. Real-time activity is activity that is happening right now. The activity could be anything such as the sale of widgets. Once the activity is complete, there is data about it.

Data warehousing captures business activity data. Real-time data warehousing captures business activity data as it occurs. As soon as the business activity is complete and there is data about it, the completed activity data flows into the data warehouse and becomes available instantly. In other words, real-time data warehousing is a framework for deriving information from data as the data becomes available.

What are slowly changing dimensions? SCD stands for Slowly changing dimensions. Slowly changing dimensions are of three types

SCD1: only maintained updated values. Ex: a customer address modified we update existing record with new address.

SCD2: maintaining historical information and current information by using
A) Effective Date
B) Versions
C) Flags
or combination of these
scd3: by adding new columns to target table we maintain historical information and current information

What are Semi-additive and factless facts and in which scenario will you use such kinds of fact tables? Snapshot facts are semi-additive, while we maintain aggregated facts we go for semi-additive.

EX: Average daily balance

A fact table without numeric fact columns is called factless fact table.

Ex: Promotion Facts

While maintain the promotion values of the transaction (ex: product samples) because this table doesn’t contain any measures.

Differences between star and snowflake schemas? Star schema - all dimensions will be linked directly with a fat table.
Snow schema - dimensions maybe interlinked or may have one-to-many relationship with other tables.

What is a Star Schema? Star schema is a type of organising the tables such that we can retrieve the result from the database easily and fastly in the warehouse environment.Usually a star schema consists of one or more dimension tables around a fact table which looks like a star,so that it got its name.

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