Data Warehouse Interview Questions

What's A Data warehouse

Answer1:
A Data warehouse is a repository of integrated information, available for queries and analysis. Data and information are extracted from heterogeneous sources as they are generated. This makes it much easier and more efficient to run queries over data that originally came from different sources". Another definition for data warehouse is: " A data warehouse is a logical collection of information gathered from many different operational databases used to create business intelligence that supports business analysis activities and decision-making tasks, primarily, a record of an enterprise's past transactional and operational information, stored in a database designed to favour efficient data analysis and reporting (especially OLAP)". Generally, data warehousing is not meant for current "live" data, although 'virtual' or 'point-to-point' data warehouses can access operational data. A 'real' data warehouse is generally preferred to a virtual DW because stored data has been validated and is set up to provide reliable results to common types of queries used in a business.

Answer2:
Data Warehouse is a repository of integrated information, available for queries and analysis. Data and information are extracted from heterogeneous sources as they are generated....This makes it much easier and more efficient to run queries over data that originally came from different sources.
Typical relational databases are designed for on-line transactional processing (OLTP) and do not meet the requirements for effective on-line analytical processing (OLAP). As a result, data warehouses are designed differently than traditional relational databases.


What is ODS?

1. ODS means Operational Data Store.
2. A collection of operation or bases data that is extracted from operation databases and standardized, cleansed, consolidated, transformed, and loaded into an enterprise data architecture. An ODS is used to support data mining of operational data, or as the store for base data that is summarized for a data warehouse. The ODS may also be used to audit the data warehouse to assure summarized and derived data is calculated properly. The ODS may further become the enterprise shared operational database, allowing operational systems that are being reengineered to use the ODS as there operation databases.


What is a dimension table?

A dimensional table is a collection of hierarchies and categories along which the user can drill down and drill up. it contains only the textual attributes.


What is a lookup table?

A lookUp table is the one which is used when updating a warehouse. When the lookup is placed on the target table (fact table / warehouse) based upon the primary key of the target, it just updates the table by allowing only new records or updated records based on the lookup condition.


Why should you put your data warehouse on a different system than your OLTP system?

Answer1:
A OLTP system is basically " data oriented " (ER model) and not " Subject oriented "(Dimensional Model) .That is why we design a separate system that will have a subject oriented OLAP system...
Moreover if a complex querry is fired on a OLTP system will cause a heavy overhead on the OLTP server that will affect the daytoday business directly.

Answer2:
The loading of a warehouse will likely consume a lot of machine resources. Additionally, users may create querries or reports that are very resource intensive because of the potentially large amount of data available. Such loads and resource needs will conflict with the needs of the OLTP systems for resources and will negatively impact those production systems.

What are Aggregate tables?

Aggregate table contains the summary of existing warehouse data which is grouped to certain levels of dimensions.Retrieving the required data from the actual table, which have millions of records will take more time and also affects the server performance.To avoid this we can aggregate the table to certain required level and can use it.This tables reduces the load in the database server and increases the performance of the query and can retrieve the result very fastly.


What is Dimensional Modelling? Why is it important ?

Dimensional Modelling is a design concept used by many data warehouse desginers to build thier datawarehouse. In this design model all the data is stored in two types of tables - Facts table and Dimension table. Fact table contains the facts/measurements of the business and the dimension table contains the context of measuremnets ie, the dimensions on which the facts are calculated.


Why is Data Modeling Important?

Data modeling is probably the most labor intensive and time consuming part of the development process. Why bother especially if you are pressed for time? A common response by practitioners who write on the subject is that you should no more build a database without a model than you should build a house without blueprints.

The goal of the data model is to make sure that the all data objects required by the database are completely and accurately represented. Because the data model uses easily understood notations and natural language , it can be reviewed and verified as correct by the end-users.

The data model is also detailed enough to be used by the database developers to use as a "blueprint" for building the physical database. The information contained in the data model will be used to define the relational tables, primary and foreign keys, stored procedures, and triggers. A poorly designed database will require more time in the long-term. Without careful planning you may create a database that omits data required to create critical reports, produces results that are incorrect or inconsistent, and is unable to accommodate changes in the user's requirements.


What is data mining?

Data mining is a process of extracting hidden trends within a datawarehouse. For example an insurance dataware house can be used to mine data for the most high risk people to insure in a certain geographial area.


What is ETL?

ETL stands for extraction, transformation and loading.

ETL provide developers with an interface for designing source-to-target mappings, ransformation and job control parameter.
· Extraction
Take data from an external source and move it to the warehouse pre-processor database.
· Transformation
Transform data task allows point-to-point generating, modifying and transforming data.
· Loading
Load data task adds records to a database table in a warehouse.

What does level of Granularity of a fact table signify?

Granularity
The first step in designing a fact table is to determine the granularity of the fact table. By granularity, we mean the lowest level of information that will be stored in the fact table. This constitutes two steps:

Determine which dimensions will be included.
Determine where along the hierarchy of each dimension the information will be kept.
The determining factors usually goes back to the requirements


What is the Difference between OLTP and OLAP?

Main Differences between OLTP and OLAP are:-

1. User and System Orientation

OLTP: customer-oriented, used for data analysis and querying by clerks, clients and IT professionals.

OLAP: market-oriented, used for data analysis by knowledge workers( managers, executives, analysis).

2. Data Contents

OLTP: manages current data, very detail-oriented.

OLAP: manages large amounts of historical data, provides facilities for summarization and aggregation, stores information at different levels of granularity to support decision making process.

3. Database Design

OLTP: adopts an entity relationship(ER) model and an application-oriented database design.

OLAP: adopts star, snowflake or fact constellation model and a subject-oriented database design.

4. View

OLTP: focuses on the current data within an enterprise or department.

OLAP: spans multiple versions of a database schema due to the evolutionary process of an organization; integrates information from many organizational locations and data stores


What is SCD1 , SCD2 , SCD3?

SCD Stands for Slowly changing dimensions.

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.


Why are OLTP database designs not generally a good idea for a Data Warehouse?

Since in OLTP,tables are normalised and hence query response will be slow for end user and OLTP doesnot contain years of data and hence cannot be analysed.


What is BUS Schema?

BUS Schema is composed of a master suite of confirmed dimension and standardized definition if facts.

What are the various Reporting tools in the Market?

1. MS-Excel
2. Business Objects (Crystal Reports)
3. Cognos (Impromptu, Power Play)
4. Microstrategy
5. MS reporting services
6. Informatica Power Analyzer
7. Actuate
8. Hyperion (BRIO)
9. Oracle Express OLAP
10. Proclarity


What is Normalization, First Normal Form, Second Normal Form , Third Normal Form?

1.Normalization is process for assigning attributes to entities–Reducesdata redundancies–Helps eliminate data anomalies–Produces controlledredundancies to link tables

2.Normalization is the analysis offunctional dependency between attributes / data items of userviews?It reduces a complex user view to a set of small andstable subgroups of fields / relations

1NF:Repeating groups must beeliminated, Dependencies can be identified, All key attributesdefined,No repeating groups in table

2NF: The Table is already in1NF,Includes no partial dependencies–No attribute dependent on a portionof primary key, Still possible to exhibit transitivedependency,Attributes may be functionally dependent on non-keyattributes

3NF: The Table is already in 2NF, Contains no transitivedependencies


What is Fact table?

Fact Table contains the measurements or metrics or facts of business process. If your business process is "Sales" , then a measurement of this business process such as "monthly sales number" is captured in the Fact table. Fact table also contains the foriegn keys for the dimension tables.


What are conformed dimensions?

Answer1:
Conformed dimensions mean the exact same thing with every possible fact table to which they are joined Ex:Date Dimensions is connected all facts like Sales facts,Inventory facts..etc

Answer2:
Conformed dimentions are dimensions which are common to the cubes.(cubes are the schemas contains facts and dimension tables)
Consider Cube-1 contains F1,D1,D2,D3 and Cube-2 contains F2,D1,D2,D4 are the Facts and Dimensions here D1,D2 are the Conformed Dimensions


What are the Different methods of loading Dimension tables?

Conventional Load:
Before loading the data, all the Table constraints will be checked against the data.

Direct load:(Faster Loading)
All the Constraints will be disabled. Data will be loaded directly.Later the data will be checked against the table constraints and the bad data won't be indexed.


What is conformed fact?

Conformed dimensions are the dimensions which can be used across multiple Data Marts in combination with multiple facts tables accordingly

What are Data Marts?

Data Marts are designed to help manager make strategic decisions about their business.
Data Marts are subset of the corporate-wide data that is of value to a specific group of users.

There are two types of Data Marts:

1.Independent data marts – sources from data captured form OLTP system, external providers or from data generated locally within a particular department or geographic area.

2.Dependent data mart – sources directly form enterprise data warehouses.


What is a level of Granularity of a fact table?

Level of granularity means level of detail that you put into the fact table in a data warehouse. For example: Based on design you can decide to put the sales data in each transaction. Now, level of granularity would mean what detail are you willing to put for each transactional fact. Product sales with respect to each minute or you want to aggregate it upto minute and put that data.


How are the Dimension tables designed?

Most dimension tables are designed using Normalization principles upto 2NF. In some instances they are further normalized to 3NF.

Find where data for this dimension are located.

Figure out how to extract this data.

Determine how to maintain changes to this dimension (see more on this in the next section).


What are non-additive facts?

Non-Additive: Non-additive facts are facts that cannot be summed up for any of the dimensions present in the fact table.


What type of Indexing mechanism do we need to use for a typical datawarehouse?

On the fact table it is best to use bitmap indexes. Dimension tables can use bitmap and/or the other types of clustered/non-clustered, unique/non-unique indexes.

To my knowledge, SQLServer does not support bitmap indexes. Only Oracle supports bitmaps.


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.


What is a general purpose scheduling tool?

The basic purpose of the scheduling tool in a DW Application is to stream line the flow of data from Source To Target at specific time or based on some condition.

What is ER Diagram?

The Entity-Relationship (ER) model was originally proposed by Peter in 1976 [Chen76] as a way to unify the network and relational database views.

Simply stated the ER model is a conceptual data model that views the real world as entities and relationships. A basic component of the model is the Entity-Relationship diagram which is used to visually represents data objects.

Since Chen wrote his paper the model has been extended and today it is commonly used for database design For the database designer, the utility of the ER model is:

it maps well to the relational model. The constructs used in the ER model can easily be transformed into relational tables. it is simple and easy to understand with a minimum of training. Therefore, the model can be used by the database designer to communicate the design to the end user.

In addition, the model can be used as a design plan by the database developer to implement a data model in a specific database management software.


Which columns go to the fact table and which columns go the dimension table?

The Primary Key columns of the Tables(Entities) go to the Dimension Tables as Foreign Keys.
The Primary Key columns of the Dimension Tables go to the Fact Tables as Foreign Keys.


What are modeling tools available in the Market?

here are a number of data modeling tools

Tool Name Company Name
Erwin Computer Associates
Embarcadero Embarcadero Technologies
Rational Rose IBM Corporation
Power Designer Sybase Corporation
Oracle Designer Oracle Corporation


Name some of modeling tools available in the Market?

These tools are used for Data/dimension modeling

1. Oracle Designer
2. ERWin (Entity Relationship for windows)
3. Informatica (Cubes/Dimensions)
4. Embarcadero
5. Power Designer Sybase


How do you load the time dimension?

Time dimensions are usually loaded by a program that loops through all possible dates that may appear in the data. It is not unusual for 100 years to be represented in a time dimension, with one row per day.

Explain the advanatages of RAID 1, 1/0, and 5. What type of RAID setup would you put your TX logs.

Transaction logs write sequentially and don't need to be read at all. The ideal is to have each on RAID 1/0 because it has much better write performance than RAID 5.

RAID 1 is also better for TX logs and costs less than 1/0 to implement. It has a tad less reliability and performance is a little worse generally speaking.

RAID 5 is best for data generally because of cost and the fact it provides great read capability.


What are the vaious ETL tools in the Market?

Various ETL tools used in market are:

1. Informatica
2. Data Stage
3. MS-SQL DTS(Integrated Services 2005)
4. Abinitio
5. SQL Loader
6. Sunopsis
7. Oracle Warehouse Bulider
8. Data Junction


What is VLDB?

Answer 1:
VLDB stands for Very Large DataBase.

It is an environment or storage space managed by a relational database management system (RDBMS) consisting of vast quantities of information.

Answer 2:
VLDB doesn’t refer to size of database or vast amount of information stored. It refers to the window of opportunity to take back up the database.

Window of opportunity refers to the time of interval and if the DBA was unable to take back up in the specified time then the database was considered as VLDB.

What are Data Marts ?

A data mart is a focused subset of a data warehouse that deals with a single area(like different department) of data and is organized for quick analysis


What are the steps to build the datawarehouse ?

Gathering bussiness requiremnts
Identifying Sources
Identifying Facts
Defining Dimensions
Define Attribues
Redefine Dimensions & Attributes
Organise Attribute Hierarchy & Define Relationship
Assign Unique Identifiers
Additional convetions:Cardinality/Adding ratios


What is Difference between E-R Modeling and Dimentional Modeling.?

Basic diff is E-R modeling will have logical and physical model. Dimensional model will have only physical model.

E-R modeling is used for normalizing the OLTP database design.

Dimensional modeling is used for de-normalizing the ROLAP/MOLAP design.


Why fact table is in normal form?

Basically the fact table consists of the Index keys of the dimension/ook up tables and the measures.

so when ever we have the keys in a table .that itself implies that the table is in the normal form.


What are the advantages data mining over traditional approaches?

Data Mining is used for the estimation of future. For example, if we take a company/business organization, by using the concept of Data Mining, we can predict the future of business interms of Revenue (or) Employees (or) Cutomers (or) Orders etc.

Traditional approches use simple algorithms for estimating the future. But, it does not give accurate results when compared to Data Mining.


What are the vaious ETL tools in the Market?

Various ETL tools used in market are:
Informatica
Data Stage
Oracle Warehouse Bulider
Ab Initio
Data Junction

What is a CUBE in datawarehousing concept?

Cubes are logical representation of multidimensional data.The edge of the cube contains dimension members and the body of the cube contains data values.


What is data validation strategies for data mart validation after loading process ?

Data validation is to make sure that the loaded data is accurate and meets the business requriments.

Strategies are different methods followed to meet the validation requriments


what is the datatype of the surrgate key ?

Datatype of the surrgate key is either inteeger or numaric or number


What is degenerate dimension table?

Degenerate Dimensions : If a table contains the values, which r neither dimesion nor measures is called degenerate dimensions.Ex : invoice id,empno


What is Dimensional Modelling?

Dimensional Modelling is a design concept used by many data warehouse desginers to build thier datawarehouse. In this design model all the data is stored in two types of tables - Facts table and Dimension table. Fact table contains the facts/measurements of the business and the dimension table contains the context of measuremnets ie, the dimensions on which the facts are calculated.


What are the methodologies of Data Warehousing.?

Every company has methodology of their own. But to name a few SDLC Methodology, AIM methodology are stardadly used. Other methodologies are AMM, World class methodology and many more.


What is a linked cube?

Linked cube in which a sub-set of the data can be analysed into great detail. The linking ensures that the data in the cubes remain consistent.

What is the main difference between Inmon and Kimball philosophies of data warehousing?

Both differed in the concept of building teh datawarehosue..

According to Kimball ...

Kimball views data warehousing as a constituency of Data marts. Data marts are focused on delivering business objectives for departments in the organization. And the data warehouse is a conformed dimension of the data marts. Hence a unified view of the enterprise can be obtain from the dimension modeling on a local departmental level.

Inmon beliefs in creating a data warehouse on a subject-by-subject area basis. Hence the development of the data warehouse can start with data from the online store. Other subject areas can be added to the data warehouse as their needs arise. Point-of-sale (POS) data can be added later if management decides it is necessary.

i.e.,
Kimball--First DataMarts--Combined way ---Datawarehouse

Inmon---First Datawarehouse--Later----Datamarts


What is Data warehosuing Hierarchy?

Hierarchies
Hierarchies are logical structures that use ordered levels as a means of organizing data. A hierarchy can be used to define data aggregation. For example, in a time dimension, a hierarchy might aggregate data from the month level to the quarter level to the year level. A hierarchy can also be used to define a navigational drill path and to establish a family structure.

Within a hierarchy, each level is logically connected to the levels above and below it. Data values at lower levels aggregate into the data values at higher levels. A dimension can be composed of more than one hierarchy. For example, in the product dimension, there might be two hierarchies--one for product categories and one for product suppliers.

Dimension hierarchies also group levels from general to granular. Query tools use hierarchies to enable you to drill down into your data to view different levels of granularity. This is one of the key benefits of a data warehouse.

When designing hierarchies, you must consider the relationships in business structures. For example, a divisional multilevel sales organization.

Hierarchies impose a family structure on dimension values. For a particular level value, a value at the next higher level is its parent, and values at the next lower level are its children. These familial relationships enable analysts to access data quickly.

Levels
A level represents a position in a hierarchy. For example, a time dimension might have a hierarchy that represents data at the month, quarter, and year levels. Levels range from general to specific, with the root level as the highest or most general level. The levels in a dimension are organized into one or more hierarchies.

Level Relationships
Level relationships specify top-to-bottom ordering of levels from most general (the root) to most specific information. They define the parent-child relationship between the levels in a hierarchy.

Hierarchies are also essential components in enabling more complex rewrites. For example, the database can aggregate an existing sales revenue on a quarterly base to a yearly aggregation when the dimensional dependencies between quarter and year are known.


What is the main differnce between schema in RDBMS and schemas in DataWarehouse....?

RDBMS Schema
* Used for OLTP systems
* Traditional and old schema
* Normalized
* Difficult to understand and navigate
* Cannot solve extract and complex problems
* Poorly modelled


DWH Schema
* Used for OLAP systems
* New generation schema
* De Normalized
* Easy to understand and navigate
* Extract and complex problems can be easily solved
* Very good model

What is hybrid slowly changing dimension?

Hybrid SCDs are combination of both SCD 1 and SCD 2.

It may happen that in a table, some columns are important and we need to track changes for them i.e capture the historical data for them whereas in some columns even if the data changes, we don't care.

For such tables we implement Hybrid SCDs, where in some columns are Type 1 and some are Type 2.


What are the different architecture of datawarehouse?

There are two main things

1. Top down - (bill Inmon)
2.Bottom up - (Ralph kimbol)


1.what is incremintal loading?
2.what is batch processing?
3.what is crass reference table?
4.what is aggregate fact table?

Incremental loading means loading the ongoing changes in the OLTP.

Aggregate table contains the [measure] values ,aggregated /grouped/summed up to some level of hirarchy.


what is junk dimension? what is the difference between junk dimension and degenerated dimension?

Junk dimension: Grouping of Random flags and text Attributes in a dimension and moving them to a separate sub dimension.

Degenerate Dimension: Keeping the control information on Fact table ex: Consider a Dimension table with fields like order number and order line number and have 1:1 relationship with Fact table, In this case this dimension is removed and the order information will be directly stored in a Fact table inorder eliminate unneccessary joins while retrieving order information..


What are the possible data marts in Retail sales.?

Product information,sales information


What is the definition of normalized and denormalized view and what are the differences between them?

Normalization is the process of removing redundancies.

Denormalization is the process of allowing redundancies.

What is meant by metadata in context of a Datawarehouse and how it is important?

Meta data is the data about data; Business Analyst or data modeler usually capture information about data - the source (where and how the data is originated), nature of data (char, varchar, nullable, existance, valid values etc) and behavior of data (how it is modified / derived and the life cycle ) in data dictionary a.k.a metadata. Metadata is also presented at the Datamart level, subsets, fact and dimensions, ODS etc. For a DW user, metadata provides vital information for analysis / DSS.


Differences between star and snowflake schemas?

Star schema
A single fact table with N number of Dimension

Snowflake schema
Any dimensions with extended dimensions are know as snowflake schema


Difference between Snow flake and Star Schema. What are situations where Snow flake Schema is better than Star Schema to use and when the opposite is true?

Star schema contains the dimesion tables mapped around one or more fact tables.
It is a denormalised model.
No need to use complicated joins.
Queries results fastly.
Snowflake schema
It is the normalised form of Star schema.
contains indepth joins ,bcas the tbales r splitted in to many pieces.We can easily do modification directly in the tables.
We hav to use comlicated joins ,since we hav more tables .
There will be some delay in processing the Query .


What is VLDB?

The perception of what constitutes a VLDB continues to grow. A one terabyte database would normally be considered to be a VLDB.


What's the data types present in bo?n what happens if we implement view in the designer n report

Three different data types: Dimensions,Measure and Detail.
View is nothing but an alias and it can be used to resolve the loops in the universe.


can a dimension table contains numeric values?

Yes.But those datatype will be char (only the values can numeric/char)


What is the difference between view and materialized view?

View - store the SQL statement in the database and let you use it as a table. Everytime you access the view, the SQL statement executes.

Materialized view - stores the results of the SQL in table form in the database. SQL statement only executes once and after that everytime you run the query, the stored result set is used. Pros include quick query results.