Friday, December 7, 2007

Data Warehouse Interview-4

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.

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

What is the difference between view and materalized 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.

What is aggregate table and aggregate fact table … any examples of both? Aggregate table contains summarised data. The materialized view are aggregated tables.

for ex in sales we have only date transaction. if we want to create a report like sales by product per year. in such cases we aggregate the date vales into week_agg, month_agg, quarter_agg, year_agg. to retrive date from this tables we use @aggrtegate function.

What is active data warehousing? An active data warehouse provides information that enables decision-makers within an organization to manage customer relationships nimbly, efficiently and proactively. Active data warehousing is all about integrating advanced decision support with day-to-day-even minute-to-minute-decision making in a way that increases quality of those customer touches which encourages customer loyalty and thus secure an organization’s bottom line. The marketplace is coming of age as we progress from first-generation “passive” decision-support systems to current- and next-generation “active” data warehouse implementations

Why do we override the execute ethod is struts? Plz give me the details? As part of Struts FrameWork we can decvelop the Action Servlet,ActionForm servlets(here ActionServlet means which class extends the Action class is called ActionServlet and ActionFome means which calss extends the ActionForm calss is called the Action Form servlet)and other servlets classes.

In case of ActionForm class we can develop the validate().this method will return the ActionErrors object.In this method we can write the validation code.If this method return null or ActionErrors with size=0,the webcontainer will call the execute() as part of the Action class.if it returns size > 0 it willnot be call the execute().it will execute the jsp,servlet or html file as value for the input attribute as part of the attribute in struts-config.xml file.

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