Sqlserver interview questions

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Sqlserver interview questions

  1. 1. CROSS JOIN:This join is a Cartesian join that does not necessitate any condition to join. The resultsetcontains records that are multiplication of record number from both the tables.How do you sort in sql:order by" statement can be used to sort columns returned in a SELECT statement. TheORDER BY clause is not valid in views, inline functions, derived tables, and subqueries,unless TOP is also specified.How do you select unique records in SQL?:Using the “DISTINCT” clauseselect distinct FirstName from EmployeesUnion & Union All:The difference between Union and Union all is that Union all will not eliminate duplicaterows, instead it just pulls all rows from all tables fitting your query specifics andcombines them into a table.If you know that all the records returned are unique from your union, use UNION ALLinstead, it gives faster resultsTruncate & Delete:Truncate an Delete both are used to delete data from the table. These both command willonly delete data of the specified table, they cannot remove the whole table datastructure.Both statements delete the data from the table not the structure of the table.TRUNCATE is a DDL (data definition language) command whereas DELETE is a DML(data manipulation language) command.You can use WHERE clause(conditions) with DELETE but you cant use WHERE clausewith TRUNCATE .You cannt rollback data in TRUNCATE but in DELETE you can rollbackdata.TRUNCATE removes(delete) the record permanently.A trigger doesn’t get fired in case of TRUNCATE whereas Triggers get fired in DELETEcommand.Compute Clause in SQL?:
  2. 2. Generates totals that appear as additional summary columns at the end of the result set.When used with BY, the COMPUTE clause generates control-breaks and subtotals in theresult set.USE Database;GOSELECT CustomerID, OrderDate, SubTotal, TotalDueFROM Sales.SalesOrderHeaderWHERE ID = 1ORDER BY OrderDateCOMPUTE SUM(SubTotal), SUM(TotalDue);What is Datawarehousing:In computing, a data warehouse (DW) is a database used for reporting and analysis. Thedata stored in the warehouse is uploaded from the operational systems. The data may passthrough an operational data store for additional operations before it is used in the DW forreporting.A data warehouse maintains its functions in three layers: staging, integration, and access.Staging is used to store raw data for use by developers. The integration layer is used tointegrate data and to have a level of abstraction from users. The access layer is for gettingdata out for users.The term Data Warehouse was coined by Bill Inmon in 1990, which he defined in thefollowing way: "A warehouse is a subject-oriented, integrated, time-variant and non-volatile collection of data in support of managements decision making process". Hedefined the terms in the sentence as follows:Subject Oriented:Data that gives information about a particular subject instead of about a companysongoing operations.Integrated:Data that is gathered into the data warehouse from a variety of sources and merged into acoherent whole.Time-variant:All data in the data warehouse is identified with a particular time period.Non-volatileData is stable in a data warehouse. More data is added but data is never removed. Thisenables management to gain a consistent picture of the business.What is Data Mart: A data mart is the access layer of the data warehouse environment
  3. 3. that is used to get data out to the users. The data mart is a subset of the data warehousewhich is usually oriented to a specific business line or team.A data mart is a simple form of a data warehouse that is focused on a single subject (orfunctional area), such as Sales, Finance, or Marketing. Data marts are often built andcontrolled by a single department within an organization. Given their single-subjectfocus, data marts usually draw data from only a few sources. The sources could beinternal operational systems, a central data warehouse, or external data.A data mart is a repository of data gathered from operational data and other sources thatis designed to serve a particular community of knowledge workers. In scope, the datamay derive from an enterprise-wide database or data warehouse or be more specialized.The emphasis of a data mart is on meeting the specific demands of a particular group ofknowledge users in terms of analysis, content, presentation,and ease-of-use. Users of adata mart can expect to have data presented in terms that are familiar.What are Fact Table & Dimension Tables:In data warehousing, a fact table consists of the measurements, metrics or facts of abusiness process. It is often located at the centre of a star schema or a snowflake schema,surrounded by dimension tables.Fact tables provide the (usually) additive values that act as independent variables bywhich dimensional attributes are analyzed. Fact tables are often defined by their grain.The grain of a fact table represents the most atomic level by which the facts may bedefined. The grain of a SALES fact table might be stated as "Sales volume by Day byProduct by Store". Each record in this fact table is therefore uniquely defined by a day,product and store. Other dimensions might be members of this fact table (such aslocation/region) but these add nothing to the uniqueness of the fact records. These"affiliate dimensions" allow for additional slices of the independent facts but generallyprovide insights at a higher level of aggregation (a region contains many stores).In data warehousing, a dimension table is one of the set of companion tables to a facttable.The fact table contains business facts or measures and foreign keys which refer tocandidate keys (normally primary keys) in the dimension tables.Contrary to fact tables, the dimension tables contain descriptive attributes (or fields)which are typically textual fields or discrete numbers that behave like text. Theseattributes are designed to serve two critical purposes: query constraining/filtering andquery result set labeling.Dimension attributes are supposed to be:Verbose - labels consisting of full words,Descriptive,Complete - no missing values,Discretely valued - only one value per row in dimensional table,Quality assured - no misspelling, no impossible values.
  4. 4. Snake flow schema to Design the tables:In computing, a snowflake schema is a logical arrangement of tables in amultidimensional database such that the entity relationship diagram resembles asnowflake in shape. The snowflake schema is represented by centralized fact tables whichare connected to multiple dimensions.The snowflake schema is similar to the star schema. However, in the snowflake schema,dimensions are normalized into multiple related tables, whereas the star schemasdimensions are normalized with each dimension represented by a single table. A complexsnowflake shape emerges when the dimensions of a snowflake schema are elaborate,having multiple levels of relationships, and the child tables have multiple parent tables("forks in the road"). The "snowflaking" effect only affects the dimension tables and NOTthe fact tables.Processing of ETL Dataware housing:Extract, transform and load (ETL) is a process in database usage and especially in datawarehousing that involves:Extracting data from outside sourcesTransforming it to fit operational needs (which can include quality levels)Loading it into the end target (database or data warehouse)Extract - The first part of an ETL process involves extracting the data from the sourcesystems. In many cases this is the most challenging aspect of ETL, as extracting datacorrectly will set the stage for how subsequent processes will go.Transform - The transform stage applies a series of rules or functions to the extracted datafrom the source to derive the data for loading into the end target.Load - The load phase loads the data into the end target, usually the data warehouse(DW). Depending on the requirements of the organization, this process varies widelyWhat is BCP:The bcp utility copies data between an instance of SQL Server and a data file in a user-specified format.The Bulk Copy Program (BCP) is a command-line utility that ships withMicrosoft SQL Server. With BCP, you can import and export large amounts of data in andout of SQL Server databases quickly and easily.DTS in SQL Server:Data Transformation Services, or DTS, is a set of objects and utilities to allow theautomation of extract, transform and load operations to or from a database. The objectsare DTS packages and their components, and the utilities are called DTS tools. DTS was
  5. 5. included with earlier versions of Microsoft SQL Server, and was almost always used withSQL Server databases, although it could be used independently with other databases.DTS allows data to be transformed and loaded from heterogeneous sources using OLEDB, ODBC, or text-only files, into any supported database. DTS can also allowautomation of data import or transformation on a scheduled basis, and can performadditional functions such as FTPing files and executing external programs. In addition,DTS provides an alternative method of version control and backup for packages whenused in conjunction with a version control system, such as Microsoft Visual SourceSafe .Multi dimensional Analysis:Multidimensional analysis is a data analysis process that groups data into two or morecategories: data dimensions and measurements. For example, a data set consisting of thenumber of wins for a single football team at each of several years is a single-dimensional(in this case, longitudinal) data set. A data set consisting of the number of wins for severalfootball teams in a single year is also a single-dimensional (in this case, cross-sectional)data set. A data set consisting of the number of wins for several football teams overseveral years is a two-dimensional data set.In many disciplines, two-dimensional data sets are also called panel data. While, strictlyspeaking, two- and higher- dimensional data sets are "multi-dimensional," the term"multidimensional" tends to be applied only to data sets with three or more dimensions.For example, some forecast data sets provide forecasts for multiple target periods,conducted by multiple forecasters, and made at multiple horizons. The three dimensionsprovide more information than can be gleaned from two dimensional panel data sets.Bulk Insert:The Bulk Insert task provides an efficient way to copy large amounts of data into a SQLServer table or view. For example, suppose your company stores its million-row productlist on a mainframe system, but the companys e-commerce system uses SQL Server topopulate Web pages. You must update the SQL Server product table nightly with themaster product list from the mainframe. To update the table, you save the product list in atab-delimited format and use the Bulk Insert task to copy the data directly into the SQLServer table.There are primary three ways in which we store information in OLAP:-MOLAPMultidimensional OLAP (MOLAP) stores dimension and fact data in a persistent datastore using compressed indexes. Aggregates are stored to facilitate fast data access.MOLAP query engines are usually proprietary and optimized for the storage format usedby the MOLAP data store. MOLAP offers faster query processing than ROLAP andusually requires less storage. However, it doesn’t scale as well and requires a separate
  6. 6. database for storage.ROLAPRelational OLAP (ROLAP) stores aggregates in relational database tables. ROLAP useof the relational databases allows it to take advantage of existing database resources,plus it allows ROLAP applications to scale well. However, ROLAP’s use of tables tostore aggregates usually requires more disk storage than MOLAP, and it is generally notas fast.HOLAPAs its name suggests, hybrid OLAP (HOLAP) is a cross between MOLAP and ROLAP.Like ROLAP, HOLAP leaves the primary data stored in the source database. LikeMOLAP,HOLAP stores aggregates in a persistent data store that’s separate from the primaryrelational database. This mix allows HOLAP to offer the advantages of both MOLAPand ROLAP. However, unlike MOLAP and ROLAP, which follow well-definedstandards,HOLAP has no uniform implementation.

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