SQL Questions 2

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SQL Questions 2
2012-12-30 23:47:07

SQL Stuff all DBAs should know
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  1. What is RDBMS?
    Relational Data Base Management Systems (RDBMS) are database management systems that maintaindata records and indices in tables. Relationships may be created and maintained across and among thedata and tables. In a relational database, relationships between data items are expressed by means oftables. Interdependencies among these tables are expressed by data values rather than by pointers.This allows a high degree of data independence. An RDBMS has the capability to recombine the dataitems from different files, providing powerful tools for data usage.
  2. What is normalization?
    Database normalization is a data design and organization process applied to data structures based onrules that help build relational databases. In relational database design, the process of organizing datato minimize redundancy. Normalization usually involves dividing a database into two or more tables anddefining relationships between the tables. The objective is to isolate data so that additions, deletions,and modifications of a field can be made in just one table and then propagated through the rest of thedatabase via the defined relationships.
  3. What are different normalization forms?
    • 1NF: Eliminate Repeating GroupsMake a separate table for each set of related attributes, and give each table a primary key. Each fieldcontains at most one value from its attribute domain.
    • 2NF: Eliminate Redundant DataIf an attribute depends on only part of a multi-valued key, remove it to a separate table.
    • 3NF: Eliminate Columns Not Dependent On KeyIf attributes do not contribute to a description of the key, remove them to a separate table. Allattributes must be directly dependent on the primary key
    • BCNF: Boyce-Codd Normal FormIf there are non-trivial dependencies between candidate key attributes, separate them out into distincttables.
    • 4NF: Isolate Independent Multiple RelationshipsNo table may contain two or more 1:n or n:m relationships that are not directly related.
    • 5NF: Isolate Semantically Related Multiple RelationshipsThere may be practical constrains on information that justify separating logically related many-to-manyrelationships.
    • ONF: Optimal Normal FormA model limited to only simple (elemental) facts, as expressed in Object Role Model notation.
    • DKNF: Domain-Key Normal FormA model free from all modification anomalies.
    • Remember, these normalization guidelines are cumulative. For a database to be in 3NF, it must firstfulfill all the criteria of a 2NF and 1NF database.
  4. What is Stored Procedure?
    A stored procedure is a named group of SQL statements that have been previously created and storedin the server database. Stored procedures accept input parameters so that a single procedure can beused over the network by several clients using different input data. And when the procedure ismodified, all clients automatically get the new version. Stored procedures reduce network traffic andimprove performance. Stored procedures can be used to help ensure the integrity of the database.e.g. sp_helpdb, sp_renamedb, sp_depends etc.
  5. What is Trigger?
    • A trigger is a SQL procedure that initiates an action when an event (INSERT, DELETE or UPDATE)occurs. Triggers are stored in and managed by the DBMS.Triggers are used to maintain the referential integrity of data by changing the data in a systematic fashion. A trigger cannot be called or executed;the DBMS automatically fires the trigger as a result of a data modification to the associated table.Triggers can be viewed as similar to stored procedures in that both consist of procedural logic that isstored at the database level. Stored procedures, however, are not event-drive and are not attached to aspecific table as triggers are. Stored procedures are explicitly executed by invoking a CALL to theprocedure while triggers are implicitly executed. In addition, triggers can also execute storedprocedures.
    • Nested Trigger: A trigger can also contain INSERT, UPDATE and DELETE logic within itself, so when thetrigger is fired because of data modification it can also cause another data modification, thereby firinganother trigger. A trigger that contains data modification logic within itself is called a nested trigger.
  6. What is View?
    A simple view can be thought of as a subset of a table. It can be used for retrieving data, as well asupdating or deleting rows. Rows updated or deleted in the view are updated or deleted in the table theview was created with. It should also be noted that as data in the original table changes, so does datain the view, as views are the way to look at part of the original table. The results of using a view arenot permanently stored in the database. The data accessed through a view is actually constructed usingstandard T-SQL select command and can come from one to many different base tables or even otherviews.
  7. What is Index?
    • An index is a physical structure containing pointers to the data. Indices are created in an existing tableto locate rows more quickly and efficiently. It is possible to create an index on one or more columns ofa table, and each index is given a name. The users cannot see the indexes, they are just used to speedup queries. Effective indexes are one of the best ways to improve performance in a databaseapplication. A table scan happens when there is no index available to help a query. In a table scan SQLServer examines every row in the table to satisfy the query results. Table scans are sometimesunavoidable, but on large tables, scans have a terrific impact on performance.
    • Clustered indexes define the physical sorting of a database table’s rows in the storage media. For thisreason, each database table may have only one clustered index.
    • Non-clustered indexes are created outside of the database table and contain a sorted list of referencesto the table itself.
  8. What is the difference between clustered and a non-clustered index?
    • A clustered index is a special type of index that reorders the way records in the table are physicallystored. Therefore table can have only one clustered index. The leaf nodes of a clustered index containthe data pages.
    • A nonclustered index is a special type of index in which the logical order of the index does not matchthe physical stored order of the rows on disk. The leaf node of a nonclustered index does not consist ofthe data pages. Instead, the leaf nodes contain index rows.
  9. What are the different index configurations a table can have?
    • A table can have one of the following index configurations:
    • No indexes
    • A clustered index
    • A clustered index and many nonclustered indexes
    • A nonclustered index
    • Many nonclustered indexes
  10. What is cursors?
    • Cursor is a database object used by applications to manipulate data in a set on a row-by-row basis,instead of the typical SQL commands that operate on all the rows in the set at one time.
    • In order to work with a cursor we need to perform some steps in the following order:
    • Declare cursor
    • Open cursor
    • Fetch row from the cursor
    • Process fetched row
    • Close cursor
    • Deallocate cursor
  11. What is the use of DBCC commands?
    DBCC stands for database consistency checker. We use these commands to check the consistency ofthe databases, i.e., maintenance, validation task and status checks.E.g. DBCC CHECKDB - Ensures that tables in the db and the indexes are correctly linked.DBCC CHECKALLOC - To check that all pages in a db are correctly allocated.DBCC CHECKFILEGROUP - Checks all tables file group for any damage.
  12. What is a Linked Server?
    Linked Servers is a concept in SQL Server by which we can add other SQL Server to a Group and queryboth the SQL Server dbs using T-SQL Statements. With a linked server, you can create very clean, easyto follow, SQL statements that allow remote data to be retrieved, joined and combined with local data.Storped Procedure sp_addlinkedserver, sp_addlinkedsrvlogin will be used add new Linked Server.
  13. What is Collation?
    Collation refers to a set of rules that determine how data is sorted and compared. Character data issorted using rules that define the correct character sequence, with options for specifying casesensitivity,accent marks, kana character types and character width.
  14. What are different type of Collation Sensitivity?
    • Case sensitivity
    •     A and a, B and b, etc.
    • Accent sensitivity
    •     a and á, o and ó, etc.
    • Kana Sensitivity
    •     When Japanese kana characters Hiragana and Katakana are treated differently, it is called Kanasensitive.
    • Width sensitivity
    •     When a single-byte character (half-width) and the same character when represented as a double-bytecharacter (full-width) are treated differently then it is width sensitive.
  15. What's the difference between a primary key and a unique key?
    Both primary key and unique enforce uniqueness of the column on which they are defined. But bydefault primary key creates a clustered index on the column, where are unique creates a nonclusteredindex by default. Another major difference is that, primary key doesn't allow NULLs, but unique keyallows one NULL only.
  16. How to implement one-to-one, one-to-many and many-to-many relationships whiledesigning tables?
    • One-to-One relationship can be implemented as a single table and rarely as two tables with primaryand foreign key relationships.
    • One-to-Many relationships are implemented by splitting the data into two tables with primary key andforeign key relationships.
    • Many-to-Many relationships are implemented using a junction table with the keys from both the tablesforming the composite primary key of the junction table.
  17. What is a NOLOCK?
    Using the NOLOCK query optimiser hint is generally considered good practice in order to improveconcurrency on a busy system. When the NOLOCK hint is included in a SELECT statement, no locks aretaken when data is read. The result is a Dirty Read, which means that another process could beupdating the data at the exact time you are reading it. There are no guarantees that your query willretrieve the most recent data. The advantage to performance is that your reading of data will not blockupdates from taking place, and updates will not block your reading of data. SELECT statements takeShared (Read) locks. This means that multiple SELECT statements are allowed simultaneous access, butother processes are blocked from modifying the data. The updates will queue until all the reads havecompleted, and reads requested after the update will wait for the updates to complete. The result toyour system is delay(blocking).
  18. What is difference between DELETE & TRUNCATE commands?
    Delete command removes the rows from a table based on the condition that we provide with a WHEREclause. Truncate will actually remove all the rows from a table and there will be no data in the tableafter we run the truncate command.
    • TRUNCATE is faster and uses fewer system and transaction log resources than DELETE.
    • TRUNCATE removes the data by deallocating the data pages used to store the table’s data, and only thepage deallocations are recorded in the transaction log.
    • TRUNCATE removes all rows from a table, but the table structure and its columns, constraints, indexesand so on remain. The counter used by an identity for new rows is reset to the seed for the column.You cannot use TRUNCATE TABLE on a table referenced by a FOREIGN KEY constraint.Because TRUNCATE TABLE is not logged, it cannot activate a trigger.TRUNCATE can not be Rolled back.TRUNCATE is DDL Command.TRUNCATE Resets identity of the table.
  20. DELETE
    • DELETE removes rows one at a time and records an entry in the transaction log for each deleted row.If you want to retain the identity counter, use DELETE instead. If you want to remove table definitionand its data, use the DROP TABLE statement.
    • DELETE Can be used with or without a WHERE clause
    • DELETE Activates Triggers.
    • DELETE Can be Rolled back.
    • DELETE is DML Command.
    • DELETE does not reset identity of the table.
  21. Difference between Function and Stored Procedure?
    • UDF can be used in the SQL statements anywhere in the WHERE/HAVING/SELECT section where asStored procedures cannot be.
    • UDFs that return tables can be treated as another rowset. This can be used in JOINs with other tables.Inline UDF's can be though of as views that take parameters and can be used in JOINs and otherRowset operations.
  22. When is the use of UPDATE_STATISTICS command?
    This command is basically used when a large processing of data has occurred. If a large amount ofdeletions any modification or Bulk Copy into the tables has occurred, it has to update the indexes totake these changes into account. UPDATE_STATISTICS updates the indexes on these tablesaccordingly.
  23. What types of Joins are possible with Sql Server?
    Joins are used in queries to explain how different tables are related. Joins also let you select data froma table depending upon data from another table.Types of joins: INNER JOINs, OUTER JOINs, CROSS JOINs. OUTER JOINs are further classified as LEFTOUTER JOINS, RIGHT OUTER JOINS and FULL OUTER JOINS.
  24. What is the difference between a HAVING CLAUSE and a WHERE CLAUSE?
    Specifies a search condition for a group or an aggregate. HAVING can be used only with the SELECTstatement. HAVING is typically used in a GROUP BY clause. When GROUP BY is not used, HAVINGbehaves like a WHERE clause. Having Clause is basically used only with the GROUP BY function in aquery. WHERE Clause is applied to each row before they are part of the GROUP BY function in a query.
  25. What is sub-query? Explain properties of sub-query.
    • Sub-queries are often referred to as sub-selects, as they allow a SELECT statement to be executedarbitrarily within the body of another SQL statement. A sub-query is executed by enclosing it in a set ofparentheses. Sub-queries are generally used to return a single row as an atomic value, though theymay be used to compare values against multiple rows with the IN keyword.
    • A subquery is a SELECT statement that is nested within another T-SQL statement. A subquery SELECTstatement if executed independently of the T-SQL statement, in which it is nested, will return a resultset. Meaning a subquery SELECT statement can standalone and is not depended on the statement inwhich it is nested. A subquery SELECT statement can return any number of values, and can be foundin, the column list of a SELECT statement, a FROM, GROUP BY, HAVING, and/or ORDER BY clauses of aT-SQL statement. A Subquery can also be used as a parameter to a function call. Basically a subquerycan be used anywhere an expression can be used.
    • Properties of Sub-Query
    • A subquery must be enclosed in the parenthesis.
    • A subquery must be put in the right hand of the comparison operator, and
    • A subquery cannot contain a ORDER-BY clause.
    • A query can contain more than one sub-queries.
  26. What are types of sub-queries?
    • Single-row subquery, where the subquery returns only one row.
    • Multiple-row subquery, where the subquery returns multiple rows,.and
    • Multiple column subquery, where the subquery returns multiple columns.
  27. What is SQL Profiler?
    • SQL Profiler is a graphical tool that allows system administrators to monitor events in an instance ofMicrosoft SQL Server. You can capture and save data about each event to a file or SQL Server table toanalyze later. For example, you can monitor a production environment to see which stored proceduresare hampering performance by executing too slowly.
    • Use SQL Profiler to monitor only the events in which you are interested. If traces are becoming toolarge, you can filter them based on the information you want, so that only a subset of the event data iscollected. Monitoring too many events adds overhead to the server and the monitoring process and cancause the trace file or trace table to grow very large, especially when the monitoring process takesplace over a long period of time.
  28. What is User Defined Functions?
    User-Defined Functions allow to define its own T-SQL functions that can accept 0 or more parametersand return a single scalar data value or a table data type.
  29. What kind of User-Defined Functions can be created?
    • There are three types of User-Defined functions in SQL Server 2000 and they are Scalar, Inline Table-Valued and Multi-statement Table-valued.
    • Scalar User-Defined Function.  A Scalar user-defined function returns one of the scalar data types. Text, ntext, image and timestampdata types are not supported. These are the type of user-defined functions that most developers areused to in other programming languages. You pass in 0 to many parameters and you get a returnvalue.
    • Inline Table-Value User-Defined Function.  An Inline Table-Value user-defined function returns a table data type and is an exceptional alternativeto a view as the user-defined function can pass parameters into a T-SQL select command and inessence provide us with a parameterized, non-updateable view of the underlying tables.
    • Multi-statement Table-Value User-Defined Function.  A Multi-Statement Table-Value user-defined function returns a table and is also an exceptionalalternative to a view as the function can support multiple T-SQL statements to build the final resultwhere the view is limited to a single SELECT statement. Also, the ability to pass parameters into a TSQLselect command or a group of them gives us the capability to in essence create a parameterized,non-updateable view of the data in the underlying tables. Within the create function command youmust define the table structure that is being returned. After creating this type of user-defined function,It can be used in the FROM clause of a T-SQL command unlike the behavior found when using a storedprocedure which can also return record sets.
  30. Which TCP/IP port does SQL Server run on? How can it be changed?
    SQL Server runs on port 1433. It can be changed from the Network Utility TCP/IP properties –> Portnumber.both on client and the server.
  31. What are the authentication modes in SQL Server? How can it be changed?
    Windows mode and mixed mode (SQL & Windows).  To change authentication mode in SQL Server click Start, Programs, Microsoft SQL Server and click SQLEnterprise Manager to run SQL Enterprise Manager from the Microsoft SQL Server program group.Select the server then from the Tools menu select SQL Server Configuration Properties, and choose theSecurity page.
  32. Where are SQL server users names and passwords are stored in sql server?
    They get stored in master db in the sysxlogins table.
  33. Which command using Query Analyzer will give you the version of SQL server and operatingsystem?
    SELECT SERVERPROPERTY('productversion'), SERVERPROPERTY ('productlevel'), SERVERPROPERTY('edition')
  34. What is SQL server agent?
    SQL Server agent plays an important role in the day-to-day tasks of a database administrator (DBA). Itis often overlooked as one of the main tools for SQL Server management. Its purpose is to ease theimplementation of tasks for the DBA, with its full-function scheduling engine, which allows you toschedule your own jobs and scripts.
  35. Can a stored procedure call itself or recursive stored procedure? How many level SP nesting possible?
    Yes. Because Transact-SQL supports recursion, you can write stored procedures that call themselves.Recursion can be defined as a method of problem solving wherein the solution is arrived at byrepetitively applying it to subsets of the problem. A common application of recursive logic is to performnumeric computations that lend themselves to repetitive evaluation by the same processing steps.Stored procedures are nested when one stored procedure calls another or executes managed code byreferencing a CLR routine, type, or aggregate. You can nest stored procedures and managed codereferences up to 32 levels.
  36. What is @@ERROR?
    The @@ERROR automatic variable returns the error code of the last Transact-SQL statement. If therewas no error, @@ERROR returns zero. Because @@ERROR is reset after each Transact-SQL statement,it must be saved to a variable if it is needed to process it further after checking it.
  37. What is Raiseerror?
    Stored procedures report errors to client applications via the RAISERROR command. RAISERRORdoesn't change the flow of a procedure; it merely displays an error message, sets the @@ERRORautomatic variable, and optionally writes the message to the SQL Server error log and the NTapplication event log.
  38. What is log shipping?
    Log shipping is the process of automating the backup of database and transaction log files on aproduction SQL server, and then restoring them onto a standby server. Enterprise Editions onlysupports log shipping. In log shipping the transactional log file from one server is automatically updatedinto the backup database on the other server. If one server fails, the other server will have the same dbcan be used this as the Disaster Recovery plan. The key feature of log shipping is that is willautomatically backup transaction logs throughout the day and automatically restore them on thestandby server at defined interval.
  39. What is the difference between a local and a global variable?
    • local variable is the way you normally use variables in sql Declare @effective_dt. SET @effective_dt = '01-01-2012'. It lasts for the duration of the connection, or if defined in the sql statement, for duration of the sql statement.
    • SQL Server provides a massive number of global variables, which are very effective to use in our regular Transact-SQL. Global variables represent a special type of variable. The server always maintain the values of these variables. All the global variables represent information specific to the server or a current user session. Global variable names begin with a @@ prefix. You do not need to declare them, since the server constantly maintains them. They are system-defined functions and you cannot declare them.
    • http://www.codeproject.com/Articles/39131/Global-Variables-in-SQL-Server for a list of global variables
  40. What command do we use to rename a db?
    • sp_renamedb ‘oldname’ , ‘newname’
    • If someone is using db it will not accept sp_renmaedb. In that case first bring db to single user usingsp_dboptions. Use sp_renamedb to rename database. Use sp_dboptions to bring database to multi usermode.
  41. What is sp_configure commands and set commands?
    Use sp_configure to display or change server-level settings. To change database-level settings, useALTER DATABASE. To change settings that affect only the current user session, use the SET statement.
  42. What are the different types of replication? Explain.
    • · Transactional· Snapshot· Merge
    • General concepts. Replication is the process whereby data is copied between databases on the same server or different servers connected by LANs, WANs, or the Internet. Microsoft SQL Server replication uses a publish, distributor and subscribe metaphor. Publisher is the server or database that sends its data to another server or database.Subscriber is the server or database that receives data from another server or database.Distributor is the server that manages the flow of data through the replication system. This server contains the distribution database.Publisher contain publication/publications. Publication is a collection of one or more articles that is sent to a subscriber server or database. Article is the basic unit of replication and can be a table or a subset of a table.Subscription is the group of data that a server or database will receive. This can be one or more publications. There are push and pull subscriptions. Push subscription is subscription when the publishing server will periodically push transactions out to the subscribing server or database. Pull subscription is subscription when the subscribing server will periodically connect to the distribution database and pull information.
    • Snapshot replication distributes data exactly as it appears at a specific moment in time and does notmonitor for updates to the data. Snapshot replication is best used as a method for replicating data thatchanges infrequently or where the most up-to-date values (low latency) are not a requirement. Whensynchronization occurs, the entire snapshot is generated and sent to Subscribers.
    • Transactional replication, an initial snapshot of data is applied at Subscribers, and then when datamodifications are made at the Publisher, the individual transactions are captured and propagated toSubscribers.
    •     Transactional replication is also known as dynamic replication. In transactional replication, modifications to the publication at the publisher are propagated to the subscriber incrementally.    Publisher and the subscriber are always in synchronization and should always be connected.    This type is mostly used when subscribers always need the latest data for processing.
    • Merge replication.  It allows making autonomous changes to replicated data on the Publisher and on the Subscriber. With merge replication, SQL Server captures all incremental data changes in the source and in the target databases, and reconciles conflicts according to rules you configure or using a custom resolver you create. Merge replication is best used when you want to support autonomous changes on the replicated data on the Publisher and on the Subscriber.Replication agents involved in merge replication are snapshot agent and merge agent.Implement merge replication if changes are made constantly at the publisher and subscribing servers, and must be merged in the end.By default, the publisher wins all conflicts that it has with subscribers because it has the highest priority. The conflict resolver can be customized.
  43. What are the OS services that the SQL Server installation adds?
    MS SQL SERVER SERVICE, SQL AGENT SERVICE, DTC (Distribution transac co-ordinator)
  44. What are three SQL keywords used to change or set someone’s permissions?
  45. What does it mean to have quoted_identifier on? What are the implications of having it off?
    When SET QUOTED_IDENTIFIER is ON, identifiers can be delimited by double quotation marks, andliterals must be delimited by single quotation marks. When SET QUOTED_IDENTIFIER is OFF, identifierscannot be quoted and must follow all Transact-SQL rules for identifiers. If ON then you can create a table named table and a field name named function if surrounded by double quotes. Not sure why you would do that.
  46. What are identifiers and literals?
    • An Identifier is essentially a name of a database, table, or table column
    • Literal is a term for data types such as strings, numbers, or
    • boolean values in SQL. These values are not named by you the
    • programmer; they just exist.
  47. What is the STUFF function and how does it differ from the REPLACE function?
    STUFF function to overwrite existing characters. Using this syntax, STUFF(string_expression, start,length, replacement_characters), string_expression is the string that will have characters substituted,start is the starting position, length is the number of characters in the string that are substituted, andreplacement_characters are the new characters interjected into the string.REPLACE function to replace existing characters of all occurance. Using this syntaxREPLACE(string_expression, search_string, replacement_string), where every incidence ofsearch_string found in the string_expression will be replaced with replacement_string.
  48. Using query analyzer, name 3 ways to get an accurate count of the number of records in atable?
    SELECT * FROM table1SELECT COUNT(*) FROM table1SELECT rows FROM sysindexes WHERE id = OBJECT_ID(table1) AND indid < 2
  49. How to rebuild Master Database?
    • Rebuilding or Restoring The SQL Server 2008 Master Database Here you will find information on how to rebuild the SQL Server 2008 Master Database. One of the improvements to SQL Server 2008 is that when you install it the SQL databases are copied from the installation media to the Program Files directory on your server and placed in the following location: Microsoft SQL ServerMSSQL10.MSSQLSERVERMSSQLBinnTemplates In the previous versions of SQL server if you had to rebuild your Master database you had to have the installation media in order to do so. With SQL Server 2008 this is no longer the case. The Master, Model and MsdbData primary data files and their transaction log files are all locally available. 
    • Follow the steps below to rebuild the Master Database. 
    • 1. From a command prompt window change to the following directory: "C:Program FilesMicrosoft SQL Server100Setup BootstrapRelease" 
    • 2. Next paste the following into the command prompt window and press “Enter”: Setup.Exe /Action=RebuildDatabase /InstanceName=MSSQLSERVER /SqlSysAdminAccounts=Admin 
    • The instance name will be MSSQLSERVER unless you are using a named instance and in that case change MSSQLSERVER to your instance name and the SqlSysAdminAccounts will be an account with SysAdmin rights then and press “Enter” again. 
    • Note: You can also use the /Quiet switch but for greater control I do not use it.
    • When the rebuild has completed review the Summary.Txt file in the Program FilesMicrosoft SQL Server100Setup BootstrapLog directory and you can also review the files in then YYYYMMDD_xxxxxx directory as well.
  50. What is the basic functions for master, msdb, model, tempdb databases?
    • The Master database holds information for all databases located on the SQL Server instance and is theglue that holds the engine together. Because SQL Server cannot start without a functioning masterdatabase, you must administer this database with care.
    • The msdb database stores information regarding database backups, SQL Agent information, DTSpackages, SQL Server jobs, and some replication information such as for log shipping.
    • The tempdb holds temporary objects such as global and local temporary tables and stored procedures.
    • The model is essentially a template database used in the creation of any new user database created inthe instance.
  51. What are primary keys and foreign keys?
    • Primary keys are the unique identifiers for each row. They must contain unique values and cannot benull. Due to their importance in relational databases, Primary keys are the most fundamental of all keysand constraints. A table can have only one Primary key.
    • Foreign keys are both a method of ensuring data integrity and a manifestation of the relationshipbetween tables.
  52. What is data integrity? Explain constraints?
    • Data integrity is an important feature in SQL Server. When used properly, it ensures that data is accurate, correct, and valid. It also acts as a trap for otherwise undetectable bugs within applications.
    • A PRIMARY KEY constraint is a unique identifier for a row within a database table. Every table shouldhave a primary key constraint to uniquely identify each row and only one primary key constraint can becreated for each table. The primary key constraints are used to enforce entity integrity.
    • A primary key is a column, or a combination of columns, that can uniquely identify a row. It is a special case of unique key. A table can have at most one primary key, but more than one unique key. When you specify a unique key on a column, no two distinct rows in a table can have the same value.
    • A FOREIGN KEY constraint prevents any actions that would destroy links between tables with thecorresponding data values. A foreign key in one table points to a primary key in another table. Foreignkeys prevent actions that would leave rows with foreign key values when there are no primary keys with that value. The foreign key constraints are used to enforce referential integrity.
    • A CHECK constraint is used to limit the values that can be placed in a column. The check constraintsare used to enforce domain integrity.
    • A NOT NULL constraint enforces that the column will not accept null values. The not null constraintsare used to enforce domain integrity, as the check constraints.
  53. What are the properties of the Relational tables?
    • Values Are Atomic This property implies that columns in a relational table are not repeating group or arrays. The key benefit of the one value property is that it simplifies data manipulation logic. Such tables are referred to as being in the “first normal form” (1NF).
    • Column Values Are of the Same Kind In relational terms this means that all values in a column come from the same domain. A domain is a set of values which a column may have. This property simplifies data access because developers and users can be certain of the type of data contained in a given column. It also simplifies data validation. Because all values are from the same domain, the domain can be defined and enforced with the Data Definition Language (DDL) of the database software.
    • Each Row is Unique This property ensures that no two rows in a relational table are identical; there is at least one column, or set of columns, the values of which uniquely identify each row in the table. Such columns are called primary keys. This property guarantees that every row in a relational table is meaningful and that a specific row can be identified by specifying the primary key value.
    • The Sequence of Columns is Insignificant This property states that the ordering of the columns in the relational table has no meaning. Columns can be retrieved in any order and in various sequences. The benefit of this property is that it enables many users to share the same table without concern of how the table is organized. It also permits the physical structure of the database to change without affecting the relational tables.
    • The Sequence of Rows is Insignificant This property is analogous the one above but applies to rows instead of columns. The main benefit is that the rows of a relational table can be retrieved in different order and sequences. Adding information to a relational table is simplified and does not affect existing queries.
    • Each Column Has a Unique Name Because the sequence of columns is insignificant, columns must be referenced by name and not by position. A column name need not be unique within an entire database but only within the table to which it belongs.
  54. What is De-normalization?
    De-normalization is the process of attempting to optimize the performance of a database by addingredundant data. It is sometimes necessary because current DBMSs implement the relational modelpoorly. A true relational DBMS would allow for a fully normalized database at the logical level, whileproviding physical storage of data that is tuned for high performance. De-normalization is a techniqueto move from higher to lower normal forms of database modeling in order to speed up database access.
  55. How to get @@error and @@rowcount at the same time?
    If @@Rowcount is checked after Error checking statement then it will have 0 as the value of@@Recordcount as it would have been reset.And if @@Recordcount is checked before the error-checking statement then @@Error would get reset.To get @@error and @@rowcount at the same time do both in same statement and store them in localvariable. SELECT @RC = @@ROWCOUNT, @ER = @@ERROR
  56. What is Identity?
    • CREATE TABLE dbo.Yaks ( YakID smallint identity(7,2), YakName char(20) )
    • The identity clause specifies that the column YakID is going to be an identity column. The first record added will automatically be assigned a value of 7 (the seed) and each subsequent record will be assigned a value 2 higher (the increment) than the previous inserted row. Most identity columns I see are specified as IDENTITY(1,1) but I used IDENTITY(7,2) so the difference would be clear. If you don't specify the identity and seed they both default to 1. Identity columns can be int, bigint, smallint, tinyint, or decimal or numeric with a scale of 0 (i.e. no places to the right of the decimal).
  57. What is a Scheduled Jobs or What is a Scheduled Tasks?
    Scheduled tasks let user automate processes that run on regular or predictable cycles. User canschedule administrative tasks, such as cube processing, to run during times of slow business activity.User can also determine the order in which tasks run by creating job steps within a SQL Server Agentjob. E.g. Back up database, Update Stats of Tables. Job steps give user control over flow of execution. If one job fails, user can configure SQL Server Agent to continue to run the remaining tasks or to stopexecution.
  58. What is a table called, if it does not have neither Cluster nor Non-cluster Index? What is itused for?
    Unindexed table or Heap. Microsoft Press Books and Book On Line (BOL) refers it as Heap.A heap is a table that does not have a clustered index and, therefore, the pages are not linked bypointers. The IAM pages are the only structures that link the pages in a table together.Unindexed tables are good for fast storing of data. Many times it is better to drop all indexes from tableand than do bulk of inserts and to restore those indexes after that.
  59. What is BCP? When does it used?
    BCP (bulk copy program). BCP is a utility that installs with SQL Server and can assist with large data transfers.
  60. Can we rewrite subqueries into simple select statements or with joins?
    Subqueries can often be re-written to use a standard outer join, resulting in faster performance. As wemay know, an outer join uses the plus sign (+) operator to tell the database to return all non-matchingrows with NULL values. Hence we combine the outer join with a NULL test in the WHERE clause toreproduce the result set without using a sub-query.
  61. Can SQL Servers linked to other servers like Oracle?
    • SQL Server can be lined to any server provided it has OLE-DB provider from Microsoft to allow a link.E.g. Oracle has a OLE-DB provider for oracle that Microsoft provides to add it as linked server to SQLServer group.
    • An OLE DB provider is a software component enabling an OLE DB consumer to interact with a data source. OLE DB providers are analogous to ODBC drivers, JDBC drivers, and ADO.NET data providers
  62. How to know which index a table is using?
    SELECT table_name,index_name FROM user_constraints
  63. How to copy the tables, schema and views from one SQL server to another?
    Microsoft SQL Server 2000 Data Transformation Services (DTS) is a set of graphical tools andprogrammable objects that lets user extract, transform, and consolidate data from disparate sourcesinto single or multiple destinations.
  64. What is Self Join?
    This is a particular case when one table joins to itself, with one or two aliases to avoid confusion. A selfjoin can be of any type, as long as the joined tables are the same. A self join is rather unique in that itinvolves a relationship with only one table. The common example is when company have a hierarchalreporting structure whereby one member of staff reports to another.
  65. What is Cross Join?
    A cross join that does not have a WHERE clause produces the Cartesian product of the tables involvedin the join. The size of a Cartesian product result set is the number of rows in the first table multipliedby the number of rows in the second table. The common example is when company wants to combineeach product with a pricing table to analyze each product at each price.
  66. Which virtual table does a trigger use?
    Inserted and Deleted.
  67. List few advantages of Stored Procedure.
    • · Stored procedure can reduced network traffic and latency, boosting application performance.
    • · Stored procedure execution plans can be reused, staying cached in SQL Server's memory,reducing server overhead.
    • · Stored procedures help promote code reuse.
    • · Stored procedures can encapsulate logic. You can change stored procedure code withoutaffecting clients.
    • · Stored procedures provide better security to your data.
  68. What is DataWarehousing?
    • · Subject-oriented, meaning that the data in the database is organized so that all the dataelements relating to the same real-world event or object are linked together;
    • · Time-variant, meaning that the changes to the data in the database are tracked and recordedso that reports can be produced showing changes over time;
    • · Non-volatile, meaning that data in the database is never over-written or deleted, oncecommitted, the data is static, read-only, but retained for future reporting;
    • · Integrated, meaning that the database contains data from most or all of an organization'soperational applications, and that this data is made consistent.
  69. What is OLTP(OnLine Transaction Processing)?
    In OLTP - online transaction processing systems relational database design use the discipline of datamodeling and generally follow the Codd rules of data normalization in order to ensure absolute dataintegrity. Using these rules complex information is broken down into its most simple structures (a table)where all of the individual atomic level elements relate to each other and satisfy the normalizationrules.
  70. How do SQL server 2000 and XML linked? Can XML be used to access data?FOR XML (ROW, AUTO, EXPLICIT)
    You can execute SQL queries against existing relational databases to return results as XML rather thanstandard rowsets. These queries can be executed directly or from within stored procedures. To retrieveXML results, use the FOR XML clause of the SELECT statement and specify an XML mode of RAW, AUTO,or EXPLICIT.
    OPENXML is a Transact-SQL keyword that provides a relational/rowset view over an in-memory XMLdocument. OPENXML is a rowset provider similar to a table or a view. OPENXML provides a way toaccess XML data within the Transact-SQL context by transferring data from an XML document into therelational tables. Thus, OPENXML allows you to manage an XML document and its interaction with therelational environment.
  72. What is an execution plan? When would you use it? How would you view the execution plan?
    An execution plan is basically a road map that graphically or textually shows the data retrieval methodschosen by the SQL Server query optimizer for a stored procedure or ad-hoc query and is a very usefultool for a developer to understand the performance characteristics of a query or stored procedure sincethe plan is the one that SQL Server will place in its cache and use to execute the stored procedure orquery. From within Query Analyzer is an option called "Show Execution Plan" (located on the Querydrop-down menu). If this option is turned on it will display query execution plan in separate windowwhen query is ran again.
  73. What are the 3 normal forms?
    • No repeating elements or groups of elements. Basically you're not listing a bunch of items associated with a single order. No column can contain a list, like you're trying to put 3 items in one attribute. You have to give each item it's own row to be NF1. Each row of data must also have a unique identifier (or Primary Key). this can be concatenated, in that 2 keys make the row unique. Order_id and Item_id.
    • No partial dependencies on a concatenated key. This means that for a table that has a concatenated primary key,each column in the table that is not part of the primary key must depend upon the entire concatenated key for its existence. If any column only depends upon one part of the concatenated key, then we say that the entire table has failed Second Normal Form and we must create another table to rectify the failure. ie Order date relies on order_id, but not item_id.  An order date needs and order, but no item. Give item_id and all it's dependencies (whole or part) a separate table. NF2 just makes you deal with 2 PK tables. If partially dependent then something moves to a diff table.
    • No dependencies on non-key attributes. If customer information is still part of an orders table, for example. customer name would rely on customer_id, not the order_id, so it's dependent on a non-key attribute. order_id would be the PK key attribute.