1. What are the different types of Joins?
Ans. Joins as used to combine the contents of two or more tables and produce a result set that incorporates rows and columns from each table. Tables are typically joined using data that they have in common Join conditions can be specified in either the FROM or WHERE clauses; specifying them in the FROM clause is recommended. WHERE and HAVING clauses can also contain search conditions to further filter the rows selected by the join conditions.

Joins can be categorized as:

Inner joins – (the typical join operation, which uses some comparison operator like = or <>). An inner join is a join in which the values in the columns being joined are compared using a comparison operator Inner joins use a comparison operator to match rows from two tables based on the values in common columns from each table.

Equi Join – It returns all the columns in both tables, and returns only the rows for which there is an equal value in the join column

   1: ( SELECT * FROM authors AS a 
   2:     INNER JOIN publishers AS p 
   3:     ON a.city = p.city 
   4:     ORDER BY a.au_lname DESC )

Self Join – A table can be joined to itself in a self-join

Outer joins – Outer joins can be a left, right, or full outer join. Outer joins are specified with one of the following sets of keywords when they are specified in the FROM clause:

LEFT JOIN or LEFT OUTER JOIN – The result set of a left outer join includes all the rows from the left table specified in the LEFT OUTER clause, not just the ones in which the joined columns match. When a row in the left table has no matching rows in the right table, the associated result set row contains null values for all select list columns coming from the right table.

RIGHT JOIN or RIGHT OUTER JOIN -  A right outer join is the reverse of a left outer join. All rows from the right table are returned. Null values are returned for the left table any time a right table row has no matching row in the left table.

FULL JOIN or FULL OUTER JOIN -   A full outer join returns all rows in both the left and right tables. Any time a row

has no match in the other table, the select list columns from the other table contain null values. When there is a match between the tables,the entire result set row contains data values from the base tables.

Cross joins – Cross joins return all rows from the left table, each row from the left table is combined with all rows from the right table. Cross joins are also called Cartesian products.

2. What is diff. between left and Right outer Join?

Ans. The result set of a left outer join includes all the rows from the left table specified in the LEFT OUTER clause , while in case of Right outer join all the rows from the right table are returned in the result set.

3. Why we use Unicode In Sql server?

Ans.Using Unicode data types, a column can store any character that is defined by the Unicode Standard, which includes all of the characters that are defined in the various character sets. Unicode data types take twice as much storage space as non-Unicode data types.

Unicode data is stored using the nchar, nvarchar, and ntext data types in SQL Server. Use these data types for columns that store characters from more than one character set. The SQL Server Unicode data types are based on the National Character data types in the SQL-92 standard

4.What is Fill Factor and its value when the index is created.

Ans.An option used when creating an index to reserve free space on each page of the index. FILLFACTOR accommodates future expansion of table data and reduces the potential for page splits. FILLFACTOR is a value from 1 through 100 that specifies the percentage of the index page to be left empty.Default is 0.

A fill factor value of 0 does not mean that pages are 0 percent full. It is treated similarly to a fill factor value of 100 in that SQL Server creates clustered indexes with full data pages and non clustered indexes with full leaf pages.If you set fill factor to 100, SQL Server creates both clustered and nonclustered indexes with each page 100 percent full. Setting fill factor to 100 is suitable only for read-only tables, to which additional data is never added.

5.What are the different types of File backup options?

Ans. All data and objects in the database, such as tables, stored procedures, triggers, and views, are stored only within the following operating system files:

Primary

This file contains the startup information for the database and is used to store data. Every database has one primary data file.

Secondary

These files hold all of the data that does not fit in the primary data file. If the primary file can hold all of the data in the database, databases do not need to have secondary data files. Some databases may be large enough to need multiple secondary data files or to use secondary files on separate disk drives to spread data across multiple disks.

Transaction Log

These files hold the log information used to recover the database. There must be at least one log file for each database. File groups allow files to be grouped together for administrative and data allocation/placement purposes

Rules of Files and Filegroups

Rules for designing files and filegroups include:

• A file or filegroup cannot be used by more than one database. For example, file sales.mdf and sales.ndf, which contain data and objects from the sales database, cannot be used by any other database.

• A file can be a member of only one filegroup.

• Data and transaction log information cannot be part of the same file or filegroup.

• Transaction log files are never part of any filegroups.

Types of BackUP

Database

Transaction log

Differential

Filegroup & Backup

6.What is Differential File group backup?

Ans.Differential database backup records only those data changes made to the database after the last full database backup. A differential database backup is smaller and takes less time to complete than a database backup. By creating differential database backups more frequently than database backups, you can decrease the amount of data you risk losing.

7.What is Collate?

Ans.A clause that can be applied to a database definition or a column definition to define the collation, or to a character string expression to apply a collation cast. The COLLATE clause can be applied only for the char, varchar, text, nchar, nvarchar, and ntext data types. The physical storage of character strings in Microsoft® SQL Server™ 2000 is controlled by collations. A collation specifies the bit patterns that represent each character and the rules by which characters are sorted and compared

8. What is Code Page?

Ans.A character set that a computer uses to interpret and display characters, often to handle international characters. Essentially, it is a table of characters and corresponding numbers in memory that the computer uses to display data properly. Different languages and locales may use different code pages.

9.When we should go for with Recompile option in Stored Procedures?

Ans.When stored procedures take parameters whose values differ widely between executions of the stored procedure, resulting in different execution plans to be created each time.  If the parameter you are supplying is atypical or if the data has significantly changed since the stored procedure was created. Use of this option is uncommon, and causes the stored procedure to execute more slowly because the stored procedure must be recompiled each time it is executed. Creating a stored procedure that specifies the WITH RECOMPILE option in its definition indicates that SQL Server does not cache a plan for this stored procedure; the stored procedure is recompiled each time it is executed.

10.What is difference between Having and Group by Clause?

Ans. The HAVING clause sets conditions on the GROUP BY clause similar to the way WHERE interacts with SELECT. The WHERE search condition is applied before the grouping operation occurs; the HAVING search condition is applied after the grouping operation occurs. The HAVING syntax is exactly like the WHERE syntax, except HAVING can contain aggregate functions.

HAVING clauses can reference any of the items that appear in the select list.

• The WHERE clause is used to filter the rows that result from the operations specified in the FROM clause.

• The GROUP BY clause is used to group the output of the WHERE clause.

• The HAVING clause is used to filter rows from the grouped result

Where – Groupby – Having – Order By

11.What are Isolation levels?

Ans. An isolation level determines the degree to which data is isolated for use by one process and guarded against interference from other processes.

Read Committed - SQL Server acquires a share lock while reading a row into a cursor but frees the lock immediately after reading the row. Because a shared lock request is blocked by an exclusive lock, a cursor is prevented from reading a row that another task has updated but not yet committed. Read committed is the default isolation level setting for both SQL Server and ODBC.

Read Uncommitted - SQL Server requests no locks while reading a row into a cursor and honors no exclusive locks. Cursors can be populated with values that have already been updated but not yet  committed. The user is bypassing all of SQL Server’s locking transaction control mechanisms.

Repeatable Read or Serializable – SQL Server requests a shared lock on each row as it is read into the cursor as in READ COMMITTED, but if the cursor is opened within a transaction, the shared locks are held until the end of the transaction instead of being freed after the row is read. This has the same effect as specifying HOLDLOCK on a SELECT statement.

12.When should we go for NOLOCK hint?

Ans. A range of table-level locking hints can be specified using the SELECT, INSERT, UPDATE, and DELETE statements to direct Microsoft® SQL Server™ 2000 to the type of locks to be used. Table-level locking hints can be used when a finer control of the types of locks acquired on an object is required. These locking hints override the current transaction isolation level for the session.

NOLOCK Do not issue shared locks and do not honor exclusive locks. When this option is in effect, it is possible to read an uncommitted transaction or a set of pages that are rolled back in the middle of a read. Dirty reads are possible. Only applies to the SELECT statement.

13. What are diff. types of Database import?

Ans.BCP, DTS, BulkCopy Command

14.Diff. between BCP and DTS, which one is advantageous?

Ans. BCP is a command prompt utility. bcp provides for running bulk copies in .bat and .cmd scripts. bcp is used to bulk copy large files into tables or views in SQL Server databases. BCP is non logged operation.

DTS allows the user to program transformations through two different interfaces.

BCP is extremely fast and has a minimal overhead, but it also has a very rigid (and sometimes unforgiving) syntax. Then along came Data Transformation Services (DTS), an improved method for importing and exporting data between heterogeneous data sources. Whether you want to move data from a legacy system on a onetime basis or continually move data back and forth for data warehousing, DTS should be your first choice. With DTS you don’t need to struggle with BCP anymore. DTS is extremely flexible and surprisingly fast, and you can use the technology to copy and transform data to or from any OLE DB or ODBC data source.

BCP is designed to work with flat files, while DTS can work with "any" database. using BCP, you have a much more mature tool, and clearer error messages than with DTS.

15.What are DBCC COMMANDS?

Ans. The Transact-SQL programming language provides DBCC statements that act as the "database consistency checker" for Microsoft® SQL Server™. These statements check the physical and logical consistency of a database. Many DBCC statements can fix detected problems. These database consistency-checking statements are grouped into these categories.

Maintenance Statements:

Maintenance tasks on a database, index, or filegroup.

DBCC DBREPAIR

DBCC SHRINKFILE

DBCC DBREINDEX

DBCC UPDATEUSAGE

DBCC SHRINKDATABASE   

Miscellaneous Statements:

Miscellaneous tasks such as enabling row-level locking or removing a dynamic-link library (DLL) from memory

DBCC dllname (FREE)

DBCC TRACEOFF

DBCC HELP

DBCC TRACEON

DBCC PINTABLE

DBCC UNPINTABLE

DBCC ROWLOCK  

Status Statements:

Status Checks

DBCC INPUTBUFFER

DBCC SHOW_STATISTICS

DBCC OPENTRAN

DBCC SQLPERF

DBCC OUTPUTBUFFER

DBCC TRACESTATUS

DBCC PROCCACHE

DBCC USEROPTIONS

DBCC SHOWCONTIG  

Validation Statements:

Validation operations on a database, table, index, catalog, filegroup, system tables, or allocation of database pages.

DBCC CHECKALLOC

DBCC CHECKTABLE

DBCC CHECKCATALOG

DBCC NEWALLOC

DBCC CHECKDB

DBCC TEXTALL

DBCC CHECKFILEGROUP

DBCC TEXTALLOC

DBCC CHECKIDENT  

16. What are System Stored Procedures?

SQL Server-supplied, precompiled collection of Transact-SQL statements. System stored procedures are provided as shortcuts Ans.for retrieving information from system tables or mechanisms for accomplishing database administration and other tasks that involve updating system tables. The names of all system stored procedures begin with sp_. System stored procedures are located in the master database and are owned by the system administrator, but many of them can be run from any database. If a system stored procedure is executed in a database other than master, it operates on the system tables in the database from which it is executed.

17. Can we prefix user defined SP’s with sp_, if so what will happen if u call that SP?

Ans. SQL Server always looks for stored procedures beginning with sp_ in this order:

i. Look for the stored procedure in the master database first.

ii. Look for the stored procedure based on any qualifiers provided (database name or owner).

iii. Look for the stored procedure using dbo as the owner, if one is not specified.

Therefore, although the user-created stored procedure prefixed with sp_ may exist in the current database, the master database is always checked first, even if the stored procedure is qualified with the database name Important : If any user-created stored procedure has the same name as a system stored procedure, the user-created stored procedure will never be executed

18. What are Indexes?

Ans.Microsoft SQL Server index is a structure associated with a table that speeds retrieval of the rows in the table. An index contains keys built from one or more columns in the table. These keys are stored in a structure that allows SQL Server to find the row or rows associated with the key values quickly and efficiently. If a table is created with no indexes, the data rows are not stored in any particular order. This structure is called a heap.

The two types of SQL Server indexes are:

• Clustered

Clustered indexes sort and store the data rows in the table based on their key values. Because the data rows are stored in sorted order on the clustered index key, clustered indexes are efficient for finding rows. There can only be one clustered index per table, because the data rows themselves can only be sorted in one order. The data rows themselves form the lowest level of the clustered index.

The only time the data rows in a table are stored in sorted order is when the table contains a clustered index. If a table has no clustered index, its data rows are stored in a heap.

• Non clustered

Nonclustered indexes have a structure that is completely separate from the data rows. The lowest rows of a nonclustered index contain the nonclustered index key values and each key value entry has pointers to the data rows containing the key value. The data rows are not stored in order based on the nonclustered key. The pointer from an index row in a nonclustered index to a data row is called a row locator. The structure of the row locator depends on whether the data pages are stored in a heap or are clustered. For a heap, a row locator is a pointer to the row. For a table with a clustered index, the row locator is the clustered index key

19.When will we go for clustered and Non-clustered Indexes?

Ans.

Indexes assist when a query:

• Searches for rows that match a specific search key value (an exact match query). An exact match comparison is one in which the query uses the WHERE statement to specify a column entry with a given value.

For example: WHERE emp_id = ‘VPA30890F’

• Searches for rows with search key values in a range of values (a range query). A range query is one in which the query specifies any entry whose value is between two values.

For example: WHERE job_lvl BETWEEN 9 and 12

or, WHERE job_lvl >= 9 and job_lvl <= 12

• Searches for rows in a table T1 that match, based on a join predicate, a row in another table T2 (an index nested loops join).

• Produces sorted query output without an explicit sort operation, in particular for sorted dynamic cursors.

• Scans rows in a sorted order to permit an order-based operation, such as merge join and stream aggregation, without an explicit sort operation.

• Scans all rows in a table with better performance than a table scan, due to the reduced column set and overall data volume to be scanned (a covering index for the query at hand).

• Searches for duplicates of new search key values in insert and update operations, to enforce PRIMARY KEY and UNIQUE constraints.

• Searches for matching rows between two tables for which a FOREIGN KEY constraint is defined. Queries using LIKE comparisons can benefit from an index if the pattern starts with a specific character string, for example ‘abc%’, but not if the pattern starts with a wildcard search, for example ‘%xyz’.

• Columns that contain a high number of distinct values, such as a combination of last name and first name (if a clustered index is used for other columns). If there are very few distinct values, such as only 1 and 0, no index should be created.

• Queries that do not return large result sets.

• Columns frequently involved in search conditions of a query (WHERE clause) that return exact matches.

• Decision Support System applications for which joins and grouping are frequently required. Create multiple nonclustered indexes on columns involved in join and grouping operations, and a clustered index on any foreign key columns.

• Columns that contain a limited number of distinct values, such as a state column that contains only 50 distinct state codes. However, if there are very few distinct values, such as only 1 and 0, no index should be created.

• Queries that return a range of values using operators such as BETWEEN, >, >=, <, and <=.

• Columns that are accessed sequentially.

• Queries that return large result sets.

• Columns that are frequently accessed by queries involving join or GROUP BY clauses; typically these are foreign key columns. An index on the column(s)

specified in the ORDER BY or GROUP BY clause eliminates the need for SQL Server to sort the data because the rows are already sorted. This improves query

performance.

• OLTP-type applications where very fast single row lookup is required, typically by means of the primary key. Create a clustered index on the primary key.

Clustered indexes are not a good choice for:

• Columns that undergo frequent changes because this results in the entire row moving (because SQL Server must keep the row’s data values in physical order). This is an important consideration in high-volume transaction processing systems where data tends to be volatile.

• Covered queries. The more columns within the search key, the greater the chance for the data in the indexed column to change, resulting in additional I/O.

20. What is Referential Integrity?

Ans.Referential integrity preserves the defined relationships between tables when records are entered or deleted. In SQL Server, referential integrity is based on relationships between foreign keys and primary keys or between foreign keys and unique keys. Referential integrity ensures that key values are consistent across tables. Such consistency requires that there be no references to nonexistent values and that if a key value changes, all references to it change consistently throughout the database.When you enforce referential integrity, SQL Server

prevents users from:

Adding records to a related table if there is no associated record in the primary table.

Changing values in a primary table that result in orphaned records in a related table.

Deleting records from a primary table if there are matching related records.

21.What are the Features of Normalized and De-normalized Databases?

Ans.Normalizing a logical database design involves using formal methods to separate the data into multiple, related tables. A greater number of narrow tables (with fewer columns) is characteristic of a normalized database.  A few wide tables (with more columns) is characteristic of an unnormalized database. Some of the benefits of normalization include:

• Faster sorting and index creation.

• A larger number of clustered indexes.

• Narrower and more compact indexes.

• Fewer indexes per table, which improves the performance of INSERT, UPDATE, and DELETE statements.

• Fewer NULL values and less opportunity for inconsistency, which increase database compactness.

22. When will you go for De-normalization?

Ans.To introduce redundancy into a table in order to incorporate data from a related table. The related table can then be eliminated. Denormalization can improve efficiency and performance by reducing complexity in a data warehouse schema.

23.Union and Union ALL, which will give better performance and why?

Ans.The UNION operator allows you to combine the results of two or more SELECT statements into a single result set. The result sets combined using UNION must all have the same structure. They must have the same number of columns, and the corresponding result set columns must have compatible data types.

The result set column names of a UNION are the same as the column names in the result set of the first SELECT statement in the UNION. The result set column names of the other SELECT statements are ignored.

By default, the UNION operator removes duplicate rows from the result set. If you use ALL, all rows are included in the results and duplicates are not removed

UNION ALL is faster because it doesn’t try to eliminate the duplicates. In fact, as a "duplicate" is only determined by comparing the whole row (all the columns returned), you should use UNION ALL all the time, except if you WANT to eliminate the duplicates by using UNION

24.What is the Diff. between drop and detach database?

Ans. Drop Database: Removes one or more databases from SQL Server. Removing a database deletes the database and the disk files used by the database.

A database that has been dropped can be re-created only by restoring a backup. You cannot drop a database currently in use (open for reading or writing by any user). Whenever a database is dropped, the master database should be backed up .

Detach Database

Detaches a database from a server and, optionally, runs UPDATE STATISTICS on all tables before detaching.Detaching a database removes the database from SQL Server, but leaves the database intact within the data and transaction log files that compose the database. These data and transaction log files can then be used to attach the database to any computer running SQL Server, including the server from which the database was detached. This makes the database available in exactly the same state it was in when it was detached.

25.What are Instead of Triggers?

Ans.INSTEAD OF triggers are executed instead of the triggering action (for example, INSERT, UPDATE, DELETE). They can also be defined on views, in which case they greatly extend the types of updates a view can support. The trigger executes in place of the triggering action. INSTEAD OF triggers can be specified on both tables and views. You can define only one INSTEAD OF trigger for each triggering action (INSERT, UPDATE, and DELETE). INSTEAD OF triggers can be used to perform enhance integrity checks on the data values supplied in INSERT

and UPDATE statements. INSTEAD OF triggers also let you specify actions that allow views, which would normally not support updates, to be updatable.

26. Solve the Query: Customer Table

Name Phoneno

Abc       123

Pqr        234

Rst        235

Xyz        997

a.Write a query to find out customers and count of phno with more than one phnumber

Ans. 

   1: Select  name, Count(Phoneno) from customer having count(Phoneno) > 1

b.Add a row pqr  phno 234. Now write a query to get the count of phno and name with no duplicate values

Ans. 

   1: Select [Name],phoneno,count(phoneno) as countofPNo from customer group by [Name], phoneno

27.Explain about DTS.

Ans. Data Transformation Services (DTS) provides the functionality to import, export, and transform data between SQL Server and any OLE DB, ODBC, or text file format. Using DTS, it is possible to:

Build data warehouses and data marts in Microsoft SQL Server by importing and transferring data from multiple heterogeneous sources interactively or automatically on a regularly scheduled basis.

Create custom transformation objects that can be integrated into third-party products.

Access applications using third-party OLE DB providers.

This allows applications, for which an OLE DB provider exists, to be used as sources and destinations of data. DTS also provides support for:

• High-speed nonlogged inserts (bcp) into SQL Server version 7.0.

• Creating customized transformations.

• Transferring complete database objects between source and destination SQL Server 7.0 data sources. The Transfer SQL Server Objects task can be used to transfer all of the metadata and data for some or all of the objects in one SQL

Server 7.0 database to another SQL Server 7.0 database.

For example, the Transfer SQL Server Objects task can be used to move a table with all of its associated index, constraint, rule, default, and trigger definitions and the existing rows in the table. The Transfer SQL Server Objects task also can be used to transfer the definitions of objects such as views and stored procedures.

28.What is OpenRowset?

Ans.It includes all connection information necessary to access remote data from an OLE DB data source.

This method is an alternative to accessing tables in a linked server and is a one-time, ad hoc method of connecting and accessing remote data using OLE DB. The OPENROWSET function can be referenced in the FROM clause of a query as though it is a table name. The OPENROWSET function can also be referenced as the target table of an INSERT, UPDATE, or DELETE statement, subject to the capabilities of the OLE DB provider. Although the query may return multiple result sets, OPENROWSET returns only the first one.

Ex:

   1: SELECT a.*  FROM OPENROWSET   ('SQLOLEDB','seattle1';'sa';'MyPass',
   2: 'SELECT * FROM pubs.dbo.authors ORDER BY au_lname, au_fname') AS a

29.What are stored procedures?

Ans.When you use Transact-SQL programs, two methods are available for storing and executing the programs. You can store the programs locally and create applications that send the commands to SQL Server and process the results, or you can store the programs as stored procedures in SQL Server and create applications that execute the stored procedures and process the results. Stored procedures in SQL Server are similar to procedures in other programming languages in that they can:

Accept input parameters and return multiple values in the form of output parameters to the calling procedure or batch.

Contain programming statements that perform operations in the database, including calling other procedures.

Return a status value to a calling procedure or batch to indicate success or failure (and the reason for failure).

You can use the Transact-SQL EXECUTE statement to run a stored procedure. Stored procedures are different from functions in that they do not return values in place of their names and they cannot be used directly in an expression.The benefits of using stored procedures in SQL Server rather than Transact-SQL programs stored locally on client computers are:

They allow modular programming.

You can create the procedure once, store it in the database, and call it any number of times in your program. Stored procedures can be created by a person who specializes in database programming, and they can be modified independently of the program source code.

They allow faster execution. If the operation requires a large amount of Transact-SQL code or is performed repetitively, stored procedures can be faster than batches of Transact-SQL code. They are parsed and optimized when they are created, and an in-memory version of the procedure can be used after the procedure is executed the first time. Transact-SQL statements repeatedly sent from the client each time they run are compiled and optimized every time they are executed by SQL Server.

They can reduce network traffic. An operation requiring hundreds of lines of Transact-SQL code can be performed through a single statement that executes the code in a procedure, rather than by sending hundreds of lines of code over the network.

They can be used as a security mechanism. Users can be granted permission to execute a stored procedure even if they do not have permission to execute the procedure’s statements directly.

A SQL Server stored procedure is created with the Transact-SQL CREATE PROCEDURE statement and can be modified with the ALTER PROCEDURE statement. The stored procedure definition contains two primary components: the specification of the procedure name and its parameters, and the body of the procedure, which contains Transact-SQL statements that perform the procedure’s operations.

Extended Stored Procedures

Extended stored procedures allow you to create your own external routines in a programming language such as C. The extended stored procedures appear to users as normal stored procedures and are executed in the same way. Parameters can be passed to extended stored procedures, and they can return results and return status. Extended stored procedures can be used to extend the capabilities of Microsoft® SQL Server™. Extended stored procedures are dynamic link libraries (DLLs) that SQL Server can dynamically load and execute. Extended stored procedures run directly in the address space of SQL Server and are programmed using the SQL Server Open Data Services API. After an extended stored procedure has been written, members of the sysadmin fixed server role can register the extended stored procedure with SQL Server and then grant permission to other users to execute the procedure. Extended stored procedures can be added only to the master database

Remote Stored Procedure

A collection of SQL statements and optional control-of-flow statements stored under a name on a remote server. Remote stored procedures can be called by clients or SQL Server

Distributed Queries

Distributed queries access data from multiple heterogeneous data sources, which can be stored in either the same or different computers. SQL Server supports distributed queries

by using OLE DB, the Microsoft specification of an application programming interface (API) for universal data access.

Distributed queries provide SQL Server users with access to:

• Distributed data stored in multiple computers that are running SQL Server.

• Heterogeneous data stored in various relational and non-relational data sources that can be accessed using an OLE DB provider. OLE DB providers expose their data in tabular objects called rowsets. SQL Server version 7.0 allows rowsets from OLE DB providers to be referenced in Transact-SQL statements as if they were a SQL Server table.

30. Solve the Query

       Name         Tel

        ABC          100

        DEF           200

        HIG            300

        ABC           100

        DEF            400

        PQR            500

Write a query to list all the customers and the no. of telephones they have, only if they have more than one unique telephone no   

Ans. 

   1: Select Name, count(phoneno) as noofPhoneno from customers group by Name having noofPhoneno > 1

31. Solve Query. Consider the following table:

     Sernum          Day  Temp.

        1                Mon      10

        2                Tue       12

        3                Wed        9

        4                Thurs     15

Write a query to list a new column with the difference in temp of the days Mon and Tue,Tue and Wed and soon. (Don’t use cursors)

Ans. 

   1: Select a.srno,b.srno,a.[day],b.[day],a.[temp],b.[temp], 
   2:     (a.[temp]-b.[temp])AS DiffinTemp from diff a 
   3:     inner join diff b on  a.srno=b.srno-1

32. Consider the tables Team Table

TeamId    TeamName

     1       Team1

     2        Team2

PlayerTable       

PlayId   Name

      1   Name1

      2   Name2

TeamPlayer

TeamId   PlayerId

1               1

2               1

1               2

Write a query to get Team name and Player name

Ans. 

   1: Select c.teamname,d.playername from 
   2:  (Select a.teamid,a.teamname,b.playerid from 
   3:    team a inner join teamplayer b on a.teamid=b.teamid) 
   4:     as c inner join player d on 
   5:      c.playerid=d.playerid

33.Different ways of getting count of rows from a table in queryanalyser *******

Ans. Select distinct count(*)

34. What is sp_addlinkedserver

Ans. Creates a linked server, which allows access to distributed, heterogeneous queries against OLE DB data sources. After creating a linked server with sp_addlinkedserver, this server can then execute distributed queries. If the linked server is defined as SQL Server, remote stored procedures can be executed.

A linked server configuration allows SQL Server to execute commands against OLE DB data sources on different servers.

Linked servers offer these advantages:

Remote server access.

The ability to issue distributed queries, updates, commands, and transactions on heterogeneous data sources across the enterprise. Freedom from the need to address diverse data

sources differently.

35. What are the different types of Locks?

Ans. There are three main types of locks that SQL Server

6.5 uses:

Shared locks

Update locks

Exclusive locks

Shared locks are used for operations that do not change or update data, such as a SELECT statement.

Update locks are used when SQL Server intends to modify a page, and later promotes the update page lock to an exclusive page lock before actually making the changes.

Exclusive locks are used for the data modification operations,  such as UPDATE, INSERT, or DELETE. Shared locks are compatible with other Shared locks or Update locks.

Update locks are compatible with Shared locks only.

Exclusive locks are not compatible with other lock types.

Let me to describe it on the real example. There are four processes, which attempt to lock the same page of the same table. These processes start one after another, so Process1 is the first process, Process2 is the second process and so on.

Process1 : SELECT

Process2 : SELECT

Process3 : UPDATE

Process4 : SELECT

Process1 sets the Shared lock on the page, because there are no another locks on this page.

Process2 sets the Shared lock on the page, because Shared locks are compatible with other Shared locks.

Process3 wants to modify data and wants to set Exclusive lock, but it cannot make it before Process1 and Process2 will be finished, because Exclusive lock is not compatible with other lock types. So, Process3 sets Update lock.

Process4 cannot set Shared lock on the page before Process3 will be finished. So, there is no Lock starvation. Lock starvation occurs when read transactions can monopolize a table or page, forcing a write transaction to wait indefinitely. So, Process4 waits before Process3 will be finished.After Process1 and Process2 were finished, Process3 transfer Update lock into Exclusive lock to modify data. After Process3 was finished, Process4 sets the Shared lock on the page to select data.

Locking optimizer hints

There are six Locking optimizer hints in SQL Server 7.0:

NOLOCK

HOLDLOCK

UPDLOCK

TABLOCK

PAGLOCK

TABLOCKX

READCOMMITTED

READUNCOMMITTED

REPEATABLEREAD

SERIALIZABLE

READPAST

ROWLOCK

NOLOCK is also known as "dirty reads". This option directs SQL Server not to issue shared locks and not to honor exclusive locks. So, if this option is specified, it is possible to read an uncommitted transaction. This results in higher concurrency and in lower consistency.

HOLDLOCK directs SQL Server to hold a shared lock until

completion of the transaction in which HOLDLOCK is used.

You cannot use HOLDLOCK in a SELECT statement that includes the FOR BROWSE option. HOLDLOCK is equivalent to SERIALIZABLE.

UPDLOCK instructs SQL Server to use update locks instead of shared locks while reading a table and holds them until the

end of the command or transaction.

TABLOCK takes a shared lock on the table that is held until

the end of the command. If you also specify HOLDLOCK, the lock is held until the end of the transaction.

PAGELOCK is used by default. Directs SQL Server to use shared page locks.

TABLOCKX takes an exclusive lock on the table that is held

until the end of the command or transaction.

READCOMMITTED

Perform a scan with the same locking semantics as a transaction running at the READ COMMITTED isolation level. By default, SQL Server operates at this isolation level.

READUNCOMMITTED

Equivalent to NOLOCK.

REPEATABLEREAD

Perform a scan with the same locking semantics as a transaction running at the REPEATABLE READ isolation level. 

SERIALIZABLE

Perform a scan with the same locking semantics as a transaction running at the SERIALIZABLE isolation level. Equivalent to HOLDLOCK.

READPAST

Skip locked rows. This option causes a transaction to skip over rows locked by other transactions that would ordinarily appear in the result set, rather than block the transaction waiting for the other transactions to release their locks on these rows. The READPAST lock hint applies only to transactions operating at READ COMMITTED isolation and will read only past row-level locks. Applies only to the SELECT statement.  You can only specify the READPAST lock in the READ COMMITTED or REPEATABLE READ isolation levels.

ROWLOCK

Use row-level locks rather than use the coarser-grained page and table-level locks.

You can specify one of these locking options in a SELECT statement.

This is the example:

   1: SELECT au_fname FROM pubs..authors (holdlock)

Lock Escalation

You can customize locking by setting Lock Escalation level. The Lock Escalation level determines, when SQL Server applies table locks instead of page locks, and it affects all users of SQL Server. So it’s escalation from the page to the table level locking.

There are three Lock Escalation options:

LE threshold maximum

LE threshold minimum

LE threshold percent

LE threshold maximum is the maximum number of page locks to hold before escalating to a table lock. The default value is 200.

LE threshold minimum is the minimum number of page locks required before escalating to a table lock. The default value is 20.

LE threshold percent is the percentage of page locks needed on a table before escalating to a table lock. The default value is 0, it means that a table lock will be occur only when the LE threshold maximum will be exceeded.

You can configure Lock Escalation levels by using the sp_configure system stored procedure. This is the example to set LE threshold maximum to 250:

   1: EXEC sp_configure 'LE threshold maximum', 250
   2: RECONFIGURE WITH OVERRIDE

Deadlocks

Deadlock occurs when two users have locks on separate objects and each user wants a lock on the other’s object.

For example, User1 has a lock on object "A" and wants a lock on object "B" and User2 has a lock on object "B" and wants a lock on object "A".

You can decide which connection will be the candidate for deadlock victim by using SET DEADLOCK_PRIORITY. In other case, SQL Server selects the deadlock victim by choosing the process that completes the circular chain of locks. So, in a multiuser situation, your application should check the message 1205 to indicate that the transaction was rolled back, and if it’s so, restart the transaction.

Note. To reduce the chance of a deadlock, you should minimize the size of transactions and transaction times.

36. What’s a Primary Key?

Ans.The column or combination of columns that uniquely identifies one row from any other row in a table. A primary key (PK) must be nonnull and must have a unique index.

A primary key is commonly used for joins with foreign keys (matching nonprimary keys) in other tables. Primary Key creates Cluster Index.