SQL Info

What is De-normalization? De-normalization is the process of attempting to optimize the performance of a database by adding redundant data. It is sometimes necessary because current DBMSs implement the relational model poorly. A true relational DBMS would allow for a fully normalized database at the logical level, while providing physical storage of data that is tuned for high performance. De-normalization is a technique to move from higher to lower normal forms of database modeling in order to speed up database access. 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 local variable. SELECT @RC = @@ROWCOUNT, @ER = @@ERROR

What is a table called, if it does not have neither Cluster nor Non-cluster Index? What is it used 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 by pointers. 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 table and than do bulk of inserts and to restore those indexes after that.

How do you load large data to the SQL server database? BulkCopy is a tool used to copy huge amount of data from tables. BULK INSERT command helps to Imports a data file into a database table or view in a user-specified format. 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 we may know, an outer join uses the plus sign (+) operator to tell the database to return all non-matching rows with NULL values. Hence we combine the outer join with a NULL test in the WHERE clause to reproduce the result set without using a sub-query.

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 without affecting clients. · Stored procedures provide better security to your data.

What is DataWarehousing? · Subject-oriented, meaning that the data in the database is organized so that all the data elements 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 recorded so that reports can be produced showing changes over time; · Non-volatile, meaning that data in the database is never over-written or deleted, once committed, 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's operational applications, and that this data is made consistent. What is OLTP(OnLine Transaction Processing)? In OLTP - online transaction processing systems relational database design use the discipline of data modeling and generally follow the Codd rules of data normalization in order to ensure absolute data integrity. Using these rules complex information is broken down ...

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 using sp_dboptions. Use sp_renamedb to rename database. Use sp_dboptions to bring database to multi user mode. What is sp_configure commands and set commands? Use sp_configure to display or change server-level settings. To change database-level settings, use ALTER DATABASE. To change settings that affect only the current user session, use the SET statement.

What is @@ERROR? The @@ERROR automatic variable returns the error code of the last Transact-SQL statement. If there was 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. What is Raiseerror? Stored procedures report errors to client applications via the RAISERROR command. RAISERROR doesn't change the flow of a procedure; it merely displays an error message, sets the @@ERROR automatic variable, and optionally writes the message to the SQL Server error log and the NT application event log.

Where are SQL server users names and passwords are stored in sql server? They get stored in master db in the sysxlogins table. Which command using Query Analyzer will give you the version of SQL server and operating system? SELECT SERVERPROPERTY('productversion'), SERVERPROPERTY ('productlevel'), SERVERPROPERTY ('edition') What is SQL server agent? SQL Server agent plays an important role in the day-to-day tasks of a database administrator (DBA). It is often overlooked as one of the main tools for SQL Server management. Its purpose is to ease the implementation of tasks for the DBA, with its full-function scheduling engine, which allows you to schedule your own jobs and scripts.

Inline Table-Value User-Defined Function An Inline Table-Value user-defined function returns a table data type and is an exceptional alternative to a view as the user-defined function can pass parameters into a T-SQL select command and in essence 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 exceptional alternative to a view as the function can support multiple T-SQL statements to build the final result where the view is limited to a single SELECT statement. Also, the ability to pass parameters into a TSQL select 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 you must define the table structure that is being returned. After creating this type of user-defined function, It can be...

What is User Defined Functions? User-Defined Functions allow to define its own T-SQL functions that can accept 0 or more parameters and return a single scalar data value or a table data type. 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.

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. 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.

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 SELECT statement. HAVING is typically used in a GROUP BY clause. When GROUP BY is not used, HAVING behaves like a WHERE clause. Having Clause is basically used only with the GROUP BY function in a query. WHERE Clause is applied to each row before they are part of the GROUP BY function in a query. What is sub-query? Explain properties of sub-query.

Difference between Function and Stored Procedure? UDF can be used in the SQL statements anywhere in the WHERE/HAVING/SELECT section where as Stored 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 other Rowset operations.

What is a NOLOCK? Using the NOLOCK query optimiser hint is generally considered good practice in order to improve concurrency on a busy system. When the NOLOCK hint is included in a SELECT statement, no locks are taken when data is read. The result is a Dirty Read, which means that another process could be updating the data at the exact time you are reading it. There are no guarantees that your query will retrieve the most recent data. The advantage to performance is that your reading of data will not block updates from taking place, and updates will not block your reading of data. SELECT statements take Shared (Read) locks. This means that multiple SELECT statements are allowed simultaneous access, but other processes are blocked from modifying the data. The updates will queue until all the reads have completed, and reads requested after the update will wait for the updates to complete. The result to your system is delay(blocking). What is difference between DELETE & ...

What is the use of DBCC commands? DBCC stands for database consistency checker. We use these commands to check the consistency of the 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. 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 query both the SQL Server dbs using T-SQL Statements. With a linked server, you can create very clean, easy to 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. What is Collation? Collation refers to a set of rules that determine how data is sorted and compared. Character data is sorted usi...

What is Index? An index is a physical structure containing pointers to the data. Indices are created in an existing table to locate rows more quickly and efficiently. It is possible to create an index on one or more columns of a table, and each index is given a name. The users cannot see the indexes, they are just used to speed up queries. Effective indexes are one of the best ways to improve performance in a database application. A table scan happens when there is no index available to help a query. In a table scan SQL Server examines every row in the table to satisfy the query results. Table scans are sometimes unavoidable, 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 this reason, 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 references to the table itse...

What is Stored Procedure? A stored procedure is a named group of SQL statements that have been previously created and stored in the server database. Stored procedures accept input parameters so that a single procedure can be used over the network by several clients using different input data. And when the procedure is modified, all clients automatically get the new version. Stored procedures reduce network traffic and improve performance. Stored procedures can be used to help ensure the integrity of the database. e.g. sp_helpdb, sp_renamedb, sp_depends etc. 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. Trig...

What is RDBMS? Relational Data Base Management Systems (RDBMS) are database management systems that maintain data records and indices in tables. Relationships may be created and maintained across and among the data and tables. In a relational database, relationships between data items are expressed by means of tables. 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 data items from different files, providing powerful tools for data usage. What is normalization? Database normalization is a data design and organization process applied to data structures based on rules that help build relational databases. In relational database design, the process of organizing data to minimize redundancy. Normalization usually involves dividing a database into two or more tables and defining relationships between the tables. The objective is to isolate data s...

List the emps who are either ‘CLERK’ or ‘ANALYST’ in the Desc order. A) select * from emp where job = ‘CLERK’ or job = ‘ANALYST’ order by job desc; List the emps who joined on 1-MAY-81,3-DEC-81,17-DEC-81,19-JAN-80 in asc order of seniority. A) select * from emp where hiredate in (’01-may-81’,’03-dec-81’,’17-dec-81’,’19-jan-80’) order by hiredate asc;

A) select * from emp where (sal/30) >100;

Answer: select * from emp where hiredate > (’30-jun-81’) and to_char(hiredate,’YYYY’) = 1981 order by job asc;

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Answer: select * from emp where hiredate < (’01-jan-81’);

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Query for selecting all data from table write below select sql query

This article covers using the REPLACE function to selectively replace text inside a string in SQL Server. The REPLACE function is easy to use and very handy with an UPDATE statment. Replace searches for certain characters in a string and replaces them with other characters. So this statement: SELECT Replace( 'SQLTeam.com Rocks!' , 'Rocks' , 'Rolls' ) will return SQLTeam.com Rolls! REPLACE searches the the first string for any occurance of the the second string and replaces it with the third string. You can also do replacements of different sizes. For example, SELECT Replace( 'SQLTeam.com Rocks!' , 'Rocks' , 'is cool' ) gives us SQLTeam.com is cool! I replaced a five character string with a seven character string with no problem. If the string isn't found, no changes will be made. SELECT Replace( 'SQLTeam.com Rocks!' , 'Yak' , 'Tibetan bison' ) returns exactly what we started with which is ...

A TRY CATCH block can catch errors in stored procedures called by other stored procedures.  An example is: BEGIN TRY EXEC ParentError END TRY BEGIN CATCH SELECT Error_Line = ERROR_LINE(), Error_Proc = ERROR_PROCEDURE() END CATCH Assuming that the ParentError stored procedure calls the ChildError stored procedure which generates an error you'd see output like this: Error_Line Error_Proc --------- -- ------------- 5 ChildError

This article covers the basics of TRY CATCH error handling in T-SQL introduced in SQL Server 2005. It includes the usage of common functions to return information about the error and using the TRY CATCH block in stored procedures and transactions. SQL Server uses the following syntax to capture errors in Transact-SQL statements: BEGIN TRY SELECT [First] = 1 SELECT [Second] = 1/0 SELECT [Third] = 3 END TRY BEGIN CATCH PRINT 'An error occurred' END CATCH This returns the following output: First ----------- 1 (1 row(s) affected) Second ----------- (0 row(s) affected) An error occurred A key difference from SQL Server 2000 is that execution is halted when SQL Server encounters an error.  At that point execution transfers to the CATCH block.  This error isn't returned to the client application or calling program.  The TRY CATCH block consumes the error.