Wednesday, February 15, 2012

What is a 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 Third Normal Form and what is its advantage?

Third Normal Form (3NF) is most preferable normal form in RDBMS. Normalization is the process of designing a data model to efficiently store data in a database. The rules of 3NF are mentioned here

  • Make a separate table for each set of related attributes, and give each table a primary key.
  • If an attribute depends on only part of a multi-valued key, remove it to a separate table
  • If attributes do not contribute to a description of the key, remove them to a separate table.
Normalization is very close to concept of object oriented schema’s and it stores one data at only one place by removing all the redundant data. It also helps to draw the schema easier. Normalization comes at the cost of performance.

Reference : Pinal Dave (http://blog.SQLAuthority.com) , DataModel.org.

Tuesday, February 14, 2012

General Questions on SQL SERVER

DBMS – Data Base Management System
RDBMS – Relational Data Base Management System or Relational DBMS
A DBMS has to be persistent, that is it should be accessible when the program created the data ceases to exist or even the application that created the data restarted. A DBMS also has to provide some uniform methods independent of a specific application for accessing the information that is stored.
RDBMS adds the additional condition that the system supports a tabular structure for the data, with enforced relationships between the tables. This excludes the databases that don’t support a tabular structure or don’t enforce relationships between tables.
DBMS does not impose any constraints or security with regard to data manipulation it is user or the programmer responsibility to ensure the ACID PROPERTY of the database whereas the RDBMS is more with this regards because RDBMS defines the integrity constraint for the purpose of holding ACID PROPERTY. RDBMS may be or may not be Client Server Database System.
Examples :
DBMS – File System, XML
RDBMS – SQL Server, Oracle
Reference : Pinal Dave (http://blog.SQLAuthority.com)

What are the Properties of the Relational Tables?

Relational tables have the following six properties:
  • Values are atomic.
  • Column values are of the same kind.
  • Each row is unique.
  • The sequence of columns is insignificant.
  • The sequence of rows is insignificant.
  • Each column must have a unique name.

What is Normalization?

Database normalization is a data design and organization process applied to data structures based on rules that help building relational databases. In relational database design, the process of organizing data to minimize redundancy is called normalization. Normalization usually involves dividing a database into two or more tables and defining 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 the database via the defined relationships.

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 is ACID property related to Database?

ACID (an acronym for Atomicity Consistency Isolation Durability) is a concept that Database Professionals generally look for while evaluating databases and application architectures. For a reliable database, all this four attributes should be achieved.
Atomicity is an all-or-none proposition.
Consistency guarantees that a transaction never leaves your database in a half-finished state.
Isolation keeps transactions separated from each other until they are finished.

What are the Different Normalization Forms?

1NF: Eliminate Repeating Groups
Make a separate table for each set of related attributes, and give each table a primary key. Each field contains at most one value from its attribute domain.
2NF: Eliminate Redundant Data
If an attribute depends on only part of a multi-valued key, then remove it to a separate table.
3NF: Eliminate Columns Not Dependent On Key
Third Normal Form (3NF) is most preferable normal form in RDBMS. Normalization is the process of designing a data model to efficiently store data in a database. The rules of 3NF are mentioned here
  • Make a separate table for each set of related attributes, and give each table a primary key.
  • If an attribute depends on only part of a multi-valued key, remove it to a separate table
  • If attributes do not contribute to a description of the key, remove them to a separate table.
Normalization is very close to concept of object oriented schema’s and it stores one data at only one place by removing all the redundant data. It also helps to draw the schema easier. Normalization comes at the cost of performance.
BCNF: Boyce-Codd Normal Form
If there are non-trivial dependencies between candidate key attributes, then separate them out into distinct tables.
4NF: Isolate Independent Multiple Relationships
No table may contain two or more 1:n or n:m relationships that are not directly related.
5NF: Isolate Semantically Related Multiple Relationships
There may be practical constrains on information that justify separating logically related many-to-many relationships.
ONF: Optimal Normal Form
A model limited to only simple (elemental) facts, as expressed in Object Role Model notation.
DKNF: Domain-Key Normal Form
A model free from all modification anomalies is said to be in DKNF.
Remember, these normalization guidelines are cumulative. For a database to be in 3NF, it must first fulfill all the criteria of a 2NF and 1NF database.

Reference: Pinal Dave (http://blog.SQLAuthority.com)

 


Difference between Stored procedure and functions

Functions
----------
1) can be used with Select statement
2) Not returning output parameter but returns Table variables
3) You can join UDF
4) Cannot be used to change server configuration
5) Cannot be used with XML FOR clause
6) Cannot have transaction within function

Stored Procedure
-----------------
1) have to use EXEC or EXECUTE
2) return output parameter
3) can create table but won’t return Table Variables
4) you can not join SP
5) can be used to change server configuration
6) can be used with XML FOR Clause
7) can have transaction within SP

Differences between stored procedures and triggers in databases?

Stored procedures are compiled collection of programs or SQL statements that live in the database. A stored procedure can access and modify data present in many tables. Also a stored procedure is not associated with any particular database object. But triggers are event-driven special procedures which are attached to a specific database object say a table. Stored procedures are not automatically run and they have to be called explicitly by the user. But triggers get executed when the particular event associated with the event gets fired. For example in case of a database having say 200 users and the last modified timestamp need to be updated every time the database is accessed and changed. To ensure this one may have a trigger in the insert or update event. So that whenever any insert or update event of the table gets fired the corresponding trigger gets activated and updates the last modified timestamp column or field with the current time. Thus the main difference between stored procedure and trigger is that in case of stored procedure the program logic is executed on the database server explicitly under eth user’s request but in case of triggers event-driven procedures attached to database object namely table gets fired automatically when the event gets fired.

What are the difference between DDL, DML and DCL commands?


DDL


Data Definition Language (DDL) statements are used to define the database structure or schema. Some examples:
  • CREATE - to create objects in the database
  • ALTER - alters the structure of the database
  • DROP - delete objects from the database
  • TRUNCATE - remove all records from a table, including all spaces allocated for the records are removed
  • COMMENT - add comments to the data dictionary
  • RENAME - rename an object

DML


Data Manipulation Language (DML) statements are used for managing data within schema objects. Some examples:
  • SELECT - retrieve data from the a database
  • INSERT - insert data into a table
  • UPDATE - updates existing data within a table
  • DELETE - deletes all records from a table, the space for the records remain
  • MERGE - UPSERT operation (insert or update)
  • CALL - call a PL/SQL or Java subprogram
  • EXPLAIN PLAN - explain access path to data
  • LOCK TABLE - control concurrency

DCL


Data Control Language (DCL) statements. Some examples:
  • GRANT - gives user's access privileges to database
  • REVOKE - withdraw access privileges given with the GRANT command

TCL


Transaction Control (TCL) statements are used to manage the changes made by DML statements. It allows statements to be grouped together into logical transactions.
  • COMMIT - save work done
  • SAVEPOINT - identify a point in a transaction to which you can later roll back
  • ROLLBACK - restore database to original since the last COMMIT
  • SET TRANSACTION - Change transaction options like isolation level and what rollback segment to use

Converting Your DataTable into List<T>Using Linq Query

This example will help you converting your DataTable into List<T>, this topic is specially helpful for development in silverlight, as DataTables doesnot exist in it, so we need to have to convert DataTable into List<T> or ObservableCollection<T>.

Suppose following is your DataTable
DataTable dtExample = new DataTable("Example");
dtExample.Columns.Add("RollNo");
dtExample.Columns.Add("Name");
dtExample.Columns.Add("Class");


The process of converting above DataTable completes in three steps.
Step1: Make a class having properties with respect to your DataTable Columns, In the Example Table there are three Columns
>> RollNo
>> Name
>> class
So You have to Make the following Class
public class Employees
{
        public String RollNo { get ;  set ;}
        public String Name{ get ;  set ;}
        public String Class{ get ;  set ;}
}

Step2: Make a Function of return type of your above class, that will convert a DataTable Row in List<T> Row, This function will be called in Linq Query in Third Step
private Employees GetEmpDataTableRow(DataRow dr)
{
     Employees oEmp = new Employees();
     oEmp.RollNo = dr["RollNo"].ToString();
     oEmp.Name = dr["Name"].ToString();
     oEmp.Class = dr["Class"].ToString();
     return oEmp ;
}
Step3: In the last you are able to convert your DataTable dtExample into List<Employees&gt. go through the Third Step
List<Employees> LstEmployees = new List<Employees>();
LstEmployees = new List<Employees>(
               (from dRow in dtExample.AsEnumerable()
                select (GetEmpDataTableRow(dRow)))
               );
  

Now LstEmployees is your List of Type List<Employees>, which is converted from DataTable dtExample Hope this will help you and work for you, You can contact in case of any confusion. Thanks for my colleague Mr. Jamal Khan who helped in the above solution