Year: 2015

Explain ACID properties   Explain ACID properties.    ACID (Atomicity, Consistency, Isolation, Durability) is a set of properties that guarantee that database transactions are processed reliably. In the context of databases, a single logical operation on the data is called a transaction. For example, a transfer of funds from one bank account to another, even involving multiple changes such as debiting one account and crediting another, is a single transaction. Atomicity : Atomicity requires that each transaction be "all or nothing": if one part of the transaction fails, the entire transaction fails, and the database state is left unchanged. An atomic system must guarantee atomicity in each and every situation, including power failures, errors, and crashes. To the outside world, a committed transaction appears (by its effects on the database) to be indivisible ("atomic"), and an aborted transaction does not happen.   Consistency : The consistency property ensures that any…

Explain normalization.       Explain normalization.   In creating a database, normalization is the process of organizing it into tables in such a way that the results of using the database are always unambiguous and as intended. Normalization may have the effect of duplicating data within the database and often results in the creation of additional tables. (While normalization tends to increase the duplication of data, it does not introduce redundancy, which is unnecessary duplication.) Normalization is typically a refinement process after the initial exercise of identifying the data objects that should be in the database, identifying their relationships, and defining the tables required and the columns within each table.   First normal form (1NF). This is the "basic" level of normalization and generally corresponds to the definition of any database, namely:   It contains two-dimensional tables with rows and columns.   Each column corresponds to a sub-object or a…

Define XML and explain the structure of XML.       Define XML and explain the structure of XML.     XML stands for extensible markup language. XML was developed around 1996 and is a subset of SGML. It's documents conform to SGML. XML was made less complicated than SGML to enable its use on the web. XML uses the ISO 10646 (Unicode) standard for encoding characters.   XML Structure   This page provides a description of XML structure including the document parts, the prologue, and provides a simple XML example document.   Document Parts   Prolog   Document Element (root element)   The Prologue : The prologue, equivalent to the header in HTML, may include the following:   An XML declaration (optional) such as: <?xml version="1.0"?> A DTD or reference to one (optional). An example reference to an external DTD file:   <!DOCTYPE LANGLIST SYSTEM "langlist.dtd">   Processing instructions - An example processing instruction that causes style to be…

7. Explain Network and Relational data models.   Explain Network and Relational data models   The network model is a database model conceived as a flexible way of representing objects and their relationships. Its distinguishing feature is that the schema, viewed as a graph in which object types are nodes and relationship types are arcs, is not restricted to being a hierarchy or lattice. The relational model for database management is a database model based on first-order predicate logic, first formulated and proposed in 1969.  In the relational model of a database, all data is represented in terms of tuples, grouped into relations. A database organized in terms of the relational model is a relational database.   In the relational model, related records are linked together with a "key".  The purpose of the relational model is to provide a declarative method for specifying data and queries: users directly state what information the database contain…

4. Define RDBMS with example.     Define RDBMS with example RDBMS is the abbreviated form of Relational Data Base Management System.  A relational database management system (RDBMS) is a database management system (DBMS) that is based on the relational model as invented by E. F. Codd, of IBM's San Jose Research Laboratory. Many popular databases currently in use are based on the relational database model. RDBMSs have become a predominant choice for the storage of information in new databases used for financial records, manufacturing and logistical information, personnel data, and much more since the 1980s. Relational databases have often replaced legacy hierarchical databases and network databases because they are easier to understand and use. However, relational databases have been challenged by object databases, which were introduced in an attempt to address the object-relational impedance mismatch in relational database, and XML databases.   …

6. Define deadlock with example.   Define deadlock with example In a database, a deadlock is a situation that occurs when two or more different database sessions have some data locked, and each database session requests a lock on the data that another, different, session has already locked. Because the sessions are waiting for each other, nothing can get done, and the sessions just waste time instead. This scenario where nothing happens because of sessions waiting indefinitely for each other is known as deadlock. Database deadlock example:   Suppose we have two database sessions called A and B. Let’s say that session A requests and has a lock on some data – and let’s call the data Y. And then session B has a lock on some data that we will call Z. But now, lets say that session A needs a lock on data Z in order to run another SQL statement, but that lock is currently held by session B. And, let’s say that session B needs a lock on data Y, but that lock is…

5. Explain about triggers.     Explain about triggers A database trigger is stored code that is executed immediately after a predefined event. It is used to ensure the coordinated performance of related actions. Although implementation varies, all major relational databases support triggers.   A database trigger is procedural code that is automatically executed in response to certain events on a particular table in a database. Triggers can restrict access to specific data, perform logging, or audit data modifications.  

3. Define instances and schemas.   Define instances and schemas In DBMS, Instance is the information stored in the Database at a particular moment. Schema is the overall Design of the Database. In programming, we declare a variable which corresponds to "Schema". But its values changes as and when required which corresponds to "Instance".     Physical Schema describes database design at physical level while a logical schema describes the database design at the logical level. A database may also have several schemas at the view level, sometimes called subschemas, that describe different views of the database.

Explain Database users.       Database Users   Database administrators – DBA is responsible for authorizing access to the database, for coordinating and monitoring its use, and acquiring software and hardware resources as needed.   (adsbygoogle = window.adsbygoogle || []).push({}); Database designers – identify data to be stored in the database and choosing appropriate structures to represent and store the data.  Most of these functions are done before the database is implemented and populated with the data.  It is the responsibility of the database designers to communicate with all prospective users to understand their requirements and come up with a design that meets these requirements.  Database designers interact with all potential users and develop views of the database that meet the data and processing requirements of these groups.  The final database must support the requirements of all user groups.   End Users   Casual End Users – occasionally …

Explain the three levels in data abstraction.       Explain the three levels in data abstraction. Three levels of data abstraction are: 1. Physical level : How the data is stored physically and where it is stored in database. 2. Logical level: What information or data is stored in the database (like what is the data type or what is format of data. 3. View level: End users work on view level. If any amendment is made it can be saved by other name. For the database to be usable it must retrieve data efficiently. This efficiency led designer to use complex data structure in the database.