Relational Databases Relational databases have long been associated with big enterprises, but lately they've been finding their way to an audience of different users. Now, most large-scale web sites use databases for at least some portion of their content. User registration information, inventory records, and event calendars are all managed through a database. Databases are applications running on the server that provides quick and efficient access to large amounts of organized data. In addition to storing information, databases can establish a relationship among various pieces of data, provide powerful sorting, searching, and merging capabilities.
Databases can also help manage access to data by supporting multiple users with different capabilities and levels of access. A database management system, or DBMS, gives the user access to their data and helps them transform the data into information. Such database management systems include dBase, Paradox, IMS, and Oracle. These systems allow users to create, update, and extract information from their databases. Compared to a manual filing system, the biggest advantages to a computerized database system are speed, accuracy, and accessibility.
A database is a structured collection of data. Data refers to the characteristics of people, things, and events. Oracle stores each item in its own field. The name of a field usually reflects its contents.
Each DBMS has its own rules for naming the data fields.A field has little meaning unless it is seen within the context of other fields. In Oracle, the fields relating to a particular person, thing, or event are bundled together to form a single, complete unit of data, called a record. Each record is made up of a number of fields.
No two fields in a record can have the same field name. During an Oracle database design project, the analysis of your business needs identifies all the fields or attributes of interest.Relational systems started the current domination in database solutions around 1985. The most popular relational database management systems include Oracle, Ingres, and Informix. The key characteristics of a relational database are: ? Objects have a complex relationship to each other, which is the key to the way a relational database works.
Queries can be based on complex relationships between objects. ? Relational databases support sophisticated transaction-based updates, and provide update tools that make use of these. The database is essentially centralized. In some cases, there is limited replication to give copies of the entire database.
? The schema is entirely application/user defined. A relational database is a general-purpose tool, which can be used to deal with a very wide range of problems. Generally, multiple databases do not share common schema. The first characteristic is the key benefit of a relational system.Whenever there is a need to perform sophisticated analysis on data and the relationship between elements of data is non-trivial; a relational database is a good choice. The key strength of a relational database is the ability to make complex queries about the relations between objects.
Examples of relational systems include an accounting system or an enterprise resource planning system. Oracle makes it very easy to link the data in multiple tables. It stores data in two or more tables and enables you to define relationships between the tables. The link is based on one or more fields common to both tables. Common features among relational databases include: ? Persistent storage of arbitrary size ? Transaction management for concurrency ? Transaction management for recovery ? Authorization and security ? Consistency and integrity ? Query language ? Logical and physical data independence ? Application generation ? Client-server distributed database Databases support three kinds of data.
Transient data lasts only for the invocation of a single program. Stable data is recoverable data that is resilient to failure. Persistent data is accessible beyond the process that creates it. In relational databases, persistency is obtained by all data stored in tables. SQL operations are automatically applicable to these data and the database stores all changes.Database transaction is a unit on a database that takes it from one consistent state to another consistent state.
Relational database transactions are short. The system strives to complete them in the shortest possible time so that the database resources can be freed for other concurrent transactions. Transaction management serves the purpose of database recovery. The principal devices to recover the database from failure are logging and backup.
Relational databases use techniques that are typically more advanced for managing recovery. Authorization and security is achieved by providing a discretionary control over access to objects according to privileges. Consistency and integrity are obtained through the implementation of referential integrity between tables.A query language is a deceptive term standing for a database manipulation language. SQL is used to query and update database tables.
It has a closure property. It takes a table as input and returns another table as output. Data independence is the degree to which changes to logical and physical data structures do not affect existing programs. Because they are based on a logical model, relational databases exhibit a very high degree of physical independence.Changes and deletions are much more troublesome.
In relational databases, the queries are always evaluated on the server. This improves performance for typical business-like transactions that process large data volumes before returning short answers. To get data from a database, we issue queries in Structured Query Language (SQL). SQL is a simple language that specifies the syntax for constructing a search request and controlling how the results are returned. There are some variations in syntax and capabilities among database vendors, but the basic commands are generally compatible among databases.
An SQL query specifies what information you are interested in and what table it can be found in. One of the most fundamental SQL commands is SELECT, which retrieves a set of information from a table. The * character means to include results from every column in the table. The WHERE clause selects a single record from the table and looks for a particular value in a particular field. INSERT allows data to be added by adding rows to a table.
The DELETE command is used to remove rows from a table that match a specified condition.Since relational databases are beneficial in their way to establish relationships among the various tables, data can be stored in separate but interrelated tables rather than trying to put it all into one table. The use of a unique key in each table can also make database access faster and more efficient. When the database expects queries based on the contents of a particular column, it can optimize its internal index files appropriately.
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