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RDBMS (Relational Database Management System) Jargons
The most popular Relational Database is MySQL. It is an open source SQL database supporting different platforms like Windows, Linux and MAC Operating Systems. The other relational databases available are Oracle, MS SQL Server and MS Access. The features of RDBMS are
• High Availability
• High Performance
• Robust Transactions and support
• Ease of management
• Less cost
In relational database model, table is defined as the collection of data organized in terms of rows and columns. Table is the simple representation of relations. The true relations cannot have duplicate rows where as the table can have. The example of Employee table is shown below in Table 3.1.
The table consists of several rows and columns. Table can be divided into smaller parts, in terms of columns. Each column is known as attributes. In the Employee table four attributes are available namely Id, Name, Age and Salary. The attribute is defined in a table to hold values of same type. This is known as Attribute Domain. In the Employee table, the Name field will hold only characters not the numbers in it.The vertical entity in a table is known as Attribute or Column.
A single entry in a table is called as Row or Record or Tuple. Set of related data’s are represented in a row or tuple. The horizontal entity in a table is known as Record or row. See Table 3.2
The candidate key that is chosen to perform the identification task is called the primary key and any others are Alternate keys. Every tuple must have, by definition, a unique value for its primary key. A primary key which is a combination of more than one attribute is called a composite primary key.
A foreign key is a “copy” of a primary key that has been exported from one relation into another to represent the existence of a relationship between them. A foreign key is a copy of the whole of its parent primary key i.e if the primary key is composite, then so is the foreign key. Foreign key values do not (usually) have to be unique. Foreign keys can also be null. A composite foreign key cannot have some attribute(s) null and others non-null.
An attribute or group of attributes, which is sufficient to distinguish every tuple in the relation from every other one is known as Super Key. Each super key is called a candidate key. A candidate key is selected from the set of Super Key. While selecting candidate key, redundant attributes should not be taken. The candidate key is also known as minimal super keys.
A key with more than one attribute to identify rows uniquely in a table is called Composite key. This is also known as Compound Key.
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