Date post: | 13-Feb-2017 |
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Education |
Upload: | university-of-education-lower-mall-lahore |
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Organization of DataDATABASE MANAGEMENT SYSTEM
Data Hierarchy• Data is organized in a hierarchy that begins with the
smallest to the largest unit of data, as it progresses up the hierarchy.
Database
File
Record
Field
Byte
Bit
Data Hierarchy• Bit (Either 0 or 1, Smallest unit of data)• Byte (Combination of 8 bits, Each byte makes 1
letter)• Field (Combination of bytes, Category)• Record (Combination of fields, Related to one
person, employee, student, member or organization)• File (Combination of records, Related to different
persons, employees, students, members or organizations)• Database (Combination of files, Largest unit of data)
Key (Database)• A value that is uniquely identified each record in a table.Examples• Roll No.• Student’s Name• Father’s Name• Course • Session• Date of Admission Etc.
Types of Key• Candidate key is a column, or set of columns, in a table that can
uniquely identify any database record without referring to any other data. • Primary key (Each table may have one or more candidate keys, but
one candidate key is special, and it is called the primary key)• Alternative key is a key associated with one or more columns
whose values uniquely identify every row in the table, but which is not the primary key.• Super key is a combination of columns that uniquely identifies any
row. May be 2 or 3 or more, as according (Direct proportional) to data.• Foreign Key is a column (or columns) that references a column
(most often the primary key) of another table. The purpose of the foreign key is to ensure referential integrity of the data.
Candidate keys
Primary key Alternati
ve keys
Super keys
Foreign keys
Foreign keys