Rows and Tables 1 :CERTIFICATION OBJECTIVE
02.05.2010 by admin - 0 CommentPosted in Oracle
The relational paradigm models data as two-dimensional tables. A table consists of a number of rows, each consisting of a set of columns. Within a table, all the rows have the same column structure, though it is possible that in some rows some columns may have nothing in them. An example of a table would be a list of one’s employees, each employee being represented by one row. The columns might be employee number, name, and a code for the department in which the employee works. Any employees not currently assigned to a department would have that column blank. Another table could represent the departments: one row per department, with columns for the department’s code and the department’s name.
A note on terminology: what Oracle refers to as a table may also be called a relation or an entity. Rows are sometimes called records or tuples, and columns may be called attributes or fields. The number of “rows in the table” is the “cardinality of the tuples.”
Relational tables conform to certain rules that constrain and define the data. At the column level, each column must be of a certain data type, such as numeric, date-time, or character. The character data type is the most general, in that it can accept any type of data. At the row level, usually each row must have some uniquely identifying characteristic: this could be the value of one column, such as the employee number and department number in the preceding examples, which cannot be repeated in different rows. There may also be rules that define links between the tables,such as a rule that every employee must be assigned a department code that can be matched to a row in the departments table. Following are examples of the tabulated data definitions:
Departments table:
Employees table:
The tables could contain these rows:
Departments:Employees:
