- table
- column
- row
- field
Relational model is based on concepts taken from algebra. These concepts are:
- relationship
- operator acting on the relations and giving as a result of the relation
Relations are presented in tables, and select data from tables to the output of relational operators on these tables.
Relational Operators
Relational operators are defined as follows:
- SELECTION - downloading data from the relationship, as a result we get all the rows that satisfy the required condition
- PROJECTION - operation retrieve all rows, but only indicated the columns of these rows
- Cartesian product - the result of combining each of each row of two relations
- CONNECTION - combination of the two relations by some criterion combining some lines of both relations
- UNION - all rows from both the relations
- Intersection - lines common to both relations
- Set difference - lines, which occur in one, but not in other relationships
Properties of relational database
A relational database has the following properties:
- base is seen by the user as a collection of tables
- names of the tables in the database must be unique
- tables consist of rows and columns
- the language used to manipulate the database is a language based on algebra nieproceduralny relationship. Today is a standard SQL.
The tables in a relational database have the following properties:
- the rows in the table must be different
- a table has no columns of the same name
- the order of rows is not specified
- the order of columns is not specified
- field values should be elementary
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