Database Reference
In-Depth Information
Types of fields in an improperly or poorly designed database.
A multipart field (composite field), which holds two or more distinct items within
its value.
A multivalued field, which contains multiple instances of the same type of value. A
calculated field, which holds a concatenated text value or the result of a
mathematical expression.
4.5 RECORD
A
record
(a tuple) represents a unique instance of the subject of a table. It is composed
of the entire set of fields in a table, regardless of whether or not the fields contain values.
Each record should be identified throughout the database by a unique value in the primary
key field of that record. Records are a key factor in understanding table relationships. Table
relationships deals how a record in one table relates to other records in another table.
4.6 RELATIONS
A relation may be thought of as a
set of rows.
A relation may alternately be though of as a
set of columns.
There is only one data structure in the relational data model - the relation.
The relational data model consists of the following three component parts:
structure
- a uniform single data structure type called a relation
manipulations
a set of
operators that transform relations into other relations
behaviour -
general integrity rules
that guard the consistency of any database
Example 2:
Let us take a product of the domains defined in Example 1, that is
P
=
PERSONNAMES ×
CITIES × -DATES-OF-BIRTH.
P
will then contain all tuples that were obtained by com-
bining all the values from these three domains:
P={<Ajit Kumar, 03-02-1940, Hoshiarpur>, <Ajit Kumar, 03-02-1940, Jaland-
har>,……………, <Ajit Kumar, 13-02-1947, Ludhiana>,…………, < Aman, 1- Jaland-
har>,……………, <Ajit Kumar, 13-02-1947, Ludhiana>,…………, < Aman, 1- 09-1960,
Hoshairpur>,..…<Balraj, 31-12-1999, Bathinda>}
'A person with
person-name
exists, is of 50 years of age or more, was born on
date-of-birth
at
city
', the corresponding relation might only hold the following triples:
PERSON50={<Ajit Kumar, 12-10-1950, Hoshiarpur>,
<Aman, 12-11-1950, Ludhiana>,
<Akash, 13-11-1951, Jalandhar>}
