lecture 5

INTEGRITY RULES
Relational database integrity rules are very important to good database design. Many (but by no means all) RDBMS enforce integrity rules automatically. Those rules are:

1- ENTITY INTEGRITY

All primary key entries are unique, and no part of primary key may be null. Each row will have a unique identity, and foreign key values can properly reference primary key values, for example... No invoice can have a duplicate number, nor can it be null. In short, all invoices are uniquely identified by their invoice number.

2- REFERENTIAL INTEGRITY

A foreign key may have either a null entry, as long as it is not a part
of its table s primary key, or an entry that matches the primary key
value in a table to which it is related.(every non –null foreign key
value must reference an existing primary key value).It is possible for
an attribute not to have corresponding value, but it will be impossible
to have an invalid entry. for example, A Customer might not yet have
an assigned sales representative(number),but it will be impossible to
have an invalid sales representative(number).as in figure below:


To avoid nulls, some designers use special codes, known as flags, to
indicate the absence of some value.