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Entity integrity
The entity integrity is a result of the key integrity: No primary
key is allowed to be the NULL(=no
value). If NULL-values would be allowed for primary keys, than two tuples could
have NULL as key value and therefore could not be distinguishably anymore. The
key integrity would not be respected. In SQL special routines (unique, not
null) are used for key and entity integrity.
Example:
| ID | Name | Surname | Year |
|---|---|---|---|
| NULL | Miller | John | 1955 |
| NULL | Miller | John | 1985 |
Since the primary key (here ID) of both tuples is NULL, we are
not able to distinct these tuples.
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