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Database Attributes Continued...
Keys
An attribute where every value is unique.
e.g. student number uniquely identifies a particular student thus it can be used to index all the tuples in the table.
Each relation has a key (may require more than one field) thus there are no duplicates in a relation.
Null values are forbidden in the primary key.
Referential Constraint or foreign key
The information in one relation (Marks) is given meaning and completeness through reference to the primary key of another relation (student)
Also to the relation subject through the primary key of the subject relation.
Constraint - the references are significant because values in the relation Marks are equal to values present in the relation Student
Marks relation| Exam paper number | Student | Subject | Grade |
| 1 | 2001XXXX | Database | F |
| 5 | 2001XXXX | Networks | D |
| 2004 | 2001XXXX | Multimedia | HD |
| 195 | 2001XXXX | Networks | P |
| 63 | 2001XXXX | Multimedia | P |
| Number | Name | uid |
| 2001XXXX | Yu | yu01 |
| 2001XXXX | Wu | wu21 |
| 2001XXXX | John | NULL |
| 2001XXXX | Smith | srt03 |
| 2001XXXX | Owen | ojm01 |
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Page updated August 25, 2004
