 |
 |
 |
 |
 |
 |
 |
 |
Database Attrabuites
Teacher relation
| Subject | Tutor |
| Databases | Getta |
| Networks | Fulcher |
| Multimedia | McKerrow |
Prospectus relation
| Degree | Subject | Year |
| Comp Sci | Database | 2 |
| Comp Sci | Networks | 2 |
| Comp Sci | Multimedia | 4 |
| IACT | Networks | 3 |
| IACT | Multimedia | 4 |
Indexes into the student relation and the subject relation
| Student | Subject | Grade |
| 2001XXXX | Database | F |
| 2001XXXX | Networks | D |
| 2001XXXX | Multimedia | HD |
| 2001XXXX | Networks | P |
| 2001XXXX | Multimedia | P |
| Number | Name | uid |
| 2001XXXX | Yu | yu01 |
| 2001XXXX | Wu | wu21 |
| 2001XXXX | John | NULL |
| 2001XXXX | Smith | srt03 |
| 2001XXXX | Owen | ojm01 |
Schema
The database's description. - the structure
Relation(attribute, attribute) - Teacher(Subject, Tutor)
Instance
The values stored in the database.
Rows of the table vary with time.
Instance of a relation is a collection of the rows at a specific time.
Null value
Signifies the absence of information
Student(Number, Name, uid) - uid is abscent for Student John
Integrity Constraints
Property that must be satisfied by all correct database instances
a constraint is a predicate with either a TRUE or FALSE value
e.g. (mark >=85) == (grade == HD)
e.g. Marks(value, grade) - 64, HD - violates the integrity constraint
Back
Page updated August 25, 2004
