1 Relational model concepts Key constraints Referential integrity constraint Steen Jensen, autumn...
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Transcript of 1 Relational model concepts Key constraints Referential integrity constraint Steen Jensen, autumn...
The relational model
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•Relational model concepts•Key constraints•Referential integrity constraint
Steen Jensen, autumn 2013
Relational model concepts, 1
The relational model represents the database as a collection of relations
Each relation resembles a table of values
The data type describing the possible values for each column (attribute) is represented by a domain of possible values, e.g. the data type for Age could be an integer number between 15 and 80
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Figure 5.1 from chapt. 5 in Elmasri
Relational model concepts, 2
The above representation is also called a relation schema
The degree of a relation is the number of attributes of its relation schema – thus the relation STUDENT has a degree seven
Each value for a specific attribute is atomic - so composite and multivalued attributes are not allowed
The special value called NULL can be used, if the value is unknown or doesn’t apply
Some relations represent facts about entities and other relations represent facts about relationships
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Key constraints
A relation schema may have more than one key – each of the keys is called a candidate key
One of the candidate keys is chosen as the primary key, so it is possible to identify tuples in the relation – thus a primary key can’t have a NULL value
The primary key is underlined
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Referential integrity constraint, 1
Referential integrity is used to maintain the consistency among tuples in two relations
The referential integrity states, that a tuple in one relation that refers to another relation must refer to an existing tuple in that relation
To define referential integrity a foreign key is used
The relation in which the foreign key is placed is called the referencing relation, and the relation which the foreign key points to is called the referenced relation
The next slide shows an example of referential integrity
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Referential integrity constraint, 2
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Exercise in foreign keys Take a look at exercise 5.15 p. 165 in Elmasri chapter 5 (physical
page 26)
Make your solution in a format corresponding to the figure shown on the previous slide (approx. 30 minutes)1. Start by pairing up with the student next to you and suggest
the appropriate foreign keys
2. Share and discuss your solution with the other pair of students at your table
Finally the solution should be presented/discussed in class
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