1. Integrity
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2. Objectives
To develop an understanding of integrity and integrity constraints.
To develop a means of expressing constraints
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3. Integrity Constraints
Constraints allow only valid combinations of data in database.
Need to ensure data manipulation operations are correct.
Constraints can be enforced by the DBMS, application program, or user.
Universally applied to all users only when defined in DBMS logical schema.
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4. Therefore designer must be able to
DBMS should
prevent invalid operations.
or carry out compensatory actions to mitigate effects of operation.
Therefore designer must be able to
identify and reject offending operations
or identify and implement a compensatory operation to ensure database left in a valid state.
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5. Referential Integrity
No primary key attribute(s) should have a null value or hold duplicate values.
If a base relation includes a foreign key then every value of the foreign key must
be equal to a value of a primary key in a relation.
or be wholly null.
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6. SQL-92 e.g.may have this clause for table ‘employee’ for instance-
FOREIGN KEY(branch_no) REFERENCES branch ;
clause in CREATE and ALTER statements.
INSERT/UPDATE rejected if attempt made to create a foreign key value in base relation holding foreign key without a matching candidate in base relation with primary key.
E.g. cannot insert a branch_no until that value exists in primary key of table branch
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7. Referential Action
FOREIGN KEY clause has ON UPDATE and ON DELETE subclauses- referential action.
E.g.
FOREIGN KEY (staff_no) REFERENCES staff ON DELETE SET NULL;
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8. Referential Action Possibilities
CASCADE- delete from primary key table and matching rows in foreign key table. Deleted rows may have a candidate key used as a foreign key in another table so FOREIGN KEY rules for this may be triggered.
RESTRICT-can only delete from primary key table after all matching foreign key references deleted
SET NULL/DEFAULT- delete row from primary key table and set foreign key values to NULL or default value.
NO ACTION- rejects delete operation.
Similar for Update operations
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9. Example Create table Car (reg_no char(8), model varchar(20),
engine_size integer,
constraint car_key primary key (reg_no)
constraint is_costed_at foreign key (engine_size) references hire-rate,
constraint is_of_type foreign key (model) references type_of_car…
Assumes tables hire_rate and type_of_car exist
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10. What happens to Car if an engine_size is removed/updated in Hire_rate?
Is_costed_at
Car
Type_of_car
Hire_Rate
Is_of_type
0..*
1..1
Car (registration_no, model, engine_size)
Type_of_car (model, make)
Hire_rate (engine_size, daily_hire_rate)
When considering referential action the questions to ask here would be-
What happens to Car if an engine_size is removed/updated in Hire_rate?
What happens to Car if a Model is removed/updated in Type_of_Car?
What happens to foreign key values in a primary/foreign key relationship if the target (primary key) of a foreign key reference is removed/updated?
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11. uses RESTRICT referential action unless programmed otherwise.
SQL Server 7
uses RESTRICT referential action unless programmed otherwise.
This means that data/tables can only be deleted/dropped in order
i.e. data/tables at many ends (foreign key end ) of relationships first.
SQL Server 2000
CASCADE and NO ACTION available explicitly.
RESTRICT used if no referential action specified explicitly
Cascade does not apply to DROP so order of dropping tables still applies
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12. Dialogue above exists for client-side databases
With Server-side referential integrity defined via the SQL schema
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13. Equivalent dialogue server-side
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14. Summary Data Integrity is a necessary feature of any database system.
Referential integrity rules are necessary in relational database systems.
SQL provides a means of specifying foreign keys and the associated referential actions
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