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Integrity Checking

Integrity Checking. Edel Sherratt. From before. Entities all become relations (tables) Relationships some become relations (tables) some are implemented by use of primary and foreign keys Enterprise Rules

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Integrity Checking

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  1. Integrity Checking Edel Sherratt

  2. From before Entities • all become relations (tables) Relationships • some become relations (tables) • some are implemented by use of primary and foreign keys Enterprise Rules • Some are enforced by primary and foreign keys (entity integrity and referential integrity), or by uniqueness constraints • Some need additional coding • using DBMS facilities • using application code if necessary • we know which by their cardinality signatures

  3. Simple Case 1 - example ‘Many’ end contains foreign key reference to ‘one’ end Integrity rules: Every tour is provided by exactly one operator; an operator may provide no tours, or may provide several.

  4. Simple Case 1 - SQL • CREATE TABLE Operator ( • code VARCHAR(5) PRIMARY KEY, • name VARCHAR(50), • tel VARCHAR(20), • UNIQUE(code,name,tel) • ); • CREATE TABLE Tour ( • ID INTEGER PRIMARY KEY, • Start VARCHAR(20), • Visiting Varchar(20), • Operator VARCHAR(5) NOT NULL • REFERENCES Operator(code)), • UNIQUE(ID,Start,Visiting,Operator) • );

  5. Simple Case 2 - example Many-to-many relationship → relation with composite key Integrity rules: A student may take no modules, or may take one or more modules. A module may have no registered students, or may have several.

  6. Simple Case 2 - sql • CREATE TABLE module ( • module_idVARCHAR(6) PRIMARY KEY, • title TEXT); • CREATE TABLE student ( • student_id VARCHAR(10) PRIMARY KEY, • name TEXT, • scheme TEXT); • CREATE TABLE takes ( • student_idVARCHAR(10) REFERENCES student(student_id), • module_code VARCHAR(6) REFERENCES module(module_id), • PRIMARY KEY (student, module));

  7. Simple Case 3 - example • CREATE TABLE sponsor ( • Company_codeVARCHAR(6) PRIMARY KEY, • Company_name TEXT, • UNIQUE(Company_code,Company_name)); • CREATE TABLE student ( • student_id VARCHAR(10) PRIMARY KEY, • name TEXT, • scheme TEXT); • CREATE TABLE sponsoring ( • Student_id VARCHAR(10) REFERENCES student(student_id), • Company_codeVARCHAR(6) REFERENCES module(module_id), • PRIMARY KEY (student_id));

  8. Simple Case 3 - SQL New relation with same key as ‘many’ end Integrity rules: A student may have at most one sponsor. A sponsor may have no students, or may have several.

  9. Lecturer Module 1..* 1..1 1..1 1..* Problem Case - example With primary and foreign keys we cannot do better than the optional case. Plant the key of the ‘one’ side in the ‘many’ side

  10. Problem case (continued) Lecturer{name} Module{name, lecturer* references Lecturer(name)} Mandatory participation (each lecturer teaches at least one module) cannot be enforced using entity and referential integrity constraints

  11. Problem case (continued) • How can we ensure that every instance of Lecturer is involved in at least one relationship with a Module? • Cannot enforce it using primary and foreign keys • Can use functions and triggers • http://www.aber.ac.uk/~dcswww/Dept/Teaching/CourseNotes/current/CS27020/ • Lectures/integrity-checking/createMandatory.sql • Lectures/integrity-checking/createMandatoryFull.sql

  12. Mandatory participationSQL implementation CREATE TABLE lecturer ( name text PRIMARY KEY); CREATE TABLE module ( name text PRIMARY KEY, lecturer text REFERENCES lecturer (name) INITIALLY DEFERRED);

  13. A PL/pgSQL function CREATE FUNCTION check_teaching_duties () RETURNS trigger as $has_module$BEGIN IF NOT EXISTS( SELECT * FROM module WHERE module.lecturer LIKE NEW.name) THEN RAISE EXCEPTION 'Lecturer % must refer to module', NEW.name; END IF; RETURN NEW;END;

  14. A PL/pgSQL trigger $has_module$ LANGUAGE plpgsql;CREATE TRIGGER has_module AFTER UPDATE OR INSERT ON lecturer FOR EACH ROW EXECUTE PROCEDUREcheck_teaching_duties();

  15. A transaction to load data BEGIN;INSERT INTO module (name, lecturer) VALUES ('Databases', 'Hardy'), ('Law', 'Bott'), ('Maths', 'Holstein'), ('Robots', 'Hardy');INSERT INTO lecturer (name) VALUES ('Bott'), ('Hardy'), ('Holstein');COMMIT;

  16. PL/pgSQL functions Disabling and re-enabling triggers usually happens in the context of functions that carry out transactions Like adding a new lecturer and allocating a module as one transaction (example online) What about deleting a module? When should that not be allowed? Need to exercise judgement!

  17. Integrity Checking - summary

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