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Relational data integrity

Relational data integrity . Lecture 8 . Outline. integrity constraints and data definition candidate keys foreign keys nulls domains conditional expressions normal forms. Constraints. in real life systems constraints exist between data values

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Relational data integrity

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  1. Relational data integrity Lecture 8

  2. Outline • integrity constraints and data definition • candidate keys • foreign keys • nulls • domains • conditional expressions • normal forms

  3. Constraints • in real life systems constraints exist between data values • it would be useful to communicate these constraints to the database system • data is associated with a meaning • stating some constraints on data  describing a part of the meaning • 90% should be spent on integrity constraints definition

  4. Examples of possible inaccuracies how would you express, in NL, integrity constraints that would avoid the above situations?

  5. Types of integrity constraints • integrity constraint • application specific integrity constraints • examples • mechanisms • domains • conditional expressions • normal forms • generic (inherent to the relational model - resulting from definitions) • entity integrity : primary key • referential integrity : foreign key

  6. Data definition • integrity constraints definition in a relational language should include • primary key definition • candidate keys definition • foreign key definition - including foreign key rules • conditional expressions

  7. Example - SQL data definition (in brief) • CREATE TABLE <relation name> ( • @<attribute definition><conditional expression>, • <primary key definition>, • @<candidate key definition>, • @<foreign key definition>, • @<conditional expressions> • ) ; • <primary key definition> ::= PRIMARY KEY ( <set of attributes> ) • <candidate key definition > ::= CANDIDATE KEY ( <set of attributes> ) • <foreign key definition> ::= FOREIGN KEY ( <set of attributes> ) • REFERENCES <relation name> • ON DELETE <option> • ON UPDATE <option> • CREATE ASSERTION <name> CHECK <conditional expression>

  8. Candidate key - example

  9. Candidate key • candidate key • uniqueness property • irreducibility property • entity integrity constraint • simple/composite • primary/alternate

  10. Foreign key - example

  11. Foreign key • foreign key (FK) •  corresponding candidate key (CK) in another relation •  FK  CK such that FK = CK (reverse not required) • target/referenced relation/tuple | referring relation/tuple • referential integrity constraint • foreign keys and PostgreSQL

  12. Referential diagram

  13. Foreign key rules - motivation

  14. What happens if ... • short saving accounts are not offered by the bank anymore? • the “code” and name for small saving accounts is to be changed to ‘i-sav’ and ‘instant saving’ respectively? • the interest for s-sav is to be decreased by 0.3%?

  15. Foreign key rules • the modifications are performed in the REFERRED relation • rules • ON DELETE RESTRICT • ON DELETE CASCADE • ON UPDATE RESTRICT • ON UPDATE CASCADE • split into four groups: think of one example for each situation; don’t use the study guide

  16. Examples • on delete restrict • students and books • on delete cascade • employees and children • on update restrict • students and modules • on update cascade • employees and departments

  17. Nulls

  18. Nulls • representing missing/unavailable information • primary key and nulls • foreign key and nulls

  19. Domains • expressing integrity constraints on scalar values • constraints on permissible scalar values • constraints on the applicability of scalar operators • SQL • does not support domains • offers other mechanisms

  20. Domains - example of integrity constraints

  21. Conditional expressions • will be studied with SQL • pointer forward: • the university’s database:“a student has two choose two options (1/2cu courses) in the final year; if the students chooses an extra optional course than the final year project will count as only 1/2cu”

  22. FDs, MDs, and JDs • particular constraints • functional dependencies • multiple dependencies • join dependencies • expressed by means of normal forms • extensively studied in the following lectures

  23. Normal forms - example

  24. Integrity constraints - further considerations • state and transition integrity constraints • example • when and how are the integrity constraints applied • depends on the type • ‘normal forms’ and ‘domains’ - always • entity and referential - after each transaction • conditional expressions • default : after each transaction • other possibilities? • correct database = the logical AND of the set of integrity constraints is satisfied after each transaction

  25. Summary • the relational data model • data objects • operators • integrity constraints • SQL implements the relational model • the subject of the next lectures • you know what a relational model is, but do not know yet how to design one

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