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 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

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

Types of integrity constraints 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

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

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>

Candidate key - example

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

Foreign key - example

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

Referential diagram

Foreign key rules - motivation

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%?

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

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

Nulls

Nulls representing missing/unavailable information primary key and nulls foreign key and nulls

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

Domains - example of integrity constraints

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”

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

Normal forms - example

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

Summary the relational data model SQL implements the relational 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