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Relational data integrity

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Presentation on theme: "Relational data integrity"— Presentation transcript:

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
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 entity integrity constraint
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 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

17 Nulls

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

19 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

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 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

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