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