1. Domains
4/17/2019
See scm-intranet

2. Objectives To understand the idea of a domain.
To understand the idea of a domain constraint in terms of integrity constraints.
To develop a means of expressing  domain constraints. 
4/17/2019
See scm-intranet

3. These domains must be defined.
Domains represent the set of valid values from which an attribute, or set of attributes, can derive values.
These domains must be defined.
Data types alone are clearly insufficient
e.g. an order number is more than simply a string of characters or an integer.
4/17/2019
See scm-intranet

4. Sets Domains are sets. Sets can only be defined
Sets can only be defined 
by enumeration (listing every value i.e. every member of the set) 
or by producing a predicate or rule which defines a valid member of the set.
4/17/2019
See scm-intranet

5. Definition The following outlines the definition of domains in SQL-92.
The mechanisms available for definition, and capabilities vary from system to system 
4/17/2019
See scm-intranet

6. Enumeration Enumeration can written informally thus-
attribute: enumerated set
x : [0..1]
y: [a..z]
d: [U00..U99]
colour: [red,yellow,green]
4/17/2019
See scm-intranet

7. Predicates Predicates can be defined in many ways including-
English narrative
e.g. an order date must be less than a delivery date.
Masks
e.g. reg_no: (ch1)(d)(d)(d)(space)(ch2(ch2)(ch2)
Where
ch1:[a,b,c,d,e,f,g,h,j,k,l,m,n,p,q,r,s,t,u,v,w,x,y,z]
ch2:(a..z)
d:[0..9]
4/17/2019
See scm-intranet

8. order_date LT delivery_date set operations
Expressions
e.g.
order_date LT delivery_date
set operations
shipping_post_code :
select post_code from valid_post_code 
(assumes table of valid post code pre-exists).
4/17/2019
See scm-intranet

9. Implementation
Ideally, domains should be specified in SQL but this is not always possible, especially if using SQL-89.
However, domains still need to be defined in some manner as they are form important integrity constraints-in addition to primary keys, referential integrity etc..
4/17/2019
See scm-intranet

10. e.g. char, varchar, date, money, integer, decimal etc...
Every domain will have a data type as a minimum part of the definition. 
e.g. char, varchar, date, money, integer, decimal etc...
    
4/17/2019
See scm-intranet

11. Data Types and Valid Operations
Care needs to be taken with data types. Selection of a data type implies a selection of valid operations
e.g. selecting integer as a data type for a domain or attribute implies that integer arithmetic is valid for that domain/attribute.
So, if house number was defined as an integer addition of house numbers would be a valid operation.
What does it mean to add two house numbers together??
4/17/2019
See scm-intranet

12. Example Domain Definitions in SQL-92
Create Domain
e.g.
A domain is defined thus-
create domain title_type varchar(35) check (value is not null);
This definition can then be used in a SQL schema thus-
create table movie_title
(title title_type, );
4/17/2019
See scm-intranet

13. create domain revenue as decimal(9,2)
e.g.
create domain revenue as decimal(9,2)
constraint price_not_negative check (value>=0) not deferrable;
-here the domain constraint has been given a name i.e. price_not_negative
4/17/2019
See scm-intranet

14. Using Check constraints
N.B. There is no CREATE DOMAIN command in SQL*Server so check constraints must be used.
 e.g
create table movie_title
(our_cost decimal(9,2) check (our_cost<100.00), );
e.g.
(our_cost decimal(9,2) check (our_cost< and our_cost>0), );
4/17/2019
See scm-intranet

15. Other examples of Check clauses
e.g.
check (current_sale_price<=(select (max_price) from competition_prices))
check ((our_cost between .99 and ) and
(regular_rental_price between ) and
(rental_date ) );
4/17/2019
See scm-intranet

16. Check Constraints and Enumeration
e.g.
Enumeration
check movie_type in ('horror','western' );
check (answer_one, answer_two) in values('no','no'),('yes','no');
4/17/2019
See scm-intranet

17. Attribute domain constraint
Constraint Names
Constraints can be defined and given a name. They can then be referred to in any table using this name.
e.g.
constraint check_movie_type check (movie_type in ));
create table movie
(movie_type mov_type check_movie_type,…..);
         the order of items in brackets is –
   Attribute domain constraint
4/17/2019
See scm-intranet

18. Identity Property (AutoNumber)
CREATE TABLE new_employees
( id_num int IDENTITY(1,1),
fname varchar (20),
minit char(1),
lname varchar(30) )
Syntax
IDENTITY [ ( seed , increment ) ]
seed
Is the value that is used for the very first row loaded into the table.
increment
Is the incremental value that is added to the identity value of the previous row that was loaded.
If neither is specified, the default is (1,1).
If an identity column exists for a table with frequent deletions, gaps can occur between identity values. If this is a concern, do not use the IDENTITY property.
4/17/2019
See scm-intranet

19. Deferment Constraints can be deferrable or not deferrable
Default is not deferrable
deferrable- means checking deferred until transaction commited.
not deferrable- means check applied at end of every sql statement.
4/17/2019
See scm-intranet

20. Summary Domains are an important integrity constraint
Data types are not sufficient
Schema level domain implementation removes the need for checking in application programs
This can lead to reduced maintenance costs and reduced threat to database integrity
4/17/2019
See scm-intranet