L4 domain integrity

DATABASE SYSTEMS

Domains

Understanding Domain IntegrityVersion 1.0

(c) Rushdi Shams, CSE@KUET 1


What are Domains Domains are pools of values from where

values appear in the column of a table Domain of a column is KIND of value it

can represent Column: domain- it defines the kind of

values the column represents Data type and domain are two different

things Data type is a physical thing while

domain is a logical thing

3(c) Rushdi Shams, CSE@KUET

What are domains (continued) I said, domain is a all possible collection of

values that a column can contain, in this example, you have one perspective why I said so!

Say, Staffno column has 4 values in it- 01K28, 01B23, 03X98 and 09Y12 (well, I can’t say what they really mean)! The domain of Staffno is {01K28, 01B23, 03X98 and 09Y12}.


What are domains (continued)

I also said that domain represents the kind of data it represents. The following example will simplify the statement

Say, Streetname and Studentname are two columns of a table. They can both have the same data type like TEXT or CHAR or VARCHAR or whatever…can’t they? But they are different. Aren’t they?



Again, say, there are two columns in a table named rollnumber and salary having the same data type INTEGER, NUMBER or whatever…but they are different, no?

So, studentname and streetname have different domains and rollnumber and salary have different domains

In these two example, you can see the difference between data type and domain as well. Can’t you?



Doamins are thus narrower concept than data types. What is a narrower concept? Well it means, it is descriptive! Degreeawarded column can be represented by simply datatype TEXT (3). But in domains, it is represented by {BA, BS, MA, MS, PhD, LLD, MD}

But remember one thing- you don’t manipulate domains in database engine, you work with data type (as data type is physical and domain is logical thing)



Sometimes domains are large in numbers so that they become difficult to implement. Say, AGE! What if you are representing the domain AGE? You can then use the concept of range. The domain of AGE can be between 1 to 120.


Deep into domains

Now, we will dig into deep of the domain and data type concept

Sometimes, you will need to compare between columns of two tables. Then, you will need the concept of domain.

9(c) Rushdi Shams, CSE@KUET

Deep into domains (continued) SALES table has

Salesperson ID which is eventually the Employee ID of EMPLOYEE table.

So, Mr. Michael Suyama was dealing with Order ID 10643.

You may have to compare them to find out this information.

If the domain of Salesperson ID and Employee ID is not same, you could not compare them!


Deep into domains (continued)

So, Salesperson ID and Employee ID, as they are comparable, are called type-compatible

Trying to combine the relations on Employee ID and Order ID, however, would probably not result in a meaningful answer, even if the two domains were defined on the same data type.


Domain Integrity In a database the domain integrity is

defined by

1. Data type and length

2. NULL value acceptance

3. Allowable values through rules

4. Default value

Back 1

Back 2


Domain Constraints Domain constraints are rules that define legal

values for columns Sometimes more than one rule is needed for a

column to completely define the legal values Unfortunately, SQL and other database engine

generally don’t facilitate with powerful tools to recognize violations on domains.

If employee ID and Salary are both NUMBERs then, if you make a query by relating them, they will certainly give you an answer! Aweful, huh?

But some tools can still be easily manipulated to avoid the violations.


Domain Constraints (continued)

DEFAULT

You can use DEFAULT to set a column value to some specific value. In this example, the data type and length, NULL value acceptance and default value is covered (see Domain Integrity)

CREATE TABLE Course(Course_ID NUMBER(10) NOT NULL,Course_Name VARCHAR(20),Pass_Mark INTEGER(3) DEFAULT 40,PRIMARY KEY (Course_ID)

);



CHECK

This is a good tool to check if violation in putting data have occurred or not. In this example NULL value acceptance, data types and length and the allowable value by rules are covered (see Domain Integrity)

CREATE TABLE Subject(Subject_ID VARCHAR(10) NOT NULL,Subject_Name VARCHAR(20),Credit NUMBER(10)

CHECK(Credit>0 AND Credit<=10),PRIMARY KEY (Subject_ID)

);



If you use CHECK on more than one column in a table, that constraint is also called table constraint.

CREATE TABLE Lecturer(ID VARCHAR(10) NOT NULL,Name VARCHAR(20),Grade CHAR(1),Salary INTEGER(5),CHECK (Salary<50000 OR Grade<=‘B’),PRIMARY KEY (ID)

);


References

Designing Effective Database Systems by Rebecca M. Riordan, Addison Wesley Professional, 2005

Structured Query Language: A Practical Introduction by Akil I Din

Database Systems by Paul Beynon-Devies, Palgrave Macmillan, 2004

www.databasedev.co.uk

http://www.databasedev.co.uk/

L4 domain integrity

Technology

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