1 Agenda TMA02 M876 Block 4

2 Model of database development data requirements conceptual data model logical schema schema and database establishing requirements data analysis database design implementation

3 Steps for Database Design Use a relational conceptual data model to give a set of tables for an initial database design Do the tables represent the data in an acceptable way, according to the given criteria for usability, efficiency and so on? Define constituents of each table columns primary key foreign keys constraints Revise tables Implementation Yes No

4 Defining Columns Choose the most appropriate data type for a column. Before choosing the data type, we have to know:- Characteristics of the expected values for the column. Operations for the column. Data TypeCharacteristicsOperations NumericRange, Precision +, -, *, / CharacterLength|| Datetime-

5 NOT NULL Constraints NOT NULL constraints apply to Primary key Foreign key (mandatory participation condition) A row of table which may become meaningless if a column is NULL Two extreme forms of policy Every column is defined as NOT NULL  Every row is meaningful and usable Allow NULL to occur  Flexibility of entering incomplete rows of data

6 Default Values Any NOT NULL column for which there will be missing values must have a default value. Two kinds of default value:- Real value Non-real value  In formulating a query, you should consider whether the value should be including in the processing or not.

7 Codes Store alternative values instead of the actual values (e.g. S - Single, M – Married, D - Divorced) Advantage Less storage space Disadvantage Not immediately understandable Solution: create an additional code table and join this table to the other tables so that only the decodes are visible to users.

8 Domains SQL Domain  Relational Domain Comparison of two SQL columns requires only that they are of comparable data types, NOT that they are the same domain SQL domain provides a common definition, including both constraints and default values, that can be shared by a number of columns. For example, if we define a SQL domain GPA which is Numeric(3, 2), all columns of this domain will be rounded to 2 decimal places automatically when calculating GPA.

9 Defining Tables Transformation of relations to SQL tables Three issues shall be considered:- Defining the primary key Defining the foreign key Defining constraints

10 Defining Primary Keys Primary keys should be chosen according to the criteria of uniqueness, minimality and not being null. Minimality is important since long primary key will consume storage space and slow the processing associated with primary key, such as joining tables. Sometimes, primary key is replaced by surrogate (unique values generated by a DBMS) to achieve minimality.

11 Defining Foreign Keys When defining a foreign key, it is important to consider whether any referential action should be associated when a referenced row is to be deleted. There are three possible referential actions:- RESTRICT SET NULL SET DEFAULT CASCADE Participation condition at the 'many' end of the relationship represented by the foreign key determines the choice of referential action. Participation ConditionReferential Actions MandatoryRESTICT or CASCADE OptionalRESTICT or SET DEFAULT or SET NULL

12 Defining Constraints Kinds of constraints:- Primary key, expressed as PRIMARY KEY Alternate key, expressed as UNIQUE Referential constraint, expressed as FOREIGN KEY Mandatory participation condition, expressed as NOT NULL for one end and CHECK clause for the other end NOT all constraints in a conceptual data model have to be defined in a database schema. Two reasons for this: processing inconsistency and processing inefficiency.

13 Revising Tables Avoiding NULL to enhance usability. Denormalization Normalization, which divide a table into tables, is to avoid duplication of data and thus prevent anomalies. Reverse process which combines tables to provide more efficient retrieval.

14 Addition Data Derived data Additional column  Efficient  Need to maintain consistency of the derived data – use Trigger Create view  Inefficient Extra tables Snapshot  Auditing and archiving Summary table

15 Usage Define views and privileges Reason for defining a view as the means to access data in a database rather than using the base tables: Usability, because specific data required by users can be defined as a view, providing a simplified way to access the data; Flexibility, because a view enables changes to be made to base tables without affecting the users’ view of data, providing data independence; Access control, because defining users’ privileges for views, rather than base tables, allows more precise control on the data made available to users.