Identifying Entities
Houston we have a problem! Database design left very late in the process Not less important than other aspects Typically where it always gets landed in the process! It makes sense to work through each layer in turn There is a lot to think about here that is both complex and subtle Some of this will serve as revision Explore the difference between classes and entities
Document Standards Problems compounded by differing documentation standards Chen notation (Peter Chen 1976) Crows Foot Notation UML Entity Relationship Diagrams an afterthought in UML? (Relational v Object Oriented Databases)
The Chen Notation House M 1 On Site A house must be on a site A site may or may not have many houses The dots indicate the membership class of related entities
The Crows Foot Notation House No indication of membership class Site
UML Notation House Site 0. . * 1 Again no indication of membership class Pretty much identical to class diagram
Which is Best? Familiarity and personal preference What standard you are told to use Which standard the software we are using i. e. Enterprise Architect “encourages” us to use UML but there are plug-ins for other types For the examples in this module we will be using the Crows Foot notation Supported by Enterprise Architect allowing auto generated code You get to decide what you use – be consistent
What is the Difference between a Class and an Entity? It really doesn’t help much when we have two documentation standards that look so similar The rectangular boxes are inevitably going to look pretty similar until the subtleties are fully appreciated
So what are the differences? Classes do not persist Classes are ultimately implemented as objects When the function or program ends, the object ceases to exist Entities are ultimately implemented as tables Tables do not vanish once the program/function has completed
Classes have Functionality Entities do not cls. Address. Book My. Address. Book = new cls. Address. Book(); My. Address. Book. Filter. By. Post. Code(“LE 1”);
“But really – aren’t they basically the same thing? ” Compare the following… Entity framework automates much of this
Classes may have no link to any entity
Classes may manage multiple entities… Customer Name Delivery Address Post Code Widget Order line 1 Product A Unit Cost Quant Tot Order line 1 Product B Unit Cost Quant Tot Order. Total
More Complex Relationship The data for the class would be derived from multiple tables For example Customer details would come from tbl. Customer Product details would come from tbl. Product There may be a stock check facility making use of tbl. Stock Some elements of the invoice would never be stored in the system For example Order line totals and the invoice total would be calculated on the fly When the invoice is stored in the system the data contained in a single object would be distributed across multiple tables.
Objects may be generated onthe fly LINQ (Language-Integrated Query) Query Language for Objects Similar to SQL var query. London. Customers = from cust in customers where cust. City == "London" select cust;
The Difference… Entities are about storing stuff Classes are about controlling stuff
Normalisation/De-Normalisation Database normalization is the process of organizing the fields and tables of a relational database to minimize redundancy. Normalization usually involves dividing large tables into smaller (and less redundant) tables and defining relationships between them. The objective is to isolate data so that additions, deletions, and modifications of a field can be made in just one table and then propagated through the rest of the database using the defined relationships. (Wikipedea)
Things to Consider No row order significance shelf A product price 89 lard B butter 37 bread 62 milk 32 We are insisting that the rows must be sorted as above otherwise we won’t know what shelf a product is on
No Column Order Significance student# 0427 h name smith david ivan We are assuming that the columns must always be in this order otherwise we won’t know the person’s name.
Each attribute value must have only a single value car astra make vauxhall engine-size 1100, 1300
No Duplication box# contents colour 31 sugar brown 47 flour white 9 rice white 47 flour white 103 sugar white
Always Decompose Many to Many Relationships Order Product Order Line Product
One to One Relationships Do not exist (probably). A B
De-Normalisation Some times we deliberately step back from a normalised database to meet other requirements E. g. System response time
Other Places to Inform our Entity Design Smoke and mirrors prototype Class diagram Sequence diagram
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