Converting an extended e r model into a relational database design

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Converting an extended e r model into a relational database design

Converting an extended e r model into a relational database design

Shaw So is the result of your abstraction the "essential details" or something "hypothetical and incomprehensible"? Is the result of your abstraction a work of art like Michelangelo, or is it a piece of nonsense like Picasso?

Shaw points out another area of confusion: Abstraction and indirection are very different yet cooperating concepts with two completely different purposes in software development. Abstraction is used to reduce complexity.

Indirection is used to reduce coupling or dependence. The problem is that programmers frequently mix these up, using one for the other purpose and just generally screwing things up.

By not knowing the difference between the two, and not knowing when to use one vs. Just as the definition of encapsulation can be corrupted, so can the definition of abstraction. Shaw identifies the following as a prime example: Think about it, the "abstract" keyword doesn't reduce, summarize, or generalize a more concrete implementation, but rather creates an indirect path to the real implementation of that function.

If misleading or even incorrect definitions of such basic terms are used from the get-go, it is no wonder that hordes of programmers don't realise that they are being led down the wrong path?

Each database table requires its own class This biggest problem virtually everybody has with OOP is how to split the entire application into a collection of different classes.

What should be defined as a class, and what should not? What sort of class hierarchy would be best? Getting back to basics what you are trying to do is build a system where you have software objects that represent Real World RW objects.

Once you have identified which RW objects your application is supposed to deal with, then surely it follows that you must define a class for each of these RW objects from which you are able to create software objects?

In order to avoid confusion between RW objects and software objects I am going to use a different word. Another word already in use within the IT community is 'entity', so I shall use that. So an 'object' in the software is a representation of an 'entity' to the business.

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Bear in mind that unless you are developing software which directly manipulates a real-world object, such as process control, robotics, avionics or missile guidance systems, then some of the properties and methods which apply to that real-world object may be completely irrelevant in your software representation.

If, for example, you are developing an enterprise application such as Sales Order Processing which deals with entities such as Products, Customers and Orders, you are only manipulating the information about those entities and not the actual entities themselves. In pre-computer days this information was held on paper documents, but nowadays it is held in a database in the form of tables, columns and relationships.

An object in the real world may have many properties and methods, but in the software representation it may only need a small subset. For example, an organisation may sell many different products with each having different properties, but all that the software may require to maintain is an identity, a description and a price.

A real person may have operations such as stand, sit, walk, and run, but these operations would never be needed in an enterprise application. Regardless of the various operations and methods which exist in a real-world object, when an application does nothing more than interface with entities in a database the programmer would be wise to understand the following: Objects in the real world may be of different types, but all objects in a database are all of the same type - they are tables.

Objects in the real world may have many properties, but objects in a database only contain those properties which are relevant to the application, and these properties are known as columns, where each table has its own set of columns.

If, following the rules of normalisationsome columns are split off to form another table, then that other table automatically becomes a separate entity in its own right. Objects in the real world may have operations which are unique to that type of object, but every object in a database is of the same type it is a table which is subject to the same limited set of operations - Create, Read, Update and Delete CRUD.

This simple series of observations led me to the following blindingly obvious conclusions: Every table in the database is a separate entity, and as each entity requires its own class I see no reason why each table should not have its own class.

There is a lot of code which is required to manipulate the contents of a database table, and instead of duplicating this code in every table class I define it once in an abstract class and share it in every concrete table class using that OO mechanism called inheritance. The properties of a table are limited to a set of columns and do not include any other tables, so the idea of creating a single class to handle multiple tables never occurred to me.

I never bothered with Object Composition as this was never discussed in the online articles which were available in When I eventually became aware of it I dismissed it as the ravings of a lunatic as there seemed to be nothing I could with it that I could not already do, and far more easily at that, with inheritance.

In my many years of designing, building and using databases one valuable tool is the Entity Relationship Diagram ERD without which you cannot design a database that will support the needs of the business.

This is where you identify all the entities used in the business and the relationships between them. To me this seems blindingly obvious:The ETL phases. During the ETL process, data is extracted from an OLTP database, transformed to match the data warehouse schema, and loaded into the data warehouse database (Berson and Smith, , Moss, ).Many data warehouses also incorporate data from non-OLTP systems, such as text files, legacy systems, and spreadsheets.

DAM Glossary is a resource containing definitions relating to Digital Asset Management and related fields. It covers a range of different subjects including metadata, hardware, interoperability, asset .

How to Convert ER Diagram to Relational Database | Learn Databases

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Pull requests and comments always welcome.. Prelude; Target Audience; Acknowledgements; Credits; Target Version.

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