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Re: DataBase Concepts
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Posted: Dec 7, 2005 7:24 AM
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A Database is designed to fullfill business requirements or solve business problems.
A major issue is the output of the database is most important. You have to get the information out of the database i.e. reports, displays, extract files. So the database is disigned after you know what kind of information has to be extracted.
Some poor sap has to write a querry, design an interface, etc.
The first rule in design is that the user or the owner will never tell you all of the requirements. You have to dig this information out of people's heads and the technical requirements.
Some designers make a general design or a logical model which has to be based on the business requirements but not too specific. Then they identify the keys for the Physical design. And then after that the entities that might be required.
You can get rather involved with this and map out each process with sub-process steps. The process is like a design for a business requirement Like taking an order, Or writing a paycheck.
The Entity type formatting has a lot of options. It includes the obvious like number or character. Dates are often stored as just a number. Most databases have their own date type. This may be a certain number of seconds or minutes or days since a set date. So they have this internal stored version and the version that is displayed may be a conversion like mm/dd/yy or time or both or any combination.
A field may be numbered from 1 and increase by one every time you add another row. This can be handy for keys. There may be a default value or the element may allow no value (Null). The fields that are related may be forced to updated their relationships or require there to be a relationsip. For something like a social security number or a phone number there may be a specific edit pattern or a conversion string for display. Take a decimal number. Accounting may require that school credits be stored with 5 decimal places for computations and rounding, but you may only want to display 2 decimals. You have to know this if you ever write a program or do a comparison. Sometimes there is no decimal stored,but it is implied.
OK enough of that. Usually the format of the data is cataloged in the Data Dictionary or multiple dictionaries. This way when some form is designed the system knows what the correct format to display is. This is the main thing about a database it the database reads the dictionary and then instantly knows what kind of data it is. You dont have to write some long description of the data format just to output a simple report (In Theory).
Every database has its own data types or list of data types and they tend to want to use standard data types that are for the most part interchangeable. This is so the ODBC or data exchange drivers will be able to read the data and share it with some other application. i.e. download to Excel to do a pivot table.
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