Healthy, well-constructed entity classes are the foundations of an effective relational database.
A database should have a separate entity class/worksheet/table for every major component of the database
Data should not be unnecessarily duplicated in multiple entity classes/worksheets/tables
Duplicating data is a common error,
but it's easy to avoid if you structure your entity classes well
An entity class is essentially an object wrapper for a database table.
The attributes of an entity are transformed to columns on the database table.
Each entity set is a single table, analogous to an Excel worksheet, which contains entities and attributes which are analogous to rows and columns in an Excel worksheet.
|ERModel entity set||composed of entities||which have attributes|
|Excel worksheets||composed of rows||which have column header cells|
Each row contains a record which is a single entry in a table and each record is composed of several descriptive fields called attributes.
When creating databases, sketch out the different entity classes/tables you will need and the data contained in the entity classes/tables before you begin
|AutoNumber||Automatically increments||used for primary key (unique identifier)|
|Number||integers that are negative or positive||not numerals that do not have numeric values, like SSANs, PIDs, ZIP Codes, or telephone numbers|
|Text||used for words or non-numeric-value numerals|
|Currency||currency amounts||with choice of decimal places|
|Date/Time||Microsoft tools, like Excel and Access, store dates and times as a number representing the number of days since 01 January 1900, plus a fractional portion of a 24 hour day||formatting is necessary to have the numbers display as dates and times|
|Field Size||number of characters|
|Format||how the field's contents will be displayed|
|Decimal Places||when data type is numeric|
|Default Value||for new entries|
|Validation Rule||range of acceptable entries||checks values|
|Required||a yes/no field||indicates if a value is mandatory|
|Indexed||creates an index for the field||improves searching and sorting|
To distinguish one entity from another, entity sets contain a primary key attribute
the primary key is an identifier - such as a part number, a product code, or an employee ID - that is unique to each record
The primary key should be a piece of information that won't change frequently
a foreign key is a connector - it is a copy of a primary key for one entity in one entity set - that is placed as an attribute in a different entity in a different entity set to relate the two entities, in two different entity sets, together
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