Characteristics Of The Relational Data Model And Relational Database Constraints

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Register to read the introduction… Since then, the model has been implemented in a large number of commercial systems. Current popular relational DBMSs (RDBMSs) include DB2 and Informix Dynamic Server (from IBM), Oracle and Rdb (from Oracle), and SQL Server and Access (from Microsoft). Some other commercial relational DBMSs used are Sybase (also known as Sybase SQL Server), Paradox, and Foxbase. Recently, some open source relational DBMSs, such as MySQL and PostGreSQL have also grown in popularity. At the time this chapter was written, the market share varied depending on platform; with DB2, Oracle, and Microsoft SQL Server dominating the market overall. [Note to editor: Would you like to include TM symbol in all trademarks? Also the small caps formatting, such as SQL and SQL can be made consistent. Personally, I prefer SQL over SQL but am flexible]. Oracle dominates the market in the UNIX world followed by DB2, whereas Microsoft SQL Server dominates the Windows platform market with Oracle coming in second. [Note to editor: this will need to be updated at the time the book goes to …show more content…
By atomic we mean that each value in the domain is indivisible as far as the relational model is concerned. A common method of specifying a domain is to specify a data type from which the data values forming the domain are drawn, along with formatting information. It is also useful to specify a name for the domain, to help in interpreting its values. A domain is usually specified by giving it a name, identifying its data type, and stipulating its format. Some examples of domains follow:

• USA_phone_numbers: The set of ten-digit phone numbers valid in the United States. • Local_phone_numbers: The set of seven-digit phone numbers valid within a particular area code in the United States. • Social_security_numbers: The set of valid nine-digit social security numbers. • Names: The set of character strings that represent names of persons. • Grade_point_averages: Possible values of computed grade point averages; each must be a real (floating-point) number between 0 and

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