Web hosting contract - Whereas fields apply structure to records, datatypes apply
Whereas fields apply structure to records, datatypes apply structure and restrictions to fields and values in those fields. Datatypes There are many different types of datatypes, which vary often more in name than anything else with respect to different database engines. This section describes all different variations of datatypes, but without targeting any specific vendor database engine. Datatypes can be divided into three separate sections: . Simple datatypes These are datatypes applying a pattern or value limitation on a single value such as a number. . Complex datatypes These include any datatypes bridging the gap between object and relational databases, including items such as binary objects and collection arrays. Specifics on complex datatypes are not strictly necessary for this book as they are more object-oriented than relational in nature. . Specialized datatypes These are present in more advanced relational databases catering to inherently structured data such as XML documents, spatial data, multimedia objects and even dynamically definable datatypes. Simple Datatypes Simple datatypes include basic validation and formatting requirements placed on to individual values. This includes the following: . Strings A string is a sequence of one or more characters. Strings can be fixed-length strings or variable-length strings: . Fixed-length strings Afixed-length string will always store the specified length declared for the datatype. The value is padded with spaces even when the actual string value is less than the length of the datatype length. For example, the value NY in a CHAR(3) variable would be stored as NY plus a space character. Fixed-length strings are generally only used for short length strings because a variable-length string (discussed next) requires storage of both value and length. Fixed-length strings are also more efficient for ensuring fixed record lengths of key values. Figure 3-6 shows an FXCODE field representing a foreign exchange currency code, always returning three characters even when the currency code is less than three characters in length. Acase in point is the defunct currency code DM (Deutsche Marks, German currency), returning DM plus a space character, yielding a total of three characters. 42 Chapter 3
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