External and Internal Representations of Data

A data type is necessary to allow the RDBMS to accurately convert the data from an external representation, most often text in a file, to an internal representation of the data. For example, the external representation of a numeric value is a text string containing at least one numeral, and possibly a plus or minus sign or a decimal point. A text value may contain any valid text character, usually only printable characters from the ASCII set. The internal representation of the data is dependent upon the particular RDBMS and hardware being used. It is not necessary to know the exact internal representation of the data. The important thing to consider is which data type accurately represents the data for your purposes. 

The internal representation of data is not entirely unimportant. When the float data type is used, the data are typically converted to the internal floating-point representation used by the computer on which the RDBMS is installed. This may have unintended consequences because of the rounding that occurs, especially if several mathematical operations are 

Sometimes, there is no value available for a particular column or a particular row. Rather than inventing a special value to represent this, such as 99999 for numeric or for text, the relational database provides a special null value. This should be used when a value is unknown or unavailable. When actual data becomes available, the null value can be updated. 

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As with all data in an RDBMS, there is an external and internal representation of data. This was discussed in an earlier chapter for standard data types, such as text and numeric. For molecular structures, there is of course no SQL standard. When building a database containing molecular structures, a decision should first be made which internal representation will be used and which external representation. 

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