Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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CHAPTER 12 ■ DATATYPES
The “N” Variant
So, of what use are the NVARCHAR2 and NCHAR (for completeness)? They are used in systems
where the need to manage and store multiple character sets arises. This typically happens in
a database where the predominant character set is a single-byte, fixed-width one (such as
WE8ISO8859P1), but the need arises to maintain and store some multibyte data. There are many
systems that have legacy data but need to support multibyte data for some new applications,
or systems that want the efficiency of a single-byte character set for most operations (string
operations on a fixed-width string are more efficient than on a string where each character
may store a different number of bytes) but need the flexibility of multibyte data at some
points.
The NVARCHAR2 and NCHAR datatypes support this need. They are generally the same as
their VARCHAR2 and CHAR counterparts, with the following exceptions:
• Their text is stored and managed in the database’s national character set, not the
default character set.
• Their lengths are always provided in characters, whereas a CHAR/VARCHAR2 may specify
either bytes or characters.
In Oracle9i and later, the database’s national character set may take one of two values:
UTF8 or AL16UTF16 (UTF16 in 9i; AL16UTF16 in 10g). This makes the NCHAR and NVARCHAR types
suitable for storing only multibyte data, which is a change from earlier releases of the database
(Oracle8i and earlier allowed you to choose any character set for the national character set).
Binary Strings: RAW Types
Oracle supports the storage of binary data as well as text. Binary data is not subject to the
character set conversions we discussed earlier with regard to the CHAR and VARCHAR2 types.
Therefore, binary datatypes are not suitable for storing user-supplied text, but are suitable for
storing encrypted information—encrypted data is not “text,” but a binary representation of
the original text, word processing documents containing binary markup information, and so
on. Anything that should not be considered by the database to be “text” and that should not
have character set conversion applied to it should be stored in a binary datatype.
Oracle supports three datatypes for storing binary data:
• The RAW type, which we focus on in this section and is suitable for storing RAW data up to
2,000 bytes in size.
• The BLOB type, which supports binary data of much larger sizes, coverage of which we’ll
defer to the “LOB Types” section later in the chapter.
• The LONG RAW type, which is supported for backward compatibility and should not be
considered for new applications.
The syntax for the binary RAW type is straightforward:
RAW( )
For example, the following code creates a table capable of storing 16 bytes of binary information
per row: