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

CHAPTER 12 ■ DATATYPES 499
Table 12-1. Four Basic String Types
String Type
Description
VARCHAR2( ) is a number between 1 and 4,000 for up to 4,000
bytes of storage. In the following section, we’ll examine in
detail the differences and nuances of the BYTE versus CHAR
modifier in that clause.
CHAR( ) is a number between 1 and 2,000 for up to 2,000
bytes of storage.
NVARCHAR2( )
is a number greater than 0 whose upper bound is
dictated by your national character set.
NCHAR( )
is a number greater than 0 whose upper bound is
dictated by your national character set.
Bytes or Characters
The VARCHAR2 and CHAR types support two methods of specifying lengths:
• In bytes: VARCHAR2(10 byte). This will support up to 10 bytes of data, which could be as
few as two characters in a multibyte character set.
• In characters: VARCHAR2(10 char). This will support to up 10 characters of data, which
could be as much as 40 bytes of information.
When using a multibyte character set such as UTF8, you would be well advised to use the
CHAR modifier in the VARCHAR2/CHAR definition—that is, use VARCHAR2(80 CHAR), not VARCHAR2(80),
since your intention is likely to define a column that can in fact store 80 characters of data. You
may also use the session or system parameter NLS_LENGTH_SEMANTICS to change the default
behavior from BYTE to CHAR. I do not recommend changing this setting at the system level;
rather, use it as part of an ALTER SESSION setting in your database schema installation scripts.
Any application that requires a database to have a specific set of NLS settings makes for an
“unfriendly” application. Such applications, generally, cannot be installed into a database
with other applications that do not desire these settings, but rely on the defaults to be in place.
One other important thing to remember is that the upper bound of the number of bytes
stored in a VARCHAR2 is 4,000. However, even if you specify VARCHAR2(4000 CHAR), you may not
be able to fit 4,000 characters into that field. In fact, you may be able to fit as few as 1,000
characters in that field if all of the characters take 4 bytes to be represented in your chosen
character set!
The following small example demonstrates the differences between BYTE and CHAR, and
how the upper bounds come into play. We’ll create a table with three columns, the first two of
which will be 1 byte and 1 character, respectively, with the last column being 4,000 characters.
Notice that we’re performing this test on a multibyte character set database using the character
set AL32UTF8, which supports the latest version of the Unicode standard and encodes
characters in a variable-length fashion using from 1 to 4 bytes for each character: