tailieunhanh - Oracle 8 Database Administration volume 2 instruction guide phần 10

Tham khảo tài liệu 'oracle 8 database administration volume 2 instruction guide phần 10', công nghệ thông tin, cơ sở dữ liệu phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Choosing a Database and a National Character Set Character Sets and National Character Sets of a Database Database Character Sets National Character Sets Defined at creation time Defined at creation time Cannot be changed without re-creation Cannot be changed without re-creati on Store data columns of type CHAR VARCHAR2 ClOb LONG Store data columns of type NCHAR NVARCHAR2 and NCLOB Can store varying-width character sets Can store fixed-width and varying-width multibyte character sets 22-5 Copyright Oracle Corporation 1998. All rights reserved. ORACLE The CREATE DATABASE statement has the CHARACTER SET clause and the additional optional clause NATIONAL CHARACTER SET to declare the character set to be used as the database character set and the national character set. Both character sets cannot be changed after creating the database. If no NATIONAL CHARACTER SET clause is present the national character set defaults to the same as the database character set. Because the database character set is used to identify and to hold SQL and PL SQL source code it must have either EBCDIC or 7-bit ASCII as a subset whichever is native to the platform. Therefore it is not possible to use a fixed-width multibyte character set as the database character set only as the national character set. The data types NCHAR NVARCHAR2 and NCLOB are provided to declare columns as variants of the basic types CHAR VARCHAR2 and CLOB to note that they are stored using the national character set and not the database character set. To declare a fixed-length character item that uses the national character set use the data type specification NCHAR size . To declare a variable-length character item that uses the national character set use the data type specification NVARCHAR2 size . To declare a character large object CLOB item containing fixed-width multibyte characters that uses the national character set use the data type specification NCLOB size . Oracle8 Database Administration 22-9 Lesson 22 Using .