Differences between varchar2 (20) and varchar2 (20 bytes) in Oracle

Differences between varchar2 (20) and varchar2 (20 byte) in Oracle Development Environment: some table field types are defined as varchar2 (20) and some table field types are defined as varchar2 (20 byte) is varchar2 (20) the same as varchar2 (20 byte? The difference is determined by the database parameter NLS_LENGTH_SEMANTICS, which has two units: char (character) or byte. The default value of this parameter is BYTE. Therefore, by default, varchar2 (20) = varchar2 (20 bytes ). If the parameter value is CHAR, It is not equal. Suggestion: Use a unified format such as varchar2 (20) Demo: SQL> show parameter nls_length_semantics; name type value generation ----------- ------------------------------ nls_length_semantics string BYTESQL> create table tab1 (2 id number (10), 3 description varchar2 (20) 4); Table created. SQL> create table tab2 (2 id number (10), 3 description varchar2 (20 char) 4); Table created. SQL> desc tab1; Name Nu Ll? Type ----------------------------------------- -------- ------------------------------ id number (10) DESCRIPTION VARCHAR2 (20) SQL> desc tab2; Name Null? Type ----------------------------------------- -------- ---------------------------- id number (10) DESCRIPTION VARCHAR2 (20 CHAR) SQL> alter session set nls_length_semantics = char; Session altered. SQL> create table tab3 (2 id number (10), 3 description varchar2 (20) 4); Table created. SQL> desc tab1; Name Null? Type ----------------------------------------- -------- ---------------------------- id number (10) DESCRIPTION VARCHAR2 (20 BYTE) SQL> desc tab2; Name Null? Type ----------------------------------------- -------- ------------------------------ id number (10) DESCRIPTION VARCHAR2 (20) SQL> desc tab3; Name Null? Type authorization -------- ---------------------------- id number (10) DESCRIPTION VARCHAR2 (20) Note: For SYS and SYSTEM, this NLS_LENGTH_SEMANTICS is not affected. If it is a BYTE, do not modify the SYSTEM-level nls_length_se, otherwise, some finished kits, such as EBS, may not work properly.