Decode函数的语法结构如下:
decode (expression, search_1, result_1)
decode (expression, search_1, result_1, search_2, result_2)
decode (expression, search_1, result_1, search_2, result_2, ...., search_n, result_n)
decode (expression, search_1, result_1, default)
decode (expression, search_1, result_1, search_2, result_2, default)
decode (expression, search_1, result_1, search_2, result_2, ...., search_n, result_n, default)
decode函数比较表达式和搜索字，如果匹配，返回结果；如果不匹配，返回default值；如果未定义default值，则返回空值。
以下是一个简单测试，用于说明Decode函数的用法:
SQL> create table t as select username,default_tablespace,lock_date from dba_users;
Table created.
SQL> select * from t;
USERNAME DEFAULT_TABLESPACE LOCK_DATE
------------------------------ ------------------------------ ---------
SYS SYSTEM
SYSTEM SYSTEM
OUTLN SYSTEM
CSMIG SYSTEM
SCOTT SYSTEM
EYGLE USERS
DBSNMP SYSTEM
WMSYS SYSTEM 20-OCT-04
8 rows selected.
SQL> select username,decode(lock_date,null,"unlocked","locked") status from t;
USERNAME STATUS
------------------------------ --------
SYS unlocked
SYSTEM unlocked
OUTLN unlocked
CSMIG unlocked
SCOTT unlocked
EYGLE unlocked
DBSNMP unlocked
WMSYS locked
8 rows selected.
SQL> select username,decode(lock_date,null,"unlocked") status from t;
USERNAME STATUS
------------------------------ --------
SYS unlocked
SYSTEM unlocked
OUTLN unlocked
CSMIG unlocked
SCOTT unlocked
EYGLE unlocked
DBSNMP unlocked
WMSYS
8 rows selected.
