Decode函数的语法结构如下:

代码如下:decode (expression,search_1,result_1)
decode (expression,result_1,search_2,result_2)
decode (expression,result_2,….,search_n,result_n)

decode (expression,default)
decode (expression,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:

代码如下: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,”unlocked”) status from t;

USERNAME STATUS
—————————— ——–
SYS unlocked
SYSTEM unlocked
OUTLN unlocked
CSMIG unlocked
SCOTT unlocked
EYGLE unlocked
DBSNMP unlocked
WMSYS

8 rows selected.