Decode函數的語法結構如下:

復制代碼 代碼如下:

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函數比較表達式和搜索字，如果匹配，返回結果；如果不匹配，返回default值；如果未定義default值，則返回空值。
以下是一個簡單測試，用于說明Decode函數的用法:

復制代碼 代碼如下:

SQL> create table t as select username,default_tablespace,lock_date from dba_users;

Table created.

SQL> select * from t;

USERNAMEDEFAULT_TABLESPACELOCK_DATE
------------------------------ ------------------------------ ---------
SYSSYSTEM
SYSTEMSYSTEM
OUTLNSYSTEM
CSMIGSYSTEM
SCOTTSYSTEM
EYGLEUSERS
DBSNMPSYSTEM
WMSYSSYSTEM20-OCT-04

8 rows selected.

SQL> select username,decode(lock_date,null,"unlocked","locked") status from t;

USERNAMESTATUS
------------------------------ --------
SYSunlocked
SYSTEMunlocked
OUTLNunlocked
CSMIGunlocked
SCOTTunlocked
EYGLEunlocked
DBSNMPunlocked
WMSYSlocked

8 rows selected.

SQL> select username,decode(lock_date,null,"unlocked") status from t;

USERNAMESTATUS
------------------------------ --------
SYSunlocked
SYSTEMunlocked
OUTLNunlocked
CSMIGunlocked
SCOTTunlocked
EYGLEunlocked
DBSNMPunlocked
WMSYS

8 rows selected.