使用Oracle的Decode函数进行多值判断

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.