Oracle sql语句中不支持boolean类型（decode&case）
版本信息：
SQL> select * from v$version;
BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.1.0.7.0 - 64bit Production
PL/SQL Release 11.1.0.7.0 - Production
CORE 11.1.0.7.0 Production
TNS for Linux: Version 11.1.0.7.0 - Production
NLSRTL Version 11.1.0.7.0 - Production
假设我们现在想知道1>0是否为真：
直接查，不行！
SQL> select 1>0 from dual;
select 1>0 from dual
*
ERROR at line 1:
ORA-00923: FROM keyword not found where expected
用decode转，还是不行！
SQL> select decode(1>0,true,'true','false') from dual;
select decode(1>0,true,'true','false') from dual
*
ERROR at line 1:
ORA-00907: missing right parenthesis
用case转，依旧不行！
SQL> select case 1>0 when true then 'true' else 'false' end from dual;
select case 1>0 when true then 'true' else 'false' end from dual
*
ERROR at line 1:
ORA-00923: FROM keyword not found where expected
原因在于sql不支持boolean类型（手头没有其他库，不知道mysql和sqlserver是否支持）：
SQL> create or replace function is_true return boolean
is
begin
return true;
end;
/
Function created.
SQL> select is_true from dual;
select is_true from dual
*
ERROR at line 1:
ORA-06552: PL/SQL: Statement ignored
ORA-06553: PLS-382: expression is of wrong type
直接放sql语句中行不通，试着放到fuction里：
decode还是出错：
SQL> CREATE OR REPLACE FUNCTION is1gt0
RETURN VARCHAR2
IS
BEGIN
RETURN DECODE (1 > 0, TRUE, 'true', 'false');
END;
/
Warning: Function created with compilation errors.
SQL> show err;
Errors for FUNCTION IS1GT0:
LINE/COL ERROR
-------- -----------------------------------------------------------------
5/3 PL/SQL: Statement ignored
5/10 PLS-00306: wrong number or types of arguments in call to 'DECODE'
SQL>
case完美通过：
SQL> CREATE OR REPLACE FUNCTION is1gt0
RETURN VARCHAR2
IS
BEGIN
RETURN CASE 1 > 0
WHEN TRUE
THEN 'true'
ELSE 'false'
END;
END;
/
Function created.
SQL> show err;
No errors.
SQL> select is1gt0 from dual;
IS1GT0
--------------------------------------------------------------------------------
true
SQL>
小结：
1. Oracle sql语句中不支持boolean类型；
2. decode是oracle独有的；而case是标准sql，mysql和sqlserver也可以使用，而且case还能把boolean转换输出。