标签：
1.关于Union的知识
select 11 from dual union select 11 from dual
和
select 11 from dual union all select 11 from dual
有区别，Union的作用是合并查询结果 All保留重复行
2. 关于Order By的知识
2.1 select t.*, t.rowid from users t order by 1,2
--按照列号排序
2.2 select t.*, t.rowid from users t order by t.name nulls first
-- 该字段空值排前面
select t.*, t.rowid from users t order by t.name nulls last
-- 该字段空值排后面
2.3 select t.*, t.rowid from users t where t.name in（‘admin‘,‘test2‘,‘测试‘）order by instr(‘admin,test2,测试‘,t.name)
--按in顺序排序
2.3.1 select t.*, t.rowid from users t order by instr(‘admin,test2,测试‘,t.name) desc
--按in顺序排序 ps：排在最底部 各位可以按需求对 instr里面的值进行调整
3. 关于Base64位数据库加密解密的知识
select utl_raw.cast_to_varchar2(utl_encode.base64_decode(utl_raw.cast_to_raw(‘MTEuMg==‘))) as "Base64解码后数据" from dual --解密 结果11.2
select utl_raw.cast_to_varchar2(utl_encode.base64_encode(utl_raw.cast_to_raw(11.2))) from dual -- 加密 结果 MTEuMg==
4. Oracle Mod的用法 取n以内被m整除的数
select t.rnum from (SELECT rownum rnum FROM ALL_OBJECTS WHERE ROWNUM <= 10 ) t where mod(t.rnum,3)=0 ; --10以内被3整除的数 结果 3,6,9 select sum(t.rnum) as total_sum from (SELECT rownum rnum FROM ALL_OBJECTS WHERE ROWNUM <= 10 ) t where mod(t.rnum,3)=0 ; --10以内被3整除的数的和 结果 18
5.Oracle Decode的用法
select decode（AA,‘11‘,‘数据是11‘,‘22‘,‘数据是22‘,‘其他数据‘) as "数据结果" from( select 11 as AA from dual union all select 22 from dual union all select 221 from dual union all select 333 from dual )
6.Oracle 的start with 用法
select t.* from sys_tree t where t.isdel=1 start with t.code=‘100‘ connect by prior t.id = t.pid order by t.orderno --查询所有的子节点 select t.* from sys_tree t where t.isdel=1 start with t.code=‘100‘ connect by prior t.pid = t.id order by t.orderno --查询所有的父节点
标签：
原文地址：http://www.cnblogs.com/mo-ran/p/5964532.html
