Oracle查询笔记

-- tanslate(str,from_str,to_str)
-- 将str中的from_str替换成to_str
select translate(‘hello‘,‘e‘,‘o‘) t
from dual;

-- instr(str,des_str)
-- 可以实现like功能
select instr(‘hello‘,‘g‘),instr(‘hello‘,‘h‘),instr(‘hello‘,‘l‘)
from dual; 

-- decode(value,s1,r1,s2,r2,default)
-- 类似于if else
select decode(‘1‘,
       ‘1‘,‘一级部门‘,
       ‘2‘,‘二级部门‘,
       ‘其他部门‘) as dept
from dual;

-- case表达式
select case 1
         when 1 then ‘001‘
         when 2 then ‘002‘
         else ‘003‘
       end as le
from dual;

-- 层次化查询(树形结构)
select level,id,pid,name
from t_test 
start with pid is null
connect by prior id=pid;