create table salary (userid int,salary decimal(9,2));

-- mysql

insert into salary values(1,1000),(2,2000), (3,3000),(4,4000),(5,5000), (1,null),(2,500);

-- oracle

　　insert all

　　　　into salary values(1,1000)

　　　　into salary values(2,2000)

　　　　into salary values(3,3000)

　　　　into salary values(4,4000)

　　　　into salary values(5,5000)

　　　　into salary values(1,null)

　　　　into salary values(2,500)

　　　　select * from dual;

-- if(value,t f)

　　select salary, if(salary>2000,'high','low') hl from salary; --  oracke 不支持

-- case when ... then...else...end

　　select salary, case when salary<=2000 then 'low' else 'high' end hl from salary;

　　select salary, case when salary<=2000 then 'low' when salary is null then 'low' when salary<=4000 then 'min' else 'high' end hl from salary;

-- case ... when ... then...else...end

　　select salary, case salary when 1000 then 'low' when 2000 then 'mid' else 'high' end hl from salary;