--通用函数
--nvl(exp1,exp2):当exp1为空时，返回exp2
--nvl2(exp1,exp2,exp3):当exp1为空时，返回exp3；否则返回exp2
select ename,sal*12+nvl2(comm,comm,0) 年收入 from emp;
--NULLIF (expr1, expr2),如果expr1=expr2,返回null;否则,返回expr1
select nullif('abc','abc') from dual;
select nullif('abc','abcaa') from dual;
--COALESCE :找到参数列表中，第一个不为空的值
select ename,comm,sal,COALESCE(comm,sal) from emp;
--给员工涨工资,根据职位涨，总裁涨1000，经理涨600 其他人员涨400
select ename,job,sal 涨前工资, case job when 'PRESIDENT' then sal+1000
                                         when 'MANAGER'   then sal+600
                                         else sal+400
                                 end 涨后工资
from emp;

select ename,job,sal 涨前工资, decode(job,'PRESIDENT',sal+1000,
                                            'MANAGER',sal+600,
                                             sal+400) 涨后工资
from emp;