select ename,sal * 12 income from emp;
select * from emp where sal>1500;
select * from emp where is not null;
select * from emp where is null;
select * from emp where sal > 1500 and comm is not null;
select * from emp where sal > 1500 or comm is not null;
select * from emp where not (sal > 1500 or comm is not null);
select * from emp where sal>1500 and sal<3000;
select * from emp where sal between 1500 and 3000;
select * from emp where hiredate between '1 -1月 -81' and '31 -12月 -81'
select *from emp where ename='smith';
