select *from emp;//查询emp表内容

select ename, job from emp;//从emp表中查询ename和job列

select ename, sal from emp;//在表emp中查询ename和sal列

select ename, sal*12 from emp;//查询ename列，查询sal，并将sal列的每个值*12

select ename, sal as "月薪" from emp;//给sal列命 取名为"月薪"

select ename, sal as "月薪", sal*12 as "年薪" from emp;