1、delete可以直接删除数据，也可以根据条件删除数据
1.1语法格式：
delete from 表名 where 。。。。。
1.2举例：删除津贴为500的员工
delete from emp where comm=500;
1.3删除津贴为null的员工
delete from emp where comm is null;
delete from 表名 where 。。。。。
delete from emp where comm=500;
delete from emp where comm is null;