查询没有身份证号的员工信息
select * from emp where idcard is null;
查询年龄在15岁(包含) 到 20岁(包含)之间的员工信息
select * from emp where age >= 15 && age <= 20;
select * from emp where age >= 15 and age <= 20;
select * from emp where age between 15 and 20;
查询姓名为两个字的员工信息 _ %
select * from emp where name like '__';