selet name,salary from employee where salary<=300000 and salary is null;
selet name,brithday from employee where brithday<'1990-01-01';
selet name,salary from employee where salary>300000 and salary < 400000;
selet name,salary from employee where salary between 300000 and 400000;
selet name, gender, salary, dep_id from employee 
where gender='男' and salary >=400000 and dep_id='D001';