查询优化

逐步优化

• 相关子查询 20ms
  • select * from employees where hire_date = (select max(hire_date) from employees);
• 不相关子查询 15ms
  • select e1.* from employees e1 join (select hire_date from employees order by hire_date desc limit 1) e2 on e1.hire_date = e2.hire_date;
• 左连接 19ms
  • select e1.* from employees e1 left join employees e2 on e1.hire_date < e2.hire_date where e2.emp_no is null;

结果表明：以上三种方法中，不相关子查询最快