逗号连接查询(用where连接条件):
select order.id, order.orderdate,employee.id,employee.name from order,employee where order.empid=employee.id
等价于inner join查询:
select o.id, o.orderdate,e.id,e.name from order o inner join employee e on o.empid=e.id
https://blog.csdn.net/huanghanqian/article/details/52847835