select GE_employee.employeeid,GE_employee.employeename,GE_orger.dept from GE_employee left join GE_orger on GE_employee.employeeid=GE_orger.employeeid and utortype='2'select GE_employee.employeeid,GE_employee.employeename,GE_orger.dept from GE_employee left join GE_orger on GE_employee.employeeid=GE_orger.employeeid where utortype='2'像上面这两句,第一句会把utortype不等于2的全都调出来,对连接的限制在这里为什么会失效呢?
第二种,是在rep2hosp表内先做排除,再与orderdetail表连接,所以总数据数肯定跟orderdetail表里的总数据一致这里应该注意使用Left Join的使用原理
left join GE_orger on GE_employee.employeeid=GE_orger.employeeid and utortype='2'select GE_employee.employeeid,GE_employee.employeename,GE_orger.dept from GE_employee
left join GE_orger on GE_employee.employeeid=GE_orger.employeeid where utortype='2'像上面这两句,第一句会把utortype不等于2的全都调出来,对连接的限制在这里为什么会失效呢?