例如:按照departmentid查询employees员工表和departments部门表的信息方式一通用型:SELECTFROMWHERESELECTelastnameedepartmentidddepartmentnameFROMemployeesedepartmentsdwhereedepartmentidddepartmentid方式二:SELECTFROMNATURALJOIN有局限性:会自动连接两个表中相同的列可能有多个:department