返回LEFT表中的所有行，即使RIGHT表中没有记录，也返回RIGHT表中的最大到期日期

Question

在此先感谢您的帮助，我是SQL Server的新手，需要您的专家帮助，我想返回LEFT表中的所有值，即使右表中没有记录也是如此，并且MAX（MOT Expiry）或MAX（Road Tax）Expiry 。

我尝试了此查询，但无法获得所需的结果

    SELECT Vehicle.id,vehicle.registrationMark, vehicle.status, RoadTax.Expiry, MOT.Expiry
    FROM Vehicle
    LEFT JOIN RoadTax ON Vehicle.id = RoadTax.VehicleID
    LEFT JOIN MOT ON Vehicle.id = MOT.VehicleID;

为了清楚起见，请看此示例。 我想从“车辆”表中退回所有车辆的最大TAX和MOT到期时间。 即使在道路税或MOT表中也没有车辆记录。

车辆表

    ---------------------------------
    | ID | RegistrationMark | Status |
    ---------------------------------
    |  1 | ABC              | Active |
    |  2 | DEF              | Active |
    |  3 | GHI              | Active |
    ---------------------------------

道路税表

    ------------------------------
    | ID | VehicleID | Expiry     |
    ------------------------------
    |  1 | 1         | 10/10/2013 |
    |  2 | 1         | 10/10/2014 |
    |  3 | 2         | 20/12/2014 |
    ------------------------------

MOT表

    -------------------------------
    | ID | VehicleID | Expiry      |
    -------------------------------
    | 1  | 2         | 25/01/2015  |
    -------------------------------

我想要的结果

    -----------------------------------------------------------
    | ID | RegistrationMark | Status | TaxExpiry  | MOTExpiry  |
    -----------------------------------------------------------
    | 1  | ABC              | Active | 10/10/2014 | NULL       |
    | 2  | DEF              | Active | 20/12/2014 | 25/01/2015 |
    | 3  | GHI              | Active | NULL       | NULL       |
    -----------------------------------------------------------

非常感谢你的帮助。

Answer 1

好像您在查询的正确轨道上—您只需要添加聚合：

SELECT Vehicle.id,vehicle.registrationMark, vehicle.status
, Max(RoadTax.Expiry) as LatestTax_Exp
, Max(MOT.Expiry) as LatestMOT_Exp
FROM Vehicle
LEFT JOIN RoadTax ON Vehicle.id = RoadTax.VehicleID
LEFT JOIN MOT ON Vehicle.id = MOT.VehicleID;
group by vehicle.ID, vehicle.Registrationmark, vehicle.status

或者，如果我误解了您想要的东西，并且车辆可能具有多个状态或注册，请尝试：

SELECT Vehicle.id,vehicle.registrationMark, vehicle.status
, Max(RoadTax.Expiry) over (partition by Vehicle.ID) as LatestRT_Exp
, Max(MOT.Expiry) over (partition by Vehicle.ID) as LatestMOT_Exp
FROM Vehicle
LEFT JOIN RoadTax ON Vehicle.id = RoadTax.VehicleID
LEFT JOIN MOT ON Vehicle.id = MOT.VehicleID;