Question

SELECT  BusLine.LineCode AS RouteID,
  BusStation.Station AS StartStation, BusStation_1.Station AS EndStation,
  BusStation_1.StationIndex - BusStation.StationIndex AS StopCount
FROM  BusLine AS BusLine_1 
    INNER JOIN Area AS Area_1 
        ON BusLine_1.AreaID = Area_1.ID 
    INNER JOIN BusInfo AS BusInfo_1 
        ON BusLine_1.ID = BusInfo_1.BusLineID 
    INNER JOIN BusStation AS BusStation_1 
        ON BusInfo_1.ID = BusStation_1.BusInfoID 
    INNER JOIN 
        Area 
        INNER JOIN BusLine 
            ON Area.ID = BusLine.AreaID 
        INNER JOIN BusInfo 
            ON BusLine.ID = BusInfo.BusLineID 
        INNER JOIN BusStation 
            ON BusInfo.ID = BusStation.BusInfoID
        ON BusStation_1.StationIndex > BusStation.StationIndex AND   BusInfo_1.ID = BusInfo.ID
WHERE  (Area.City = 'NewYork')

我已经阅读了上面的sql十分钟，但由于这一行我无法理解：

    INNER JOIN 
        Area 
        INNER JOIN BusLine

我想知道是否有人可以解释它并重新考虑它以使其易于理解？

更新架构：

区域：

ID
Name

乘车路线：

ID
AreaID --> ID in Area
LineCode

BusInfo：

ID
BusLineID --> ID in BusLine

站点巴士：

BusInfoID --> ID in BusInfo

BTW，我正在使用Access，似乎我必须使用( )来包装innjer join。

Answer 1

原作者已经足够周到地以一种指示联接在逻辑上相关的方式缩进SQL。

可以改写为：

FROM  BusLine AS BusLine_1 
    INNER JOIN Area AS Area_1 
        ON BusLine_1.AreaID = Area_1.ID 
    INNER JOIN BusInfo AS BusInfo_1 
        ON BusLine_1.ID = BusInfo_1.BusLineID 
    INNER JOIN BusStation AS BusStation_1 
        ON BusInfo_1.ID = BusStation_1.BusInfoID 
    INNER JOIN BusStation 
        ON BusStation_1.StationIndex > BusStation.StationIndex
    INNER JOIN BusInfo 
        ON BusInfo.ID = BusStation.BusInfoID AND BusInfo_1.ID = BusInfo.ID
    INNER JOIN BusLine 
        ON BusLine.ID = BusInfo.BusLineID 
    INNER JOIN Area 
        ON Area.ID = BusLine.AreaID 
WHERE  (Area.City = 'NewYork')

编辑：我应该补充一点，嵌套的内连接语法只是一种表示优先级的方法，即这组连接应该（逻辑上）在其他连接之前执行。一些SQL引擎支持使用括号来实现此目的。在所有连接都是内连接的情况下，嵌套和非嵌套表单之间没有逻辑差异（即您提供的SQL和我上面提供的备用SQL）。

Answer 2

令你困惑的事情并不是你所强调的部分。 INNER JOIN Area首先加入上述部分，因为以下部分INNER JOIN BusLine ON Area.ID = BusLine.AreaID是带有子句的常规标准联接。

如果（如果这是一个很大的话）我已经正确地阅读了这个问题（公交线路离开纽约时他们有多少站点），这就是我的意思。试着重构它

SELECT  BusLine.LineCode AS RouteID,
  BusStation.Station AS StartStation, BusStation_1.Station AS EndStation,
  BusStation_1.StationIndex - BusStation.StationIndex AS StopCount
FROM  BusLine AS BusLine_1 
    INNER JOIN Area AS Area_1 
        ON BusLine_1.AreaID = Area_1.ID 
    INNER JOIN BusInfo AS BusInfo_1 
        ON BusLine_1.ID = BusInfo_1.BusLineID 
    INNER JOIN BusStation AS BusStation_1 
        ON BusInfo_1.ID = BusStation_1.BusInfoID 
    INNER JOIN Area 
        ON BusStation_1.StationIndex > BusStation.StationIndex AND   BusInfo_1.ID = BusInfo.ID AND Area.City = 'NewYork'
    INNER JOIN BusLine 
        ON Area.ID = BusLine.AreaID 
    INNER JOIN BusInfo 
        ON BusLine.ID = BusInfo.BusLineID 
    INNER JOIN BusStation 
        ON BusInfo.ID = BusStation.BusInfoID

请告诉我结果集是否相同！

如何编写这个sql以使其易于理解

2 个答案: