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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，但是由于这一行，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

公交线路：

ID
AreaID --> ID in Area
LineCode

公交信息：

ID
BusLineID --> ID in BusLine

公车站：

BusInfoID --> ID in BusInfo

顺便说一句，我正在使用 Access，看来我必须( )使用innjer join.

4

2 回答 2

2

原作者已经足够体贴地以一种指示连接在逻辑上相关的方式缩进 SQL。

它可以重写为：

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，嵌套的内部连接语法只是一种指示优先级的方法，即这组连接应该（逻辑上）在其他连接之前执行。一些 SQL 引擎支持为此使用括号。在所有连接都是内部连接的情况下，嵌套和非嵌套形式之间没有逻辑差异（即您提供的 SQL 和我在上面提供的替代 SQL）。

于 2013-09-03T10:05:40.720 回答
1

让您感到困惑的并不完全是您突出显示的部分。INNER JOIN Area首先与上面的部分连接，因为下面的部分INNER JOIN BusLine ON Area.ID = BusLine.AreaID是带有子句的常规标准连接。

如果（这是一个很大的如果）我已经正确阅读了这个查询（离开纽约的公交线路及其停靠站的数量），这就是我要尝试重构的内容

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！

于 2013-09-03T09:54:49.370 回答