如何使用计算列计算同一查询中使用子查询的另一列答案

【问题标题】：How to use a calculated column to calculate another column in the same query using a subquery如何使用计算列计算同一查询中使用子查询的另一列
【发布时间】：2021-05-20 07:09:04
【问题描述】：

我正在尝试根据以下查询中的 NLength 和 NSpeedLimit 字段添加一个新的计算字段 (TravelTime)

SELECT          
        sc.OBJECTID, 
        sn.Name, 
        case    when hn.side = 'Right Side' then ''
            else sc.LCity
        end as LCity, 
        case    when hn.side = 'Left Side' then ''
            else sc.RCity
        end as RCity, 
        case    when hn.side = 'Right Side' then ''
            else sc.LZip
        end as LZip, 
        case    when hn.side = 'Left Side' then ''
            else sc.RZip
        end as RZip, 
        sc.SHAPE.STLength() AS NLength,
        ISNULL(sc.SpeedLimit,1) AS NSpeedLimit

FROM            STREETNAME AS sn 
    INNER JOIN
                STREETHASSTREETNAME AS hn ON 
                         sn.GlobalID = hn.GlobalID AND 
                         hn.Role = 'Primary'
INNER JOIN      STREETCENTERLINE AS sc ON 
                hn.GlobalID = sc.GlobalID

新的计算字段是 TravelTime = NLength/(NSpeedLimit*88) 但我不能在选择语句中添加NLength/(NSpeedLimit*88) AS TravelTime。我知道我需要做一个子查询，但我不知道它应该去哪里。

【问题讨论】：

想必你用的是SQL Server，所以我加了标签。
这能回答你的问题吗？ How to use one data field into another case expression in the same query
甚至this
这能回答你的问题吗？ Possible to store value of one select column and use it for the next one?

标签： sql sql-server tsql spatial-query

【解决方案1】：

您可以使用 Temp_tables、派生表或公用表表达式 (CTE) 来获取结果。简单的方法是派生表，因为您不需要更多的编码。

  SELECT A.*
  ,  A.NLength/(A.NSpeedLimit * 88)  as [TravelTime]
  FROM
  (


  SELECT          
    sc.OBJECTID, 
    sn.Name, 
    case    when hn.side = 'Right Side' then ''
        else sc.LCity
    end as LCity, 
    case    when hn.side = 'Left Side' then ''
        else sc.RCity
    end as RCity, 
    case    when hn.side = 'Right Side' then ''
        else sc.LZip
    end as LZip, 
    case    when hn.side = 'Left Side' then ''
        else sc.RZip
    end as RZip, 
    sc.SHAPE.STLength() AS NLength,
    ISNULL(sc.SpeedLimit,1) AS NSpeedLimit

  FROM    STREETNAME AS sn 
   INNER JOIN
            STREETHASSTREETNAME AS hn ON 
                     sn.GlobalID = hn.GlobalID AND 
                     hn.Role = 'Primary'
          INNER JOIN      STREETCENTERLINE AS sc ON 
              hn.GlobalID = sc.GlobalID

            ) AS A

【讨论】：

【解决方案2】：

您可以使用apply 在FROM 子句中添加列：

SELECT . . .
       v.NLength, v.NSpeedLimit, 
        (v.NLength / (v.NSpeedLimit*88)) as TravelTime
FROM STREETNAME sn JOIN
     STREETHASSTREETNAME hn
     ON sn.GlobalID = hn.GlobalID AND 
        hn.Role = 'Primary' JOIN
     STREETCENTERLINE sc
     ON hn.GlobalID = sc.GlobalID CROSS APPLY
     (VALUES (sc.SHAPE.STLength(), COALESCE(sc.SpeedLimit, 1)
     ) v(NLength, NSpeedLimit)

【讨论】：