Question

在PostgreSQL中，您可以指定不带order by子句的frame子句。以下作品：

Option Explicit

Enum enumWatchType
    WatchExpression
    BreakWhenTrue
    BreakWhenChange
End Enum
Enum enumProceduresType
    AllProcedures
    Caller
End Enum
Enum enumModuleType
    AllModules
    CurrentModule
    ThisWorkbook
End Enum

Public testVar As Boolean
Sub HelloNewSession()
    AddWatch "testVar = True", AllProcedures, CurrentModule, BreakWhenTrue
    testVar = True
End Sub

Sub AddWatch( _
    expression As String, _
    Optional proceduresType As enumProceduresType = enumProceduresType.Caller, _
    Optional moduleType As enumModuleType = enumModuleType.CurrentModule, _
    Optional watchType As enumWatchType = enumWatchType.WatchExpression)

    Dim i As Long

    Application.SendKeys "%DA"
    Application.SendKeys getEscapedSendkeysText(expression)
    If proceduresType = enumProceduresType.AllProcedures Then
        Application.SendKeys "%p"
        For i = 1 To 1000 'You could use VBIDE to count the valid types to actually scroll up the right number of times!
            Application.SendKeys "{UP}"
        Next
    End If
    If moduleType = enumModuleType.AllModules Then
        Application.SendKeys "%m"
        For i = 1 To 1000 'You could use VBIDE to count the valid types to actually scroll up the right number of times!
            Application.SendKeys "{UP}"
        Next
    ElseIf moduleType = enumModuleType.ThisWorkbook Then
        Application.SendKeys "%m"
        For i = 1 To 1000 'You could use VBIDE to count the valid types to actually scroll up the right number of times!
            Application.SendKeys "{DOWN}"
        Next
    End If
    Select Case watchType
        Case enumWatchType.WatchExpression
            Application.SendKeys "%w"
        Case enumWatchType.BreakWhenTrue
            Application.SendKeys "%t"
        Case enumWatchType.BreakWhenChange
            Application.SendKeys "%c"
    End Select

    Application.SendKeys "~"

End Sub
Function getEscapedSendkeysText(ByVal text As String) As String
    Dim char As String, i As Long
    Const chars As String = "~%+^()[]"
    For i = 1 To Len(chars)
        char = Mid$(chars, i, 1)
        text = Replace(text, char, "{" & char & "}")
    Next
    getEscapedSendkeysText = text
End Function

在Oracle中，你不能这样做（你会得到“在窗口规范中通过表达式缺少ORDER”）。这对我来说很有意义 - 行没有排序，请求前后行是没有意义的。

阅读时docs create table dual (dummy character varying); insert into dual values ('X'); select min(dummy) OVER (ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) from dual; select min(dummy) OVER (ROWS BETWEEN 1 PRECEDING AND 10 FOLLOWING) from dual;是可选的且独立于[frame_clause]，但我不明白为什么这需要/有用。

为什么PostgreSQL允许这种行为？

Answer 1

一个window子句是SELECT阶段的一部分，在查询处理的那个阶段实际上有一个行顺序。如果您没有指定order by，那么它只是一个随机订单。

您可以做的一件事就是为方便起见，包括下一行的列，例如：

select  id
,       max(id) over (rows between 1 following and 1 following) as next_id
from    AnyTable

按顺序读取行但由于某种原因需要下一行的id的客户端可以使用它。

为什么PostgreSQL允许没有表达式的frame子句？

1 个答案: