如何在 Access SQL 中将空字符串转换为数字 null

Question

我想用暂存 table 中的数据填充 table。 staging table 中有趣的列具有数据类型 text，但在其他方面填充了可解析为双精度值或空字符串（即 "4.209"、"42" 或"")。目标 table 中的相应列的数据类型为 double.

我正在执行的SQL语句是

insert into dest (.., theColumn, ... ) select ...., theColumn, .. from src

当我执行语句（使用 ADO）时收到 Data type mismatch in criteria expression 错误。

如果我用 null 替换 theColumn，它可以正常工作。所以，我认为我应该以某种方式将空字符串转换为 nulls。这可能吗？

Answer 1

将空字符串转换为零也可能有效。

insert into dest (.., theColumn, ... ) select ...., theColumn+0, .. from src

Answer 2

您可以使用 Val 将 "" 转换为 0:

insert into dest (.., theColumn, ... ) select ...., Val(theColumn), .. from src

要插入 Null，请使用如下函数：

' Converts a value of any type to its string representation.
' The function can be concatenated into an SQL expression as is
' without any delimiters or leading/trailing white-space.
'
' Examples:
'   SQL = "Select * From TableTest Where [Amount]>" & CSql(12.5) & "And [DueDate]<" & CSql(Date) & ""
'   SQL -> Select * From TableTest Where [Amount]> 12.5 And [DueDate]< #2016/01/30 00:00:00#
'
'   SQL = "Insert Into TableTest ( [Street] ) Values (" & CSql(" ") & ")"
'   SQL -> Insert Into TableTest ( [Street] ) Values ( Null )
'
' Trims text variables for leading/trailing Space and secures single quotes.
' Replaces zero length strings with Null.
' Formats date/time variables as safe string expressions.
' Uses Str to format decimal values to string expressions.
' Returns Null for values that cannot be expressed with a string expression.
'
' 2016-01-30. Gustav Brock, Cactus Data ApS, CPH.
'
Public Function CSql( _
    ByVal Value As Variant) _
    As String

    Const vbLongLong    As Integer = 20
    Const SqlNull       As String = " Null"

    Dim Sql             As String
    Dim LongLong        As Integer

    #If Win32 Then
        LongLong = vbLongLong
    #End If
    #If Win64 Then
        LongLong = VBA.vbLongLong
    #End If

    Select Case VarType(Value)
        Case vbEmpty            '    0  Empty (uninitialized).
            Sql = SqlNull
        Case vbNull             '    1  Null (no valid data).
            Sql = SqlNull
        Case vbInteger          '    2  Integer.
            Sql = Str(Value)
        Case vbLong             '    3  Long integer.
            Sql = Str(Value)
        Case vbSingle           '    4  Single-precision floating-point number.
            Sql = Str(Value)
        Case vbDouble           '    5  Double-precision floating-point number.
            Sql = Str(Value)
        Case vbCurrency         '    6  Currency.
            Sql = Str(Value)
        Case vbDate             '    7  Date.
            Sql = Format(Value, " \#yyyy\/mm\/dd hh\:nn\:ss\#")
        Case vbString           '    8  String.
            Sql = Replace(Trim(Value), "'", "''")
            If Sql = "" Then
                Sql = SqlNull
            Else
                Sql = " '" & Sql & "'"
            End If
        Case vbObject           '    9  Object.
            Sql = SqlNull
        Case vbError            '   10  Error.
            Sql = SqlNull
        Case vbBoolean          '   11  Boolean.
            Sql = Str(Abs(Value))
        Case vbVariant          '   12  Variant (used only with arrays of variants).
            Sql = SqlNull
        Case vbDataObject       '   13  A data access object.
            Sql = SqlNull
        Case vbDecimal          '   14  Decimal.
            Sql = Str(Value)
        Case vbByte             '   17  Byte.
            Sql = Str(Value)
        Case LongLong           '   20  LongLong integer (Valid on 64-bit platforms only).
            Sql = Str(Value)
        Case vbUserDefinedType  '   36  Variants that contain user-defined types.
            Sql = SqlNull
        Case vbArray            ' 8192  Array.
            Sql = SqlNull
        Case Else               '       Should not happen.
            Sql = SqlNull
    End Select

    CSql = Sql & " "

End Function

Answer 3

使用IIf()表达式：如果theColumn包含一个表示有效数字的字符串，return该数字；否则 return 为空。

SELECT IIf(IsNumeric(theColumn), Val(theColumn), Null) FROM src

我的第一个冲动是使用 IsNumeric()。但是我意识到这是对您要求的更直接的翻译...

SELECT IIf(theColumn='', Null, Val(theColumn)) FROM src

如何在 Access SQL 中将空字符串转换为数字 null

How do I convert an empty string into a numerical null in Access SQL_

sql

ms-access

ado