如何在Access SQL_中将空字符串转换为数字null

Question

我想用暂存表中的数据填充一个表。暂存表中感兴趣的列的数据类型为text，但在其他情况下会填充可解析为双精度的值或空字符串（即"4.209"、"42"或""）。目标表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

当我执行该语句（使用ADO）时，我收到一个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，我想我应该以某种方式将空字符串转换为nulls。这可能吗？

Answer 1

使用IIf()表达式：如果theColumn包含表示有效数字的字符串，则返回该数字；否则返回Null。

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

我的第一个冲动是使用IsNumeric()。然而，我意识到这是你要求的更直接的翻译。。。

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

Answer 2

将空字符串转换为零也可以。

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

Answer 3

您可以使用Val将"转换为0:

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

要插入Null，请使用以下函数：

' 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