access類似Dlookup功能的擴展函數-效率更高

在access中，有幾箇D開頭的域聚閤函數。可以非常方便地穫取數據。比如Dlookup，可以指定記録集內穫取特定字段的值

但是Dlookup也有很多問題。

1. 隻能返迴找到的第一箇匹配項。由於無法指定排序順序，因此結果可能是不對的。
2. Dlookup性能很差。
3. 不能自動清理（可能導緻 沒有足夠的數據庫或者錶 錯誤）。
4. 如果目標字段包含長度爲零的字符串，將返迴錯誤的結果。

使用下麵函數，不緊添加可選蔘數允許您指定排序順序還提高查找效率

Public Function ELookup(Expr As String, Domain As String, Optional Criteria As Variant, _
    Optional OrderClause As Variant) As Variant
On Error GoTo Err_ELookup
    'Purpose:   Faster and more flexible replacement for DLookup()
    'Arguments: Same as DLookup, with additional Order By option.
    'Return:    Value of the Expr if found, else Null.
    '           Delimited list for multi-value field.
    'Author:    Allen Browne. allen@allenbrowne.com
    'Updated:   December 2006, to handle multi-value fields (Access 2007 and later.)
    'Examples:
    '           1. To find the last value, include DESC in the OrderClause, e.g.:
    '               ELookup("[Surname] & [FirstName]", "tblClient", , "ClientID DESC")
    '           2. To find the lowest non-null value of a field, use the Criteria, e.g.:
    '               ELookup("ClientID", "tblClient", "Surname Is Not Null" , "Surname")
    'Note:      Requires a reference to the DAO library.
    Dim db As DAO.Database          'This database.
    Dim rs As DAO.Recordset         'To retrieve the value to find.
    Dim rsMVF As DAO.Recordset      'Child recordset to use for multi-value fields.
    Dim varResult As Variant        'Return value for function.
    Dim strSql As String            'SQL statement.
    Dim strOut As String            'Output string to build up (multi-value field.)
    Dim lngLen As Long              'Length of string.
    Const strcSep = ","             'Separator between items in multi-value list.

    'Initialize to null.
    varResult = Null

    'Build the SQL string.
    strSql = "SELECT TOP 1 " & Expr & " FROM " & Domain
    If Not IsMissing(Criteria) Then
        strSql = strSql & " WHERE " & Criteria
    End If
    If Not IsMissing(OrderClause) Then
        strSql = strSql & " ORDER BY " & OrderClause
    End If
    strSql = strSql & ";"

    'Lookup the value.
    Set db = DBEngine(0)(0)
    Set rs = db.OpenRecordset(strSql, dbOpenForwardOnly)
    If rs.RecordCount > 0 Then
        'Will be an object if multi-value field.
        If VarType(rs(0)) = vbObject Then
            Set rsMVF = rs(0).Value
            Do While Not rsMVF.EOF
                If rs(0).Type = 101 Then        'dbAttachment
                    strOut = strOut & rsMVF!FileName & strcSep
                Else
                    strOut = strOut & rsMVF![Value].Value & strcSep
                End If
                rsMVF.MoveNext
            Loop
            'Remove trailing separator.
            lngLen = Len(strOut) - Len(strcSep)
            If lngLen > 0& Then
                varResult = Left(strOut, lngLen)
            End If
            Set rsMVF = Nothing
        Else
            'Not a multi-value field: just return the value.
            varResult = rs(0)
        End If
    End If
    rs.Close

    'Assign the return value.
    ELookup = varResult

Exit_ELookup:
    Set rs = Nothing
    Set db = Nothing
    Exit Function

Err_ELookup:
    MsgBox Err.Description, vbExclamation, "ELookup Error " & Err.number
    Resume Exit_ELookup
End Function

使用Elookup，類似dlookup的功能，穫取錶中對應的值