access类似DCount功能的扩展函数-统计不同值的数量

access中，使用 DCount 函数可以统计特定记录集内的总数。但是DCount还有些不足：

1. 内置函数DCount（）无法计算不同值的数量；

2. Access中的域聚合函数效率低下。

这里给出一个自定义的Ecount函数，它提供了一个额外的参数，可以计算不同的值

Public Function ECount(Expr As String, Domain As String, Optional Criteria As String, Optional bCountDistinct As Boolean) As Variant
On Error GoTo Err_Handler
    'Purpose:   Enhanced DCount() function, with the ability to count distinct.
    'Return:    Number of records. Null on error.
    'Arguments: Expr           = name of the field to count. Use square brackets if the name contains a space.
    '           Domain         = name of the table or query.
    '           Criteria       = any restrictions. Can omit.
    '           bCountDistinct = True to return the number of distinct values in the field. Omit for normal count.
    'Notes:     Nulls are excluded (whether distinct count or not.)
    '           Use "*" for Expr if you want to count the nulls too.
    '           You cannot use "*" if bCountDistinct is True.
    'Examples:  Number of customers who have a region: ECount("Region", "Customers")
    '           Number of customers who have no region: ECount("*", "Customers", "Region Is Null")
    '           Number of distinct regions: ECount("Region", "Customers", ,True)
    Dim db As DAO.Database
    Dim rs As DAO.Recordset
    Dim strSql As String

    'Initialize to return Null on error.
    ECount = Null
    Set db = DBEngine(0)(0)

    If bCountDistinct Then
        'Count distinct values.
        If Expr <> "*" Then             'Cannot count distinct with the wildcard.
            strSql = "SELECT " & Expr & " FROM " & Domain & " WHERE (" & Expr & " Is Not Null)"
            If Criteria <> vbNullString Then
                strSql = strSql & " AND (" & Criteria & ")"
            End If
            strSql = strSql & " GROUP BY " & Expr & ";"
            Set rs = db.OpenRecordset(strSql)
            If rs.RecordCount > 0& Then
                rs.MoveLast
            End If
            ECount = rs.RecordCount     'Return the number of distinct records.
            rs.Close
        End If
    Else
        'Normal count.
        strSql = "SELECT Count(" & Expr & ") AS TheCount FROM " & Domain
        If Criteria <> vbNullString Then
            strSql = strSql & " WHERE " & Criteria
        End If
        Set rs = db.OpenRecordset(strSql)
        If rs.RecordCount > 0& Then
            ECount = rs!TheCount        'Return the count.
        End If
        rs.Close
    End If

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

Err_Handler:
    MsgBox Err.Description, vbExclamation, "ECount Error " & Err.Number
    Resume Exit_Handler
End Function

如下图，可以计算字段中不重复的值，如果参数为false，即功能跟Dcount是一样的。