如何 SQL 连接表，在 Access-VBA 函数中选择最大值？

Question

我目前有以下 Access VBA 函数，其操作方式如 previous question 中所述（对于理解这个问题非常有用）：

Private Function MapFields(tableNameTemp As String, tableName As String, commonField As String, newTableName)

    Dim tableNameFieldCount As Integer
    tableNameFieldCount = GetFieldCount(tableName)
    Dim tableNameFieldsArray() As String
    ReDim tableNameFieldsArray(0 To tableNameFieldCount) As String ' since you cannot Dim against a variable size
    Call GetFields(tableName, tableNameFieldsArray)    

    sqlJoinQuery = "SELECT tbl_grp_by.*, [" + tableName + "].* " & _
    "INTO " + newTableName & _
    " FROM (SELECT Max([" + tableNameTemp + "].[Field1]) as [Field1], " & _
    "Max([" + tableNameTemp + "].[Field2]) as [Field2], " & _
    "Max([" + tableNameTemp + "].[Field3]) as [Field3], " & _
    "[" + tableNameTemp + "].[Field4] as [Field4] " & _
    "FROM [" + tableNameTemp & _
    "] INNER JOIN [" + tableName & _
    "] ON [" + tableNameTemp + "].[" + commonField + "] = [" + tableName + "].[" + commonField + "] " & _
    "GROUP BY [" + tableNameTemp + "].[" + commonField + "]) as tbl_grp_by " & _
    "INNER JOIN [" + tableName & _
    "] ON [" + tableName + "].[" + commonField + "] = tbl_grp_by.[" + commonField + "]"


    Debug.Print sqlJoinQuery
    CurrentDb.Execute sqlJoinQuery

End Function

但是，在我的数据中，Field3 不包含是或否，它可以包含多个值之一。为简单起见，假设这些值可以是以下字符串集中的任何一项：
(0, >1 million, 0001-0010)

在这种情况下，Max() Access SQL 函数将不适用于Field3，因为它们是具有用户定义层次结构的字符串。然而，我需要仍然选择最大值。我将定义从最低 (1) 到最高 (3) 的值：

1. 0
2. >1 million
3. 0001-0010

以下是 tableNameTemp 可能出现的示例：

╔════════════════════════╦════════╦════════╦══════════════════════╗
║ Field4 AKA commonField ║ Field1 ║ Field2 ║ Field3               ║
╠════════════════════════╬════════╬════════╬══════════════════════╣
║  SA12                  ║  No    ║  No    ║  0                   ║
╠════════════════════════╬════════╬════════╬══════════════════════╣
║  CY84                  ║  No    ║  No    ║  0                   ║
╠════════════════════════╬════════╬════════╬══════════════════════╣
║  CY84                  ║  Yes   ║  No    ║  0001-0010           ║
╠════════════════════════╬════════╬════════╬══════════════════════╣
║  CY84                  ║  No    ║  No    ║  >1 million          ║
╠════════════════════════╬════════╬════════╬══════════════════════╣
║  CY84                  ║  No    ║  Yes   ║  0                   ║
╠════════════════════════╬════════╬════════╬══════════════════════╣
║  EH09                  ║  Yes   ║  No    ║  >1 million          ║
╠════════════════════════╬════════╬════════╬══════════════════════╣
║  EH09                  ║  No    ║  No    ║  >1 million          ║
╚════════════════════════╩════════╩════════╩══════════════════════╝

鉴于上面的示例 tableNameTemp 值，下表显示了这些值将如何映射到 tableName 表。请注意它如何选择要映射的最大值。

╔════════════════════════╦════════╦════════╦══════════════════════╗
║ Field4 AKA commonField ║ Field1 ║ Field2 ║ Field3               ║
╠════════════════════════╬════════╬════════╬══════════════════════╣
║  SA12                  ║  No    ║  No    ║  0                   ║
╠════════════════════════╬════════╬════════╬══════════════════════╣
║  CY84                  ║  Yes   ║  Yes   ║  0001-0010           ║
╠════════════════════════╬════════╬════════╬══════════════════════╣
║  EH09                  ║  Yes   ║  No    ║  >1 million          ║
╚════════════════════════╩════════╩════════╩══════════════════════╝

由于我对 Access SQL 和 VBA 的经验都不是很丰富，所以我不知道如何才能做到这一点。我猜它可能涉及硬编码Field3 的可能值列表，然后根据它们在列表中的位置对它们进行排名（即位置 3 将包含值0）。然后，以某种方式使用它作为选择哪个值的键。如果这有任何用处，我在下面提供了它的代码：

Dim hierarchy(0 to 2) As String
hierarchy(0) = "0001-0010" ' highest value '
hierarchy(1) = ">1 million"
hierarchy(2) = "0"         ' lowest value  '

编辑
根据 Serg 的帮助更新了代码：

sqlJoinQuery = "SELECT tbl_grp_by.[Field1],tbl_grp_by.[Field2], " & _
                        "Switch( " & _
                            "tbl_grp_by.[maxfield3] = 0, '0', " & _
                            "tbl_grp_by.[maxfield3] = 1, '>1 million', " & _
                            "tbl_grp_by.[maxfield3] = 2 '0001-0010' " & _
                        ") as [Field3], " & _
                        "tbl_grp_by.[Field4], " & _
                    "[" + tableName + "].* " & _
                    "INTO [" + newTableName + "] FROM (" & _
                        "SELECT Max([" + tableNameTemp + "].[Field1]) as [Field1], " & _
                            "Max([" + tableNameTemp + "].[Field2]) as [Field2], " & _
                            "Max(Switch( " * _
                                "[" + tableNameTemp + "].[Field3] = '0' , 0, " & _
                                "[" + tableNameTemp + "].[Field3] = '>1 million' , 1, " & _
                                "[" + tableNameTemp + "].[Field3] = '0001-0010', 2 " & _
                            "))as [maxField3], " * _
                            "[" + tableNameTemp + "].[Field4] as [Field4] " * _
                        "FROM [" + tableNameTemp + "] " & _
                        "INNER JOIN [" + tableName + "] " & _
                            "ON [" + tableNameTemp + "].[" + commonField + "] = [" + tableName + "].[" + commonField + "] " & _
                        "GROUP BY [" + tableNameTemp + "].[" + commonField + "] " & _
                    ") as tbl_grp_by " & _
                    "INNER JOIN [" + tableName + "] " & _
                        "ON [" + tableName + "].[" + commonField + "] = tbl_grp_by.[" + commonField + "]"

Answer 1

编码值，在 SQL 中以这种方式解码最大值。

目前您正在构建 SQL 命令（我将表名变量替换为任意值，temp 和 tableName）

SELECT tbl_grp_by.*, [tableName].*  
INTO newTableName 
FROM (
    SELECT Max([temp].[Field1]) as [Field1], 
        Max([temp].[Field2]) as [Field2],  
        Max([temp].[Field3]) as [maxField3], 
        [temp].[Field4] as [Field4]  
    FROM [temp]
    INNER JOIN [tableName ]
       ON [temp].[commonField] = [tableName].[commonField] 
    GROUP BY [temp].[commonField]
 ) as tbl_grp_by  
INNER JOIN [tableName]
  ON [tableName].[commonField] = tbl_grp_by.[commonField]

将其构建为

SELECT tbl_grp_by.[Field1],tbl_grp_by.[Field2],
    Switch( 
        tbl_grp_by.[maxfield3] = 0, '0',
        tbl_grp_by.[maxfield3] = 1, '>1 million',
        tbl_grp_by.[maxfield3] = 2 '0001-0010' 
    ) as [Field3],   
    tbl_grp_by.[Field4],
[tableName].*  
INTO newTableName 
FROM (
    SELECT Max([temp].[Field1]) as [Field1], 
        Max([temp].[Field2]) as [Field2],  
        Max(Switch(  
            [temp].[field3] = '0' , 0,
            [temp].[field3] = '>1 million' , 1,
            [temp].[field3] = '0001-0010', 2  
         ))as [maxField3], 
        [temp].[Field4] as [Field4]  
    FROM [temp]
    INNER JOIN [tableName ]
       ON [temp].[commonField] = [tableName].[commonField] 
    GROUP BY [temp].[commonField]
 ) as tbl_grp_by  
INNER JOIN [tableName]
  ON [tableName].[commonField] = tbl_grp_by.[commonField]   

所以 [field3] 在内部查询中的 max() 下被编码，而 max 在外部查询中被解码。

Answer 2

我会考虑创建一个带有值字段的参考表，因为特别是当值随时间变化时更容易维护。

CREATE TABLE tblReference (field_txt text, val Integer);

获取具有最高值和唯一字段的 field_txt，然后将左连接（内连接）到您当前的数据集。

qry_field3_max = "SELECT [Field3],[commonField] FROM tblReference INNER JOIN (SELECT [commonField], MAX(val) as val FROM tblReference INNER JOIN tblNameTemp on tblReference.[field_txt]=tblNameTemp.[Field3] Group By [commonField]) as tbl_max_fields on tblReference.val=tbl_max_fields.val"