传递查询丢失对 SQL 所做的更改答案

【问题标题】：Pass-through query losing changes made to SQL传递查询丢失对 SQL 所做的更改
【发布时间】：2014-05-21 13:26:22
【问题描述】：

我有一个直通查询MyPassThru。这是查询 SQL 的简化版本：

SET NOCOUNT ON

DECLARE @FromDate datetime = '1/25/2014'
DECLARE @ThruDate datetime = '3/1/2014'

SELECT *
  FROM MtgDailyTrans M
  WHERE M.TransDate >= @FromDate
    AND M.TransDate <= @ThruDate

我需要更改@FromDate 和@ThruDate 参数。为此，我编写了以下函数 (source for RegExReplace)：

Private gDb As DAO.Database

Function Db() As DAO.Database
    If gDb Is Nothing Then Set gDb = CurrentDb
    Set Db = gDb
End Function

Sub MyPassThruSetDates(FromDate As Date, ThruDate As Date)
    Const FromPattern As String = "(@FromDate datetime = ')([\d/]+)'"
    Const ThruPattern As String = "(@ThruDate datetime = ')([\d/]+)'"
    Dim qd As DAO.QueryDef
    Set qd = Db.QueryDefs("MyPassThru")
    qd.SQL = RegExReplace(FromPattern, qd.SQL, "$1" & Format(FromDate, "m/d/yyyy") & "'")
    qd.SQL = RegExReplace(ThruPattern, qd.SQL, "$1" & Format(ThruDate, "m/d/yyyy") & "'")
    Set qd = Nothing
End Sub

问题是当我对MyPassThru 的SQL 进行更改然后运行MyPassThruSetDates() 过程时，我对SQL 所做的更改会回滚。为什么？

【问题讨论】：

标签： ms-access vba dao

【解决方案1】：

发生这种情况的原因是持久性Db 对象正在缓存 QueryDef 及其属性的副本（以包括 .SQL 属性）。如果您调用MyPassThruSetDates，然后更改MyPassThru 的SQL，然后再次调用MyPassThruSetDates，原始SQL 将覆盖自原始调用MyPassThruDates 以来所做的任何更改。

解决方案是刷新 QueryDefs 集合以确保它使用的是最新值：

Sub MyPassThruSetDates(FromDate As Date, ThruDate As Date)
    Const FromPattern As String = "(@FromDate datetime = ')([\d/]+)'"
    Const ThruPattern As String = "(@ThruDate datetime = ')([\d/]+)'"
    Dim qd As DAO.QueryDef
    Db.QueryDefs.Refresh   ' <--- This is the key!!!
    Set qd = Db.QueryDefs("MyPassThru")
    qd.SQL = RegExReplace(FromPattern, qd.SQL, "$1" & Format(FromDate, "m/d/yyyy") & "'")
    qd.SQL = RegExReplace(ThruPattern, qd.SQL, "$1" & Format(ThruDate, "m/d/yyyy") & "'")
    Set qd = Nothing
End Sub

要进一步解释为什么会发生这种情况，请参阅以下注释较多的测试例程：

Sub TestDbCache()
    Const QName As String = "TempQry"

    Dim qd As DAO.QueryDef, db As DAO.Database

    'First, we create a querydef
    Set db = CurrentDb
    Set qd = db.CreateQueryDef(QName, "SELECT 'original'")
    Debug.Print qd.SQL                           '--> SELECT 'original';

    'Next, we update the querydef's .SQL outside the scope of our db object
    CurrentDb.QueryDefs(QName).SQL = "SELECT 'changed'"

    'The querydef and db objects are unaware of the change to .SQL
    Debug.Print qd.SQL                           '--> SELECT 'original';
    Debug.Print db.QueryDefs(QName).SQL          '--> SELECT 'original';
    Debug.Print CurrentDb.QueryDefs(QName).SQL   '--> SELECT 'changed';

    'Refreshing the collection updates both the db and qd objects
    db.QueryDefs.Refresh
    Debug.Print qd.SQL                           '--> SELECT 'changed';
    Debug.Print db.QueryDefs(QName).SQL          '--> SELECT 'changed';

    'Note that the .SQL is "SELECT 'changed'" when we set the NewDb object
    Dim NewDb As DAO.Database
    Set NewDb = CurrentDb

    'We change the .SQL without refreshing the NewDb's QueryDefs collection
    CurrentDb.QueryDefs(QName).SQL = "SELECT 'changed again'"

    'Since the NewDb object never cached the contents of the query,
    '   it returns the correct current value of .SQL
    Debug.Print NewDb.QueryDefs(QName).SQL       '--> SELECT 'changed again';

    'The other db object has not refreshed its QueryDefs collection,
    '   so it is wrong once again
    Debug.Print qd.SQL                           '--> SELECT 'changed';
    Debug.Print db.QueryDefs(QName).SQL          '--> SELECT 'changed';
End Sub

【讨论】：