Access Cookbook, 2nd Edition [Electronic resources] نسخه متنی

Recipe 14.5 Pass Parameters to Stored Procedures from Pass-Through Queries in an MDB

14.5.1 Problem

You are calling stored procedures
that require parameters by using pass-through queries. How can you
pass parameters to the pass-through query from your form? If you
include a reference to the form in the pass-through query, you get an
error message from SQL Server.

14.5.2 Solution

Pass-through queries are not processed in the same way as regular
Access queries against linked tables. The SQL syntax you type in a
pass-through query is passed directly to SQL Server. Any references
to forms or controls on forms in a pass-through query are meaningless
to SQL Server, so you must pass the actual values for your
parameters.

A pass-through query has three important
properties:

The
SQL property contains the textual content of the pass-through query.
This must be a valid Transact-SQL statement.

The connection string contains
information that the query uses to connect to SQL Server. You can
specify a DSN, or use a string containing all the requisite
connection information, as shown in the Solution in Recipe 14.1.

The ReturnsRecords property specifies
whether or not the query returns records. An action query that just
modifies data without retrieving anything would have this property
set to No or False.

Figure 14-12 shows the properties sheet for a
pass-through query to the pubs sample database in
SQL Server.

Figure 14-12. Pass-through query properties

The most
versatile way to set these properties is to write a procedure that
sets them at runtime by using a DAO QueryDef object.
You'll then need to set parameter values to the
procedure for connection information, the SQL string that comprises
the pass-through query, and whether or not the query returns records.

To
modify a pass-through query at runtime, follow these general steps:

1. Open a new module and set a reference to the DAO object library.

2. Create a new public procedure. Here is the complete code listing:

   Public Sub acbPassThrough( _
   ByVal QueryName As String, _
   ByVal SQLStatement As String, _
   Optional ConnectStr As Variant, _
   Optional ReturnsRecords As Boolean = True)
   Dim qdf As DAO.QueryDef
   Dim strConnect As String
   Set qdf = CurrentDb.QueryDefs(QueryName)
   ' If no connection information is supplied,
   ' connection information from the query is used.
   If IsMissing(ConnectStr) Then
   strConnect = qdf.Connect
   Else
   strConnect = CStr(ConnectStr)
   End If
   ' Set query properties to parameter values.
   qdf.Connect = strConnect
   qdf.ReturnsRecords = ReturnsRecords
   qdf.SQL = SQLStatement
   ExitHere:
   Set qdf = Nothing
   Exit Sub
   HandleErr:
   MsgBox Err & ": " & Err.Description, , "Error in acbPassThrough"
   Resume ExitHere
   End Sub

3. To test the procedure, create a new query and choose Query
   SQL-Specific Pass-through from the menu.

4. Save the query, naming it qryPassThrough.

5. Create a form with text boxes and optionally a combo box to test the
   procedure. The sample form in

   14-05.MDB uses the
   byroyalty stored procedure from the
   pubs sample database. It takes an input parameter
   for the royalty percentage. You can change the values on the form
   shown in Figure 14-13 to adjust any of the arguments
   needed to call the acbPassThrough procedure.

Figure 14-13. The sample form used to test the acbPassThrough procedure

6. Write the following code in the
   Click event of the command button to pass the parameters to
   acbPassThrough:

   Private Sub cmdExecute_Click( )
   Dim strQuery As String
   Dim strSQL As String
   Dim strConnect As String
   Dim fReturnsRecs As Boolean
   strQuery = Me.lblQuery.Caption
   strConnect = Me.lblConnection.Caption
   fReturnsRecs = CBool(Me.ckReturnsRecords)
   strSQL = "EXEC byroyalty " & Me.cboParameter
   Call acbPassThrough(strQuery, strSQL, strConnect, fReturnsRecs)
   Me.RecordSource = strQuery
   Me.txtAuID.Visible = True
   End Sub

7. Test the procedure by clicking the "Execute
   byroyalty" command button on the form.

14.5.3 Discussion

The acbPassThrough procedure can modify any saved pass-through query
by using the DAO QueryDef object:

Dim qdf As DAO.QueryDef
Dim strConnect As String
Set qdf = CurrentDb.QueryDefs(QueryName)

There is an optional parameter for the ConnectStr argument. If a
connection string is not supplied, the one saved with the QueryDef
object is used:

If IsMissing(ConnectStr) Then
strConnect = qdf.Connect
Else
strConnect = CStr(ConnectStr)
End If

The properties for the query are then set to the values passed into
the procedure:

qdf.Connect = strConnect
qdf.ReturnsRecords = ReturnsRecords
qdf.SQL = SQLStatement

This actually permanently saves changes to the queryif you
open the query in design view after executing the procedure,
you'll see the last properties that were set.

The values on the form are simply collected from the relevant text
boxes and combo boxes, and passed to the procedure. Then the form is
requeried and the new results of the pass-through query are loaded as
the record source of the form.

Access lets you create ad hoc queries by
using the CreateQueryDef syntax and specifying an
empty string for the parameter name. However, using a previously
saved query eliminates the overhead of creating a new object from
scratch and then discarding it.

The result set returned from a pass-through query is always read-only.