How To Create a Parameter Query via Data Access Objects

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A parameter query is created in a program by using the CreateQueryDef() function. Following is the syntax for the CreateQueryDef() function: The sqltext string is optional or it can be defined by using the .SQL property of the QueryDef. To create a parameter query, place the PARAMETERS statement in the sqltext string. Here is the syntax for the PARAMETERS statement: The following table lists the appropriate Microsoft Jet SQL data type that should be used with the PARAMETERS statement as well as the corresponding Microsoft Access field type, Visual Basic variable type, and constant value from the DATACONS.TXT file.

NOTE: Visual Basic 4.0 and later and Office 95 and later versions of Basic support additional data types (Boolean, Byte, Byte Array). The table below lists equivalent Basic types for earlier versions of Basic:

   Microsoft      Microsoft        Visual
   Access SQL     Access Field     Basic Type      DATACONS.TXT Constant
   ---------------------------------------------------------------------
   Bit            Yes/No           Integer/Boolean   DB_BOOLEAN = 1
   Byte           Byte             Integer/Byte      DB_BYTE = 2
   Short          Integer          Integer           DB_INTEGER = 3
   Long           Long Integer     Long              DB_LONG = 4
   Currency       Currency         Double            DB_CURRENCY = 5
   IEEESingle     Single           Single            DB_SINGLE = 6
   IEEEDouble     Double           Double            DB_DOUBLE = 7
   DateTime       Date/Time        Variant           DB_DATE = 8
   Binary         Binary           String/Byte Array
   Text           Text             String            DB_TEXT = 10
   LongBinary     OLE Object       String/Byte Array DB_LONGBINARY = 11
   LongText       Memo             String            DB_MEMO = 12
				

Following the PARAMETERS statement in the sqltext string, place the query. The query can refer to the parameter (parametertext) named in the PARAMETERS statement. Wherever the query refers to a parameter, the current value will be substituted when the query is executed.

For example, if the query text is: and the parameter i was set to 42 in the program, the parameter i would be substituted and the resulting query would be equivalent to:

Multiple Parameters in a PARAMETERS Statement

It is also possible to have multiple parameters in a PARAMETERS statement. To do this, use commas to separate the parameters as follows: Prior to executing the query, set the parameters using this syntax: In the previous example, you would use QD!i=42 before executing the query.

Once the parameters are set, you are ready to execute the query. There are three methods (Execute, CreateDynaset, and CreateSnapshot) supported by a QueryDef that will cause the query to be executed.

Example Parameter Queries

The following example illustrates the use of a short parameter in a query. The example has two parts. The first part creates a new QueryDef for BIBLIO.MDB (the sample Microsoft Access database that ships with Visual Basic) and should be executed only once. The second part uses the QueryDef to create a snapshot, which is then displayed. To test the example, place each of the following code segments in a CommandButton Click event procedure.

NOTE: There are some minor differences between the older DAO syntax used by Visual Basic 3.0 and Access 1.x and the newer syntax used by the other products this article applies to. Old and New DAO syntax are indicated by comments:

   'Create QueryDef "by date"
   Dim Db As Database
   Dim Qd As QueryDef
   Set Db = OpenDatabase("C:\VB\BIBLIO.MDB")                 ' Old
   Set Db = DBEngine(0).OpenDatabase("C:\VB4-32\BIBLIO.MDB") ' New
   Set Qd = Db.CreateQueryDef("By date") 'Create the query "By date"
   QdText = "PARAMETERS dp Short; "
   QdText = QdText & "SELECT * from Titles WHERE [Year Published] = dp"
   Qd.SQL = QdText
   Print Qd.SQL
   Qd.Close

   ' Create Snapshot from QueryDef
   Dim Rs As Snapshot                   ' Old
   Set Qd = Db.OpenQueryDef("By Date")  ' Old
   Dim Rs As Recordset                  ' New
   Set Qd = Db.QueryDefs("By Date")     ' New
   Qd!dp = 1991                         'Set the value of the dp parameter
   Set Rs = Qd.CreateSnapshot()                ' Old
   Set Rs = Qd.OpenRecordset(dbOpenSnapshot)   ' New
   Do Until Rs.EOF
      For i = 1 To Rs.Fields.Count - 1
         Print Rs(i);                   'Display results of query
      Next
      Print
      Rs.MoveNext
   Loop
   Rs.Close
   Qd.Close
   Db.Close

The second example shows how to use an action parameter query. Note that action queries are invoked with the Execute method, not CreateDynaset or CreateSnapshot, because they do not return records:

   'Create QueryDef
   Dim Db As Database
   Dim Qd As QueryDef
   Set Db = OpenDatabase("C:\VB\BIBLIO.MDB")                 ' Old
   Set Db = DBEngine(0).OpenDatabase("C:\VB4-32\BIBLIO.MDB") ' New
   Set Qd = Db.CreateQueryDef("Delete by name") 'Create the query
   QdText = "PARAMETERS p1 Text; "
   QdText = QdText & "DELETE * FROM Authors WHERE Author = p1;"
   Qd.SQL = QdText
   Print Qd.SQL
   Qd.Close

   ' Execute the QueryDef
   Set Qd = Db.OpenQueryDef("Delete by name")  ' Old
   Set Qd = Db.QueryDefs("Delete by name")     ' New
   Qd!p1 = "Bob"
   Qd.Execute           'Perform the action query
   Qd.Close
   Db.Close