INF: Use of SQLBindParameter with VB and an ODBC driver

MORE INFORMATION

Declarations required to call ODBC API from VB 3.0:

   Declare Function GetFocus Lib "User" () As Integer
   Declare Function GetParent Lib "User" (ByVal hWnd%) As Integer
   Declare Function SQLAllocConnect Lib "odbc.dll" (ByVal henv&, phdbc&)
   As Integer
   Declare Function SQLAllocEnv Lib "odbc.dll" (phenv&) As Integer
   Declare Function SQLAllocStmt Lib "odbc.dll" (ByVal hdbc&, phstmt&) As
   Integer
   Declare Function SQLDisconnect Lib "odbc.dll" (ByVal hdbc&) As Integer
   Declare Function SQLError Lib "odbc.dll" (ByVal henv&, ByVal hdbc&,
   ByVal hstmt&, ByVal szSqlState$, pfNativeError&, ByVal szErrorMsg$,
   ByVal cbErrorMsgMax%, pcbErrorMsg%) As Integer
   Declare Function SQLExecDirect Lib "odbc.dll" (ByVal hstmt&, ByVal
   szSqlStr$, ByVal cbSqlStr&) As Integer
   Declare Function SQLFetch Lib "odbc.dll" (ByVal hstmt&) As Integer
   Declare Function SQLFreeConnect Lib "odbc.dll" (ByVal hdbc&) As Integer
   Declare Function SQLFreeEnv Lib "odbc.dll" (ByVal henv&) As Integer
   Declare Function SQLFreeStmt Lib "odbc.dll" (ByVal hstmt&, ByVal
   fOption%) As Integer
				

You should pay particular attention to the declaration of any string variable that will need to be passed to ODBC.DLL. This includes rgbValue in both SQLBindParameter() and SQLGetData(). The declaration should be "rgbValue as Any" to work. This is the most flexible way to declare a function variable for a Windows DLL function that will be passed from a VB program in a Windows DLL because it allows for both string and numeric values to be passed. When a string value is passed from a VB program to the Windows DLL and the DLL call expects a 'char *', then the VB program should declare the variable being passed as a VB String variable, and then pass it with the ByVal indicator. This is what is shown in the example here. If the VB variable to be passed had been a numeric value, then the variable that would be passed could be declared as long or integer, and during the call to the DLL, the variable would go into the call without the ByVal modifier.

The reason this happens is that in all cases, the C function expects a pointer to a memory location. From VB's perspective, a String variable refers to a pointer to a pointer to a memory location containing a string. This is why ByVal used in front of the variable during the function call gives a pointer to a memory location containing a string. On the other hand, a numeric variable in VB refers to a pointer to a memory location containing a numeric value. That is why the ByVal modifier is not necessary for the function call when a numeric VB variable is being passed in.

   Declare Function SQLGetData Lib "odbc.dll" (ByVal hstmt&, ByVal
   icol%, ByVal fCType%, rgbValue As Any, ByVal cbValueMax&, pcbValue As
   Long) As Integer
   Declare Function SQLBindParameter Lib "odbc.dll" (ByVal hstmt&, ByVal
   ipar%, ByVal fParamType%, ByVal fCType%, ByVal fSqlType%, ByVal
   cbColDef&, ByVal ibScale%, rgbValue As Any, ByVal cbValueMax&,
   pcbValue As Long) As Integer

   Declare Function SQLDriverConnect Lib "odbc.dll" (ByVal hdbc&, ByVal
   hWnd%, ByVal szCSIn$, ByVal cbCSIn%, ByVal szCSOut$, ByVal cbCSMax%,
   cbCSOut%, ByVal fDrvrComp%) As Integer
   Declare Function SQLParamData Lib "odbc.dll" (ByVal hstmt&, prgbValue
   As Any) As Integer
   Declare Function SQLPutData Lib "odbc.dll" (ByVal hstmt&, rgbValue As
   Any, ByVal cbValue&) As Integer

   Const SQL_ERROR = -1
   Const SQL_C_DEFAULT = 99

   Const SQL_CHAR = 1
   Const SQL_C_CHAR = 1

   Const SQL_NEED_DATA = 99

   Const SQL_LONGVARCHAR = -1
   Const SQL_BINARY = -2
   Const SQL_VARBINARY = -3
   Const SQL_LONGVARBINARY = -4

   Const SQL_C_BINARY = SQL_BINARY

   Const SQL_DRIVER_PROMPT = 2

   Const SQL_LEN_DATA_AT_EXEC_OFFSET = -100

   Sub Command3D1_Click()

				

This sample VB code illustrates the use of the ODBC API call SQLBindParameter(). It has been kept as simple as possible with a minimal amount of code and declarations.

    Dim henv As Long
    Dim hdbc As Long
    Dim hstmt As Long
    Dim nstatus%
    Dim buffer As String
    Dim param As String

    nstatus = SQLAllocEnv(henv)

    nstatus = SQLAllocConnect(henv, hdbc)
    If (nstatus = SQL_ERROR) Then
        MsgBox "Couldn't allocate memory for connection"
    End If
				

The parameters passed to SQLDriverConnect() after the second one are not really used for anything. The call expects the variables as output variables so we pass them to the call to problems.

   nstatus = SQLDriverConnect(hdbc, GetParent(GetFocus()), S$,
   Len(S$), Server, Len(Server), cbOut%, SQL_DRIVER_PROMPT)
    If (nstatus = SQL_ERROR) Then
        MsgBox "Couldn't connect"
    End If
				

In this sample, you are connecting to a SQL Server database and define this table:

CREATE TABLE model01 (col1 char(30),col2 char(30))
				

after you have executed the following insert statement:

INSERT INTO model01 VALUES ("Kate Moss","Elite")
				

For simplicity, assume only one row in the table.

    sSQLString = "Select * from model01 where col2 = ?"

    leng = Len(sSQLString)

    nstatus = SQLAllocStmt(hdbc, hstmt)
    If (nstatus = SQL_ERROR) Then
        MsgBox "Couldn't allocate memory for statement"
    End If
				

At this point, assign a value to the variable "param" that is going to be bound to the SQL statement via SQLBindParameter(). After this binding, the SQL statement becomes:

select * from model01 where col2 = 'Elite'
				

It is not a requirement of the VB programmer to put the single quotation marks into the parameter marker (param). This is handled by ODBC.

param = "Elite"
				

In this particular call to SQLBindParameter, you are binding a VB string variable to a column on the SQL Server database server that is a char(30). The '1' in the third parameter of this call indicates that this parameter is of type SQL_PARAM_INPUT. For more information, refer to the definition of fParamType in the description of SQLBindParameter() in the "Microsoft ODBC 2.0 Programmer's Reference and SDK Guide."

The sixth parameter, cbColDef, is indicating that you will be binding to a column that is 30 bytes long on SQL Server. This could also be determined with SQLColumns() but is assumed here to keep this example small and simple.

The 7th parameter (ibScale) is zero because Appendix D of the ODBC 2.0 Programmer's Reference states that this is the case for char datatypes. The 8th parameter is a pointer to the VB SQLGetData() and the VB variable "param". Note that the ByVal in the passing of the VB variable called buffer is very important for this call to succeed. The 9th parameter is 300 because you should not return more than 300 bytes through this parameter. And the final parameter indicates that the parameter is a null-terminated string (SQL_NTS as defined in C:\ODBCSDK\INCLUDE\SQL.H).

nstatus = SQLBindParameter(hstmt, 1, 1, SQL_C_CHAR, SQL_CHAR, 30,
   0, ByVal param, 300, -3)
    If (nstatus = SQL_ERROR) Then
        MsgBox "error on Bind"
    End If
				

Send the SQL statement, which is now in the form:

   select * from model01 where col2 = 'Elite'
				

    nstatus = SQLExecDirect(hstmt, sSQLString, leng)
    If (nstatus = SQL_ERROR) Then
        MsgBox "Error on execdirect"
    End If
				

Fetch back the first row of data. To keep this example simple, assume that this is the only row of data in the table that is of concern and fetch that. Normally, you should keep calling SQLFetch() and SQLGetData() in a loop until SQLFetch() returns 100 as a return code(SQL_NO_DATA_FOUND as defined in SQL.H).

    nstatus = SQLFetch(hstmt)
    If (nstatus = SQL_ERROR) Then
        MsgBox "Error on fetch"
    End If
				

It is important to initialize the VB variable that is being passed into SQLGetData() to return the data that was just fetched back. Otherwise, it may not be returned correctly or at all.

buffer = String$(300, 0)
				

Bring the data returned to the client workstation via SQLFetch() into the VB program via SQLGetData() and the VB variable "buffer". Note that the ByVal in the passing of the VB variable called buffer is very important for this call to succeed.

   nstatus = SQLGetData(hstmt, 1, SQL_C_CHAR, ByVal buffer, Len(buffer),
   300
   )
    If (nstatus = SQL_ERROR) Then
        MsgBox "Error on getdata"
    End If
				

Print the data just fetched back onto the screen.

     MsgBox "buffer contains => " + buffer
				

Here, do a quick cleanup of the environment. Notice that you do not need to call SQLFreeStmt():

    nstatus = SQLDisconnect(hdbc)
    nstatus = SQLFreeConnect(hdbc)
    nstatus = SQLFreeEnv(henv)

    MsgBox "Finished"
   End Sub