HOWTO: RDO: Use ODBC Escape Clauses in Your SQL Syntax

An ODBC escape clause is a way to talk to the ODBC driver you are using in a DBMS independent manner. The escape clause is represented by a pair of curly braces ({}) surrounding the standard form of the SQL syntax and a one- character or two-character token that specifies the type of the escape clause.

For example, if you want to specify a date value and you don't know which ODBC driver you will be using, you can use the Escape clause in this construct, {d 'yyyy-mm-dd'} and it will be translated by the ODBC driver manager into the form the backend can utilize. The following select statement will work with any ODBC driver: "SELECT * FROM table1 WHERE datefield = {d '1995-09-12'}" There are also other areas where you can use ODBC escape clauses:

Date and Time

where [.f...] allows you to specify fractions of a second if you wish.

Stored Procedures

For stored procedures a parameter marker (?) must be used for the return value (if any) and any output arguments because it is bound to a program variable. Input arguments can be either literals or parameters, for example, "{? = call procedure_name(arg1, arg2, ?)}" or "{call procedure_name(arg1, arg2, ?)}".

String Functions

This example will retrieve a resultset where the first three characters of author field is 'dat' from the Pubs database. "Select * from authors where {fn LEFT(author, 3)} = 'dat'".

This example will retrieve a resultset with the author field all in uppercase letters. It is necessary to alias the column with the phrase "as AUTHORS" because the RDC control doesn't know what you are asking for in an escape clause so if you do not alias the column, the RDC control will refer to is as "Expr1000". MSRDC1.SQL = "Select {fn UCASE(author)} as AUTHORS from authors"

Math Functions

System Functions

This example returns all the orders entered by the current user: "Select ordnum From orders Where employee = {fn User()}".

Time and Date Functions

Data Type Conversion Functions

The data type conversion function converts a data type to a different data type on the server. This example shows the simple conversion of a date column to a character string. It also shows how powerful the use of scalar functions in expressions can be. In this case, the query returns all orders made in the 1990s "Select ordnum From orders Where {fn convert(orddate, SQL_CHAR)} like '199%'".

For complete documentation see the "Microsoft ODBC 2.0 Programmer's Reference and SDK Guide."

Sample Program

This example uses the escape clause string function and assumes that you already have an ODBC data source set up. This example uses a "DSN-less" ODBC connection so you will not need to set up a DSN with the ODBC Admin utility.

1. Start a new project in Visual Basic. Form1 is created by default.
2. Add a command button to Form1.
3. Paste the following code into the General Declarations section of form1.

      Private Sub Command1_Click()
        Dim cn As rdoConnection
        Dim en As rdoEnvironment
        Dim rs As rdoResultset
        'establish connection
        Set en = rdoEngine.rdoEnvironments(0)
        en.CursorDriver = rdUseOdbc
        'Below should be modified to match your data source
        Dim cnStr As String
        cnStr = "driver={SQL Server};server=myserver;" & _
          "database=pubs;uid=myuid;pwd=mypwd"
        Set cn = en.OpenConnection(DSName:="", Prompt:=rdDriverNoPrompt, _
          Connect:=cnStr)
        Dim sql As String
        sql = "Select * from authors where {fn LEFT(au_lname, 3)} = 'gre'"
        Set rs = cn.OpenResultset(Name:=sql, Type:=rdOpenDynamic, _
          Option:=rdAsyncEnable)
        'check if still executing for asynchronous query option
        While rs.StillExecuting
          DoEvents
        Wend
        If rs.RowCount > 0 Then
          While Not rs.EOF
            Me.Print rs("au_lname")
            rs.MoveNext
          Wend
        End If
        MsgBox Str(rs.RowCount) & " rows returned."
      End Sub
   					

4. Note that you will need to change your DRIVER, SERVER, DATABASE, UID, and PWD in the OpenConnection method. You will also need to modify the SQL statement contained in the Command1_Click event to match your own SQL data source.
5. Start the program or press the F5 key.
6. You can then click on the Command1 button to start the query which will display the number of rows returned.

(Hitchhiker's Guide to Visual Basic and SQL Server, Microsoft Press. ISBN: 1-55615-906-4.).