SQL: Customizing Your Recordset's SQL Statement (ODBC)

This topic explains:

• How the framework constructs a SQL statement

• How to override the SQL statement

SQL Statement Construction

Your recordset bases record selection primarily on a SQL SELECT statement. When you declare your class with a wizard, it writes an overriding version of the GetDefaultSQL member function that looks something like this (for a recordset class called CAuthors).

CString CAuthors::GetDefaultSQL()
{
    return "AUTHORS";
}

By default, this override returns the table name you specified with the wizard. In the example, the table name is "AUTHORS." When you later call the recordset's Open member function, Open constructs a final SELECT statement of the form:

SELECT rfx-field-list FROM table-name [WHERE m_strFilter]
       [ORDER BY m_strSort]

where table-name is obtained by calling GetDefaultSQL and rfx-field-list is obtained from the RFX function calls in DoFieldExchange. This is what you get for a SELECT statement unless you replace it with an overriding version at run time, although you can also modify the default statement with parameters or a filter.

To override the default SELECT statement, pass a string containing a complete SELECT statement when you call Open. Instead of constructing its own default string, the recordset uses the string you supply. If your replacement statement contains a WHERE clause, do not specify a filter in m_strFilter because you would then have two filter statements. Similarly, if your replacement statement contains an ORDER BY clause, do not specify a sort in m_strSort so that you will not have two sort statements.

You might also encounter special syntactic requirements for operations such as outer joins, depending on your DBMS. Use ODBC functions to obtain this information from your driver for the DBMS. For example, call ::SQLGetTypeInfo for a particular data type, such as SQL_VARCHAR, to request the LITERAL_PREFIX and LITERAL_SUFFIX characters. If you are writing database-independent code, see Appendix C: SQL Grammar in the ODBC Programmer's Reference for detailed syntax information.

A recordset object constructs the SQL statement that it uses to select records unless you pass a custom SQL statement. How this is done depends mainly on the value you pass in the lpszSQL parameter of the Open member function.

The general form of a SQL SELECT statement is:

SELECT [ALL | DISTINCT] column-list FROM table-list
    [WHERE search-condition][ORDER BY column-list [ASC | DESC]]

One way to add the DISTINCT keyword to your recordset's SQL statement is to embed the keyword in the first RFX function call in DoFieldExchange. For example:

...
    RFX_Text(pFX, "DISTINCT CourseID", m_strCourseID);
...

Overriding the SQL Statement

The following table shows the possibilities for the lpszSQL parameter to Open. The cases in the table are explained following the table.

The lpszSQL Parameter and the Resulting SQL String

* m_nFields must be less than or equal to the number of columns specified in the SELECT statement. The data type of each column specified in the SELECT statement must be the same as the data type of the corresponding RFX output column.

Case 1 lpszSQL = NULL

The recordset selection depends on what GetDefaultSQL returns when CRecordset::Open calls it. Cases 2 through 5 describe the possible strings.

Case 2 lpszSQL = a Table Name

The recordset uses record field exchange (RFX) to build the column list from the column names provided in the RFX function calls in the recordset class's override of DoFieldExchange. If you used a wizard to declare your recordset class, this case has the same result as case 1 (provided that you pass the same table name you specified in the wizard). If you do not use a wizard to write your class, case 2 is the simplest way to construct the SQL statement.

The following example constructs a SQL statement that selects records from an MFC database application. When the framework calls the GetDefaultSQL member function, the function returns the name of the table, SECTION.

CString CEnrollSet::GetDefaultSQL()
{
    return "SECTION";
}

To obtain the names of the columns for the SQL SELECT statement, the framework calls the DoFieldExchange member function.

void CEnrollSet::DoFieldExchange(CFieldExchange* pFX)
{
    pFX->SetFieldType(CFieldExchange::outputColumn);
    RFX_Text(pFX, "CourseID", m_strCourseID);
    RFX_Text(pFX, "InstructorID", m_strInstructorID);
    RFX_Text(pFX, "RoomNo", m_strRoomNo);
    RFX_Text(pFX, "Schedule", m_strSchedule);
    RFX_Text(pFX, "SectionNo", m_strSectionNo);
}

When complete, the SQL statement looks like this:

SELECT CourseID, InstructorID, RoomNo, Schedule, SectionNo
    FROM SECTION

Case 3 lpszSQL = a SELECT/FROM Statement

You specify the column list by hand rather than relying on RFX to construct it automatically. You might want to do this when:

• You want to specify the DISTINCT keyword following SELECT.

  Your column list should match the column names and types in the same order as they are listed in DoFieldExchange.

• You have reason to manually retrieve column values using the ODBC function ::SQLGetData rather than relying on RFX to bind and retrieve columns for you.

  You might, for example, want to accommodate new columns a customer of your application added to the database tables after the application was distributed. You need to add these extra field data members, which were not known at the time you declared the class with a wizard.

  Your column list should match the column names and types in the same order as they are listed in DoFieldExchange, followed by the names of the manually bound columns. For more information, see Recordset: Dynamically Binding Data Columns (ODBC).

• You want to join tables by specifying multiple tables in the FROM clause.

  For information and an example, see Recordset: Performing a Join (ODBC).

Case 4 lpszSQL = SELECT/FROM Plus WHERE and/or ORDER BY

You specify everything: the column list (based on the RFX calls in DoFieldExchange), the table list, and the contents of a WHERE and/or an ORDER BY clause. If you specify your WHERE and/or ORDER BY clauses this way, do not use m_strFilter and/or m_strSort.

Case 5 lpszSQL = a Stored Procedure Call

If you need to call a predefined query (such as a stored procedure in a Microsoft SQL Server database), you must write a CALL statement in the string you pass to lpszSQL. The wizards do not support declaring a recordset class for calling a predefined query. Not all predefined queries return records.

If a predefined query does not return records, you can use the CDatabase member function ExecuteSQL directly. For a predefined query that does return records, you must also manually write the RFX calls in DoFieldExchange for any columns the procedure returns. The RFX calls must be in the same order and return the same types, as the predefined query. For more information, see Recordset: Declaring a Class for a Predefined Query (ODBC).

See also

SQL: SQL and C++ Data Types (ODBC)
SQL: Making Direct SQL Calls (ODBC)