Recordset: How Recordsets Select Records (ODBC)
This topic applies to the MFC ODBC classes.
This topic explains:
Your role and your options in selecting records.
How a recordset constructs its SQL statement and selects records.
What you can do to customize the selection.
Recordsets select records from a data source through an ODBC driver by sending SQL statements to the driver. The SQL sent depends on how you design and open your recordset class.
Your Options in Selecting Records
The following table shows your options in selecting records.
How and When You Can Affect a Recordset
When you |
You can |
|---|---|
Declare your recordset class with the Add Class wizard |
Specify which table to select from. Specify which columns to include. |
Complete your recordset class implementation |
Override member functions such as OnSetOptions (advanced) to set application-specific options or to change defaults. Specify parameter data members if you want a parameterized recordset. |
Construct a recordset object (before you call Open) |
Specify a search condition (possibly compound) for use in a WHERE clause that filters the records. See Recordset: Filtering Records (ODBC). Specify a sort order for use in an ORDER BY clause that sorts the records. See Recordset: Sorting Records (ODBC). Specify parameter values for any parameters you added to the class. See Recordset: Parameterizing a Recordset (ODBC). |
Run the recordset's query by calling Open |
Specify a custom SQL string to replace the default SQL string set up by the wizard. See CRecordset::Open in the Class Library Reference and SQL: Customizing Your Recordset's SQL Statement (ODBC). |
Call Requery to requery the recordset with the latest values on the data source |
Specify new parameters, filter, or sort. See Recordset: Requerying a Recordset (ODBC). |
How a Recordset Constructs Its SQL Statement
When you call a recordset object's Open member function, Open constructs a SQL statement using some or all of the following ingredients:
The lpszSQL parameter passed to Open. If not NULL, this parameter specifies a custom SQL string or part of one. The framework parses the string. If the string is a SQL SELECT statement or an ODBC CALL statement, the framework uses the string as the recordset's SQL statement. If the string does not begin with "SELECT" or "{CALL", the framework uses what is supplied to construct a SQL FROM clause.
The string returned by GetDefaultSQL. By default, this is the name of the table you specified for the recordset in the wizard, but you can change what the function returns. The framework calls GetDefaultSQL — if the string does not begin with "SELECT" or "{CALL", it is assumed to be a table name, which is used to construct a SQL string.
The field data members of the recordset, which are to be bound to specific columns of the table. The framework binds record columns to the addresses of these members, using them as buffers. The framework determines the correlation of field data members to table columns from the RFX or Bulk RFX function calls in the recordset's DoFieldExchange or DoBulkFieldExchange member function.
The filter for the recordset, if any, contained in the m_strFilter data member. The framework uses this string to construct a SQL WHERE clause.
The sort order for the recordset, if any, contained in the m_strSort data member. The framework uses this string to construct a SQL ORDER BY clause.
Tip
To use the SQL GROUP BY clause (and possibly the HAVING clause), append the clauses to the end of your filter string.
The values of any parameter data members you specify for the class. You set parameter values just before you call Open or Requery. The framework binds the parameter values to "?" placeholders in the SQL string. At compile time, you specify the string with placeholders. At run time, the framework fills in the details based on the parameter values you pass.
Open constructs a SQL SELECT statement from these ingredients. See Customizing the Selection for details about how the framework uses the ingredients.
After constructing the statement, Open sends the SQL to the ODBC Driver Manager (and the ODBC Cursor Library if it is in memory), which sends it on to the ODBC driver for the specific DBMS. The driver communicates with the DBMS to carry out the selection on the data source and fetches the first record. The framework loads the record into the field data members of the recordset.
You can use a combination of these techniques to open tables and to construct a query based on a join of multiple tables. With additional customization, you can call predefined queries (stored procedures), select table columns not known at design time and bind them to recordset fields or you can perform most other data-access tasks. Tasks you cannot accomplish by customizing recordsets can still be accomplished by calling ODBC API functions or directly executing SQL statements with CDatabase::ExecuteSQL.
Customizing the Selection
Besides supplying a filter, a sort order, or parameters, you can take the following actions to customize your recordset's selection:
Pass a custom SQL string in lpszSQL when you call Open for the recordset. Anything you pass in lpsqSQL takes precedence over what the GetDefaultSQL member function returns.
For more information, see SQL: Customizing Your Recordset's SQL Statement (ODBC), which describes the types of SQL statements (or partial statements) you can pass to Open and what the framework does with them.
Note
If the custom string you pass does not begin with "SELECT" or "{CALL", MFC assumes it contains a table name. This also applies to the next bulleted item.
Alter the string that the wizard writes in your recordset's GetDefaultSQL member function. Edit the function's code to change what it returns. By default, the wizard writes a GetDefaultSQL function that returns a single table name.
You can have GetDefaultSQL return any of the items that you can pass in the lpszSQL parameter to Open. If you do not pass a custom SQL string in lpszSQL, the framework uses the string that GetDefaultSQL returns. At a minimum, GetDefaultSQL must return a single table name. But you can have it return multiple table names, a full SELECT statement, an ODBC CALL statement, and so on. For a list of what you can pass to lpszSQL — or have GetDefaultSQL return — see SQL: Customizing Your Recordset's SQL Statement (ODBC).
If you are performing a join of two or more tables, rewrite GetDefaultSQL to customize the table list used in the SQL FROM clause. For more information, see Recordset: Performing a Join (ODBC).
Manually bind additional field data members, perhaps based on information you obtain about the schema of your data source at run time. You add field data members to the recordset class, RFX or Bulk RFX function calls for them to the DoFieldExchange or DoBulkFieldExchange member function, and initializations of the data members in the class constructor. For more information, see Recordset: Dynamically Binding Data Columns (ODBC).
Override recordset member functions, such as OnSetOptions, to set application-specific options or to override defaults.
If you want to base the recordset on a complex SQL statement, you need to use some combination of these customization techniques. For example, perhaps you want to use SQL clauses and keywords not directly supported by recordsets or perhaps you are joining multiple tables.