Recordset: Obtaining SUMs and Other Aggregate Results (ODBC)
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This article applies to the MFC ODBC classes. For DAO recordsets, see the article DAO Recordset.
This article explains how to obtain aggregate results using the following SQL keywords:
SUM Calculates the total of the values in a column with a numeric data type.
MIN Extracts the smallest value in a column with a numeric data type.
MAX Extracts the largest value in a column with a numeric data type.
AVG Calculates an average value of all the values in a column with a numeric data type.
COUNT Counts the number of records in a column of any data type.
You use these SQL functions to obtain statistical information about the records in a data source rather than to extract records from the data source. The recordset that is created usually consists of a single record (if all columns are aggregates) that contains a value. (There might be more than one record if you used a GROUP BY clause.) This value is the result of the calculation or extraction performed by the SQL function.
****Tip ****To add an SQL GROUP BY clause (and possibly a HAVING clause) to your SQL statement, append it to the end of m_strFilter. For example:
m_strFilter = "sales > 10 GROUP BY SALESPERSON_ID";
You can limit the number of records you use to obtain aggregate results by filtering and sorting the columns.
****Caution ****Some aggregation operators return a different data type from the column(s) over which they are aggregating.
SUM and AVG may return the next larger data type (for example, calling with int returns LONG or double).
COUNT usually returns LONG regardless of target column type.
MAX and MIN return the same data type as the columns they calculate.
For example, ClassWizard creates long``m_lSales to accommodate a Sales column, but you’ll need to replace this with a double m_dblSumSales data member to accommodate the aggregate result. See the example that follows.
To obtain an aggregate result for a recordset
Create a recordset containing the column(s) from which you want to obtain aggregate results.
Modify the function for the recordset. Replace the string representing the column name (the second argument of the RFX function call(s)) with a string representing the aggregation function on the column. For example, replace
RFX_Long(pFX, "Sales", m_lSales);with
RFX_Double(pFX, "Sum(Sales)", m_dblSumSales)Open the recordset. The result of the aggregation operation will be left in
m_dblSumSales.
****Note ****ClassWizard actually assigns data member names without Hungarian prefixes. For example, the wizard would produce m_Sales for a Sales column, rather than the m_lSales name used earlier for illustration.
If you’re using a class to view the data, you’ll have to change the DDX function call to display the new data member value; in this case, changing it from
DDX_FieldText(pDX, IDC_SUMSALES, m_pSet->m_lSales, m_pSet);
to
DDX_FieldText(pDX, IDC_SUMSALES, m_pSet->m_dblSumSales, m_pSet);