Specifying selection, sorting, and grouping criteria

In the SELECT statement associated with a DataWindow object, you can add selection, sorting, and grouping criteria that are added to the SQL statement and processed by the DBMS as part of the retrieval.

If you have defined retrieval arguments, you reference them in the WHERE or HAVING clause. In SQL statements, variables (called host variables) are always prefaced with a colon to distinguish them from column names.

For example, if the DataWindow object is retrieving all rows from the Department table where the dept_id matches a value provided by the user at runtime, your WHERE clause will look something like this:

WHERE dept_id = :Entered_id

where Entered_id was defined previously as an argument in the Specify Retrieval Arguments dialog box.

You can limit the rows that are retrieved into the DataWindow object by specifying selection criteria that correspond to the WHERE clause in the SELECT statement.

For example, if you are retrieving information about employees, you can limit the employees to those in Sales and Marketing, or to those in Sales and Marketing who make more than $50,000.

You can sort the rows that are retrieved into the DataWindow object by specifying columns that correspond to the ORDER BY clause in the SELECT statement.

For example, if you are retrieving information about employees, you can sort on department, and then within each department, you can sort on employee ID.

1. Click the Sort tab to make the Sort view available (or select View>Sort if the Sort view is not currently displayed).

   The columns you selected display in the order of selection. You might need to scroll to see your selections.

2. Drag the first column you want to sort on to the right side of the Sort view.

   This specifies the column for the first level of sorting. By default, the column is sorted in ascending order. To specify descending order, clear the Ascending check box.

3. Continue to specify additional columns for sorting in ascending or descending order as needed.

   You can change the sorting order by dragging the selected column names up or down. With the following sorting specification, rows will be sorted first by department ID, then by employee ID:

   

4. Define limiting (Where view), grouping (Group view), and limiting groups (Having view) criteria as appropriate.

5. Click the SQL Select button to return to the DataWindow painter.

You can group the retrieved rows by specifying groups that correspond to the GROUP BY clause in the SELECT statement. This grouping happens before the data is retrieved into the DataWindow object. Each group is retrieved as one row into the DataWindow object.

For example, if in the SELECT statement you group data from the Employee table by department ID, you will get one row back from the database for every department represented in the Employee table. You can also specify computed columns, such as total and average salary, for the grouped data. This is the corresponding SELECT statement:

SELECT dept_id, sum(salary), avg(salary)
FROM employee
GROUP BY dept_id

If you specify this with the Employee table in the EAS Demo DB, you get five rows back, one for each department.

For more about GROUP BY, see your DBMS documentation.

1. Click the Group tab to make the Group view available (or select View>Group if the Group view is not currently displayed).

   The columns in the tables you selected display in the left side of the Group view. You might need to scroll to see your selections.

2. Drag the first column you want to group onto the right side of the Group view.

   This specifies the column for grouping. Columns are grouped in the order in which they are displayed in the right side of the Group view.

3. Continue to specify additional columns for grouping within the first grouping column as needed.

   To change the grouping order, drag the column names in the right side to the positions you want.

4. Define sorting (Sort view), limiting (Where view), and limiting groups (Having view) criteria as appropriate.

5. Click the Return button to return to the DataWindow painter.

If you have defined groups, you can define HAVING criteria to restrict the retrieved groups. For example, if you group employees by department, you can restrict the retrieved groups to departments whose employees have an average salary of less than $50,000. This corresponds to:

SELECT dept_id, sum(salary), avg(salary)
FROM employee
GROUP BY dept_id
HAVING avg(salary) < 50000

If you specify this with the Employee table in the EAS Demo DB, you will get three rows back, because there are three departments that have average salaries less than $50,000.