Typically, a Column in a StorageDataSet derives its values from data in a database column or as a result of being imported from a text file. A column may also derive its values as a result of a calculated expression. JBuilder supports two kinds of calculated columns: calculated and aggregated.
In order to create a calculated column, you need to create a new persistent Column object in the StorageDataSet and supply the expression to the StorageDataSet object's calcFields event handler. Calculated columns can be defined and viewed in JBuilder. The calculated values are only visible in the running application. JBuilder-defined calculated columns are not resolved to or provided from its data source, although they can be written to a text file. For more information on defining a calculated column in the designer, see Tutorial: Creating a calculated column in the designer. For more information on working with columns, see Working with columns.
The formula for a calculated column generally uses expressions involving other columns in the data set to generate a value for each row of the data set. For example, a data set might have non-calculated columns for QUANTITY and UNIT_PRICE and a calculated column for EXTENDED_PRICE. EXTENDED_PRICE would be calculated by multiplying the values of QUANTITY and UNIT_PRICE.
Calculated aggregated columns can be used to group and/or summarize data, for example, to summarize total sales by quarter. Aggregation calculations can be specified completely through property settings and any number of columns can be included in the grouping. Four types of aggregation are supported (sum, count, min, and max) as well as a mechanism for creating custom aggregation methods. For more information, see Aggregating data with calculated fields.
Calculated columns are also useful for holding lookups from other tables. For example, a part number can be used to retrieve a part description for display in an invoice line item. For information on using a calculated field as a lookup field, see Creating lookups.
Values for all calculated columns in a row are computed in the same event call.
These are the topics covered:
This tutorial builds on the example in Querying a database. The database table that is queried is EMPLOYEE. The premise for this example is that the company is giving all employees a 10% raise. We create a new column named NEW_SALARY and create an expression that multiplies the existing SALARY data by 1.10 and places the resulting value in the NEW_SALARY column. The completed project is available in the samples/borland/samples/tutorial/dataset/CalcColumn directory of your JBuilder installation under the project name CalcColumn.jpr.
Property name | Value |
calcType | calculated |
caption | NEW_SALARY |
columnName | NEW_SALARY |
dataType | BIGDECIMAL |
If you were adding more than one column, you could manually edit the setColumns() method to change the position of the new columns or any other persistent column. No data will be displayed in the calculated column in the grid in the designer. The calculations are only visible when the application is running. The data type of BIGDECIMAL is used here because that is the data type of the SALARY column which will be used in the calculation expression. Calculated columns are always read-only.
void queryDataSet1_calcFields(ReadRow readRow, DataRow dataRow, boolean boolean1) throws DataSetException{ //calculate the new salary dataRow.setBigDecimal("NEW_SALARY", readRow.getBigDecimal("SALARY").multiply(new BigDecimal(1.1))); }
This method is called for calcFields whenever a field value is saved and whenever a row is posted. This event passes in an input which is the current values in the row (readRow, or changedRow), an output row for putting any changes you want to make to the row (dataRow, or calcRow), and a boolean (boolean1, or isPosted) that indicates whether the row is posted in the DataSet or not. You may not want to recalculate fields on rows that are not posted yet.
import java.math.BigDecimal;
When the application is running, the values in the calculated column will automatically adjust to changes in any columns referenced in the calculated expression.
You can use the aggregation feature of a calculated column to summarize your data in a variety of ways. Columns with a calcType of aggregated have the ability to
The aggDescriptor property is used to specify columns to group, the column to aggregate, and the aggregation operation to perform. The aggDescriptor is described in more detail below. The aggregation operation is an instance of one of these classes: CountAggOperator, SumAggOperator, MaxAggOperator, MinAggOperator, or a custom aggregation class that you define.
Creating a calculated aggregated column is simpler than creating a calculated column, because no event method is necessary (unless you are creating a custom aggregation component). The aggregate can be computed for the entire data set, or you can group by one or more columns in the data set and compute an aggregate value for each group. The calculated aggregated column is defined in the data set being summarized, so every row in a group will have the same value in the calculated column (the aggregated value for that group). The column is hidden by default. You can choose to show the column or show its value in another control, which is what we do in the following tutorial section.
For this option | Make this choice |
---|---|
Connection URL | jdbc:odbc:DataSet Tutorial |
Username | SYSDBA |
Password | masterkey |
Click the Test Connection button to test the connection and ensure its validity. If the connection is successful, click OK. If not successful, see Troubleshooting JDBC connections in the tutorials.
For this option | Make this choice |
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Database | database1 |
SQL Statement | select cust_no, PO_NUMBER, SHIP_DATE, TOTAL_VALUE from SALES |
Place SQL text in resource bundle | unchecked |
Click the Test Query button to test the query and ensure its validity. If successful, click OK. If not successful, review the SQL statement for errors.
Property name | Value |
caption | GROUP_TOTAL |
columnName | GROUP_TOTAL |
currency | True |
dataType | BIGDECIMAL |
calcType | aggregated |
A new column is instantiated and the following code is add to the jbInit() method. To view the code, select the Source tab. To view more code, use Alt+Z to toggle between the standard view and an expanded view of code.
column1.setCurrency(true); column1.setCalcType(borland.jbcl.dataset.CalcType.AGGREGATE); column1.setCaption("GROUP_TOTAL"); column1.setColumnName("GROUP_TOTAL"); column1.setDataType(borland.jbcl.util.Variant.BIGDECIMAL);
No data will be displayed in the TextFieldControl in the designer. The calculations are only visible when the application is running. The data type of BIGDECIMAL is used here because that is the data type of the TOTAL_VALUE column which will be used in the calculation expression. Aggregated columns are always read-only.
Property name | Value |
columnName | PO_NUMBER |
dataType | STRING |
This and the next two steps ensure the columns that will display in the grid are persistent. Persistent columns are enclosed in brackets in the Component tree. Also, when you add more than one column, you can manually edit the setColumns() method in the Source code pane to change the order of display of the columns.
Property name | Value |
columnName | CUST_NO |
dataType | INT |
Property name | Value |
columnName | SHIP_DATE |
dataType | TIMESTAMP |
In the agg property editor,
Based on above selections, you will have a sum of all sales to a given customer.
This step generate the following source code in the jbInit() method:
column1.setAgg(new borland.jbcl.dataset.AggDescriptor(new String[] {"CUST_NO"}, "TOTAL_VALUE", new borland.jbcl.dataset.SumAggOperator()));
When the agg property editor looks like the one below, click OK.
When the application is running, the values in the aggregated field will automatically adjust to changes in the TOTAL_VALUE field. Also, the value that displays in the TextFieldControl will display the aggregation for the CUST_NO of the currently selected row.
The agg property editor provides a simple interface for creating and modifying AggDescriptor objects. An AggDescriptor object's constructor requires the following information:
The agg property editor uses CMT to extract possible column names for use as grouping columns, and presents them as a list of Available Columns. Only non-calculated, non-aggregate column names are allowed in the list of grouping columns.
If the DataSet for whose Column the agg property is being defined has a MasterLink descriptor (i.e., is a detail DataSet), the linking column names will be added by default to the list of grouping columns when defining a new AggDescriptor.
The buttons beneath the list of grouping columns and available columns can be used to move the highlighted column name of the list above the button to the opposite list. Also, double-clicking on a column name in a list will move the column name to the opposite list. Entries within both lists are read-only; there should be no way to edit the contents of either list. Note that since the ordering of column names is insignificant within a group, a column name is always appended to the end of its destination list. An empty (null) group is allowed.
The Aggregate Column choice control will contain the list of all non-aggregate column names for the current DataSet, obtained also from CMT. Although the current set of AggOperators provided with JBCL does not provide support for non-numeric aggregate column types, we do not restrict columns in the list to numeric types, since it's possible that a user's customized AggOperator could support string and date types.
The Aggregate Operation choice control displays the list of AggOperators built into JBCL as well as any user-defined AggOperators detectable by CMT within the same class context as the AggDescriptor's Column.
Users desiring to perform calculations on aggregated values (e.g., the sum of line items ordered multiplied by a constant) should check the Calculated Aggregate check box. Doing so disables the Aggregate Column and Aggregate Operation choice controls, and substitutes their values with 'null' in the AggDescriptor constructor, signifying a calculated aggregate type. When the Calculated Aggregate check box is unchecked, the Aggregate Column and Aggreage Operation choice controls are enabled.
Clicking the OK button assigns an AggDescriptor created with the settings defined in the dialog box to the Column's agg property. Clicking the Cancel button restores the Column's agg property to its previous state. Clicking the Help button activates the Help system, displaying "Agg property editor" as the current help page.
To use an aggregation method other than the ones provided by JBuilder, you can create a custom aggregation event handler. One way to create a custom aggregation event handler is to code the calcAggAdd and calcAggDelete events through the UI Designer. calcAggAdd and calcAggDelete are StorageDataSet events that are called after the AggOperator is notified of an update operation. A typical use for these events is for totalling columns in a line items table (like SALES). The dollar amounts can be totalled using a built-in SumAggOperator. Additional aggregated columns can be added with the AggDescriptor's aggOperator property set to null. These additional columns might be for applying a tax or discount percentage on the subtotal, calculating shipping costs, and then calculating a final total.
You can also create a custom aggregation class by implementing a custom aggregation operator component by extending from AggOperator and implementing the abstract methods. The advantage of implementing a component is reusability in other DataSets. You may wish to create aggregation classes for calculating an average, standard deviation, or variance.
Check the Borland JBuilder technical publications Web site at http://www.borland.com/techpubs/borland/jbuilder/ for updates to this topic that may include a tutorial or sample custom aggregations. Check the samples directory in case a sample custom aggregation example is created after this documentation was written.