Topic group

To select a subset of rows from your table, you have to identify the criteria by which rows qualify for selection.

For example, if you want to calculate the average sales volume of customers in Texas, your criteria might be that the STATE_PROV field in the Customer table contain the value "TX". Rows that fail to meet this criteria are ignored.

Another row selection technique is to specify a condition by writing an expression in the Command window that defines which rows qualify for processing.

Setting For conditions

Use the For condition to select rows that appear throughout a table rather than in a contiguous group. For conditions, check all rows in the table when determining which rows qualify for processing. Processing starts at the top of the table and goes to the bottom (unless you limit it using one of the options discussed in the previous section).

dBASE Plus compares each row with the condition you specify to determine whether to process a row. For example, to count the number of customer orders that exceed $10,000, you might specify the following For condition: TOT_INV > 10000.

Setting While conditions

Use the While condition to select a series of rows that appear consecutively in a table. While conditions check only the current row and subsequent rows when determining which rows qualify for processing. Processing starts at the current row rather than at the top of the table. Therefore, the row pointer must be at the first qualified row before processing; otherwise, no rows can be selected.

This method works best when you use an index whose key matches fields in the condition. That way, you can quickly find the first row in the series, then process rows sequentially. Processing ends when the key no longer matches the condition.

For example, to do a calculation only on rows of customers in Texas, it would speed up processing to apply an index on STATE_PROV, which would group all the TX rows together. Search for the first TX row, and then enter STATE_PROV="TX" in the While field to process from the current row through the last TX row.

You can also create or modify an index to include a subset of rows. In this case enter a For condition that specifies STATE_PROV="TX". When that index is active, only TX rows are selected.