You use an aggregate function, or set function, such as sum or count, to perform a calculation on data read in from one column, up to the occurrence of a break in another column. For instance, in the section, Population Example in the appendix, "Report-Writer Report Examples," the regional population subtotals represent use of the sum aggregate on each of the columns tot, tot_18 to 65, tot_under18, and tot_over65 up to a break in region. Additionally, the population totals at the end of the report represent use of the sum aggregate for the same columns up to a break in "report."

You specify which data must be used in the calculation by naming the column containing that data as a parameter of the aggregate function. In the POPULATION example, the columns containing the relevant data are tot, tot_18 to 65, tot_under18, and tot_over65. You indicate the cut-off point for the data to be included in each calculation by placing the aggregate function within the footer section for a particular column or section of the report. The aggregate value is calculated each time a break occurs in the specified footer.

Aggregates can be non-unique or unique, simple, or cumulative. A non-unique aggregate performs a calculation based on every value read in from the aggregate column up to a break in the specified footer. A unique aggregate performs a calculation on each break value in the aggregate column, up to a break in the specified footer. (Depending on how the data is sorted and where the aggregate is specified, the break values can or cannot be the actual unique values in a column.)

A simple aggregate produces a single value, calculated on all the values in the aggregate column up to a break in the specified footer. A cumulative aggregate calculates a running total for each value in the aggregate column up to the break containing the aggregate instruction. Simple and cumulative aggregates can be either non-unique or unique. Aggregate types are discussed in more detail later in this section.

The following aggregates are allowed: