Aggregate Functions Part 2

Aggregate functions are more useful combined with the 'groupBy' method, so that each group can have its own aggregate result. Similar to the simple aggregate functions, the 'aggregate' method is needed to get the aggregate results, which will return a list of 'AggregateResult' items.

List results =
    new Query('Account').
    selectField('Rating').
    count('Name', 'countName').
    max('NumberOfEmployees', 'maxEmployees').
    groupBy('Rating').
    aggregate();

This example tries to group the accounts based on their rating, and for each rating group it tries to find the maximum number of employees.

for (AggregateResult result : results) {
    System.debug('Rating: ' + result.get('Rating'));
    System.debug('maxEmployees: ' + result.get('maxEmployees'));
}

Note that we can only select fields that appear in the group by method. In this example, only the 'Rating' field appears in the group by clause, so only the 'Rating' field can be selected.