SOQL AggregateResult

SOQL AggregateResult

When using aggregate functions like COUNT(), SUM(), AVG(), etc. with GROUP BY,
the query does not return a list of sObjects like Account but instead a list of AggregateResult objects.

List<Account> = accList = [SELECT AccountSource, COUNT(Id) SourceCount 
                           FROM Account 
                           GROUP BY AccountSource];

If you try to assign the result directly to a List<Account>, you will get an error:
Illegal assignment from List<AggregateResult> to List<Account>

Thus, you must store the results in a List<AggregateResult> and then extract the values using aliases with .get('aliasName').

Example

public class AccountAggregateResult {
    public static void getAccountSourceCount() {
        // Query with GROUP BY and alias for COUNT(Id)
        List<AggregateResult> accList = [
            SELECT AccountSource, COUNT(Id) SourceCount 
            FROM Account 
            GROUP BY AccountSource
        ];

        // Loop through results
        for (AggregateResult ar : accList) {
            // Retrieve values using .get('fieldOrAliasName')
            System.debug('Source: ' + ar.get('AccountSource'));
            System.debug('Count: ' + ar.get('SourceCount'));
        }
    }
}