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Example 1:
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AggregateResult[] groupedResults = [SELECT Campaignid, AVG(Amount) FROM Opportunity GROUP BY CampaignId];
for (AggregateResult ar : groupedResuits) {
System.debug ('Campaign ID' + ar.get('CampaignId'-);
System.debug ('Average amount' + ar.get('exprd'));
}

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Example 2:
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AggregateResult[] groupedResults = [SELECT Campaignid, AVG(Amount) theAverage FROM Opportunity GROUP BY Campaignid];
for (AggregateResult ar : groupedResuits) {
System.debug(*Campaign ID' + ar.get('CampaignId'));
System.debug ('Average amount' + ar.get('theAverage'));
}

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Example 3:
=====================
AggregateResult[] groupedResults = [SELECT CampaignId, AVG(Amount) FROM Opportunity GROUP BY CampaignId];
for (AggregateResult ar : groupedResuits) {
System.debug(*Campaign ID' + ar.get('CampaignId'));
System.debug('Average amount' + ar.get.AVG(-);
}

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Example 4:
=====================
AggregateResult[] groupedResults = [SELECT CampaigniId, AVG(Amount) theAverage FROM Opportunity GROUP BY Campaignid];
for (AggregateResult ar : groupedResults) {
System.debug('Campaign ID' + ar.get('CampaignId'));
System.debug ('Average amount' + ar.theAverage-;
}...