SOQL Aggregate functions in Apex

Aggregate functions in salesforce include AVG(), COUNT(), MIN(), MAX(), SUM(). The functions like  SUM() and MAX() in SOQL allow to roll up and summarize the data in a query. The GROUP BY clause in a SOQL query is to avoid iterating through individual query results and used to specify a group of records instead of processing many individual records.

A query that includes an aggregate function returns its results in an array of AggregateResult objects. AggregateResult is a read-only sObject and is only used for query results. The values in the AggregateResult object can be accessed much like a map calling a “get” method with the name of the column.

The example Apex Trigger used below is based on the Payment custom object that has a lookup to the Contact and Project objects. Aggregate functions like sum(), max() are used in the query. The sum() is used for the amount field to get the sum of the payment amount made for the project by a contact, max() is to get the last payment date made for the project. The AggregateResult query is used to capture the result of the query and to get the value using an alias name.

Paymentrigger: 

Trigger paymentrigger on payment__c (after insert,after update,after unDelete,after delete) { 
    map<id,project__c> updateMap =new map<id,project__c>(); 
    map<id,contact> updateMap1 =new map<id,contact>(); 
    set connameset=new set(); 
    set projnameset=new set(); 
    if(Trigger.isInsert|| Trigger.isUpdate|| Trigger.isUnDelete){ 
        for(payment__c pay:Trigger.new){ 
            if(pay.project__c != null) 
                projnameset.add(pay.project__c); 
            if(pay.Contact__c != null) 
                connameset.add(pay.Contact__c); 
        } 
    } 
    if(Trigger.isDelete){ 
        for(payment__c pays:Trigger.old){ 
            if(pays.project__c!=null) 
                projnameset.add(pays.project__c); 
            if(pays.contact__c!=null) 
               connameset.add(pays.contact__c);             
        } 
    }     
    List AggregateResultList = [select project__c,Sum(amount__c)amt,max (Payment_date__c) maxDate from payment__c where project__c in:projnameset group by project__c]; 
    if(AggregateResultList != null && AggregateResultList.size() > 0){ 
        for(AggregateResult aggr:AggregateResultList){             
            project__c pr1=new project__c(); 
            pr1.recentpay__c=(date)aggr.get('maxDate'); 
            pr1.Id=(id)aggr.get('project__c'); 
            pr1.total_amount__c=(decimal)aggr.get('amt'); 
            updateMap.put(pr1.Id, pr1); 
        } 
    }else { 
        for(id idSet:projnameset){ 
            project__c pr1=new project__c(); 
            pr1.recentpay__c=null; 
            pr1.Id=idSet; 
            pr1.total_amount__c=0; 
            updateMap.put(pr1.Id, pr1); 
        } 
    } 
    update updateMap.values();     
    list AggregateResultList1=[select Contact__c,sum(amount__c)amts from payment__c where Contact__c in:connameset group by Contact__c]; 
    if(AggregateResultList1!= null && AggregateResultList1.size() > 0){ 
        for(AggregateResult agg:AggregateResultList1){ 
            Contact con=new Contact(); 
            con.Id=(id)agg.get('Contact__C'); 
            con.total_amount__c=(decimal)agg.get('amts'); 
            updatemap1.put(con.Id, con); 
        } 
    }else{ 
        for(id idset1:connameset){ 
            Contact con=new Contact(); 
            con.Id=idset1; 
            con.total_amount__c=0; 
            updatemap1.put(con.Id, con); 
        } 
    } 
    update updatemap1.values();     
} 

The above screenshots show two payment records and the red highlight area shows the payment date and the amount paid for the project bsnl by the contact karthi.

The above screenshot shows the project record with recent pay field with the date retrieved from the aggregate result of the query by using a max() to get the last payment date made for the project bsnl. The total amount field shows the total amount paid for the project by using the sum() function.

Reference Link: 

1. https://developer.salesforce.com/docs/atlas.en- us.apexcode.meta/ apexcode/langCon_apex _SOQL_agg_fns.htm

2. https://developer. salesforce.com/docs/atlas.en-us.soql_sosl.meta/soql_sosl/ sforce_api_calls_soql_select _agg_functions.htm

3. https://developer. salesforce.com/docs/atlas.en-us.api.meta/api/sforce_ api_calls_query_ aggregateresult.htm