DML requires SObject or SObject list type: String (line 31)

Question

For the below code I got this error on line 31 i.e upsert z;

global class updatezone implements Database.Batchable < sObject > , Database.stateful {

  string query;
  string z;

    //Start Method....   
    global Database.querylocator start(Database.BatchableContext bc) {
        query = 'SELECT id,No_of_Agents__c,No_of_Female_Agents__c,No_of_Male_Agents__c,Agents_40_years_and_above__c,Agents_below_40_years__c,X1_star_Agents__c,X2_star_Agents__c,X3_star_Agents__c,X4_star_Agents__c,X5_star_Agents__c' +
            ' FROM Zone__C ';
        return Database.getQueryLocator(query);
    }
    //Execute Method....
    global void execute(Database.BatchableContext bc, List < Zone__C > scope) {

        for (zone__c z: [SELECT Id, Name__C, No_of_Agents__c, No_of_Female_Agents__c, No_of_Male_Agents__c, Agents_40_years_and_above__c, Agents_below_40_years__c, X1_star_Agents__c, X2_star_Agents__c, X3_star_Agents__c, X4_star_Agents__c, X5_star_Agents__c FROM
                Zone__C ]) 
 {
            z.No_of_Agents__C = [SELECT count() FROM Agent__C WHERE Zones__C =: z.Name__C];
            z.Agents_below_40_years__C = [SELECT count() FROM Agent__C WHERE Age__C <= 40 AND Zones__C =: z.Name__C];
            z.Agents_40_years_and_above__c = [SELECT count() FROM Agent__C WHERE Age__C > 40 AND Zones__C =: z.Name__C];
            z.No_of_Male_Agents__C = [SELECT count() FROM Agent__C WHERE Gender__C =: 'Male' AND Zones__C =: z.Name__C ]; 
            z.No_of_Female_Agents__C = [SELECT count() FROM Agent__C WHERE Gender__C =: 'Female'AND Zones__C =: z.Name__C];               
            z.X1_star_Agents__c = [SELECT count() FROM Agent__C WHERE Ranking_Stars__C = 1 AND Zones__C =: z.Name__C];
            z.X2_star_Agents__c = [SELECT count() FROM Agent__C WHERE Ranking_Stars__C = 2 AND Zones__C =: z.Name__C];
            z.X3_star_Agents__c = [SELECT count() FROM Agent__C WHERE Ranking_Stars__C = 3 AND Zones__C =: z.Name__C];
            z.X4_star_Agents__c = [SELECT count() FROM Agent__C WHERE Ranking_Stars__C = 4 AND Zones__C =: z.Name__C];
            z.X5_star_Agents__c = [SELECT count() FROM Agent__C WHERE Ranking_Stars__C = 5 AND Zones__C =: z.Name__C];
        }

            upsert z;

    }
//Finish Method....
    public void finish(Database.BatchableContext bc) {
        Id job = bc.getJobId();
    }
}

Answer 1

You have two z variables; one inside a for loop, the other inside the class. Inside the for loop, z is a Zone__c, but outside of the loop, z is a String (line 4). Were you to remove that line, you'd get an error about z not existing. Aside from the "query-inside-a-loop" problem, you'll need to change your code so that you have a list to store the records you're processing:

Zone__c[] updates = new Zone__c[0];
for(Zone__c z: ...) {
  ...
  updates.add(z);
}
update updates;

You should be using update here, not upsert, because you're definitely modifying existing records.