How to export data in .csv by using apex code?

public class Cls_RecordsAsCSVExporter {
    public static void mainmethod(){
        List<Account> extractAccountMemList = new List<Account>();
        List<Account> AccList = [select id,name,(select id from contacts) from Account];
        for(Account acc: AccList){
            List<contact> conlist = acc.contacts;
            if(conlist.size() == 0){
                extractAccountMemList.add(acc);
            }
        }
        String generatedCSVFile ='';
        String fileRow ='';
        for(Account acc: extractAccountMemList){
            fileRow ='';
            fileRow = fileRow +','+ acc.Id;
            fileRow = fileRow +','+ acc.Name;
            fileRow = fileRow.replaceFirst(',','');
            generatedCSVFile = generatedCSVFile + fileRow + '\n';
        }
        Messaging.EmailFileAttachment csvAttachment = new Messaging.EmailFileAttachment();
        Blob csvBlob = blob.valueOf(generatedCSVFile);
        String csvName = 'acc details which doesn ot have contacts.csv';
        csvAttachment.setFileName(csvName);
        csvAttachment.setBody(csvBlob);
        Messaging.SingleEmailMessage email = new Messaging.SingleEmailMessage();
        String[] toAddresses = new String[]{'XXXXXXXXX@gmail.com'};
            String subject = 'acc details which does not have contacts CSV';
        email.setSubject(subject);
        email.setToAddresses(toAddresses);
        email.setPlainTextBody('acc details which does not have contacts CSV');
        email.setFileAttachments(new Messaging.EmailFileAttachment[]{csvAttachment});
        Messaging.SendEmailResult[] r = Messaging.sendEmail(new Messaging.SingleEmailMessage[]{email});
    }
}

Open Developer Console and call the mainmethod() it will send all your Accounts which don't have Contacts to mail id which is mentioned in the program.