Can we insert parent and child object record in single DML statement?

Try something like below: Note that reference field must be external id field on parent object. Here i have used SLASerialNumber__c  as external id field.

Contact con = new Contact(lastName='bhardwaj');
con.Email = 'naman.bhardwaj@test.com';

Account accRef = new Account(SLASerialNumber__c='12');
con.Account = accRef;

Account acc = new Account(Name='namanTestAcc', SLASerialNumber__c='12');

Database.SaveResult[] results = Database.insert(new SObject[] {acc, con});

for (Integer i = 0; i < results.size(); i++) {
if (results[i].isSuccess()) {
System.debug('Successfully Created ID: '+ results[i].getId());
}
}

Hello,

Yes, you need to make one field as an External id on your parent object.

Thanks.