NOT IN Keyword in Soql in Salesforce.

Suppose there is a requirement to fetch all accounts record which don't have any contacts,
how will we do it?

We can fetch account with their corresponding contact record in a in line query.
for example
list<Account> listOfAccounts = [SELECT id, Name,
                                                 (SELECT id,name FROM Contacts)
                                                  FROM Accounts];
list<Account> accountListwithoutContact = new list<Account>();

for(Account act :listOfAccounts ){
      if(act .Contacts !=null && act .Contacts.size() !=0){
              accountListwithoutContact.add(act)   ;
      }

}

accountListwithoutContact list will contain all account which dont have any contacts.

Second approach would be using NOT IN keyword.
list<Account> listOfAccounts = [SELECT id, Name
                                                  FROM Account
                                                  WHERE id NOT IN (SELECT AccountId FROM Contact)];
Now this is the final list which contains all accounts which don't have any contact.