i want to retrieve the attachments of SObject Account and also i was created a custom field(checkbox) named as is_read__c. Now, using SOQL is it possible to get attachments of Account from attachment SObject where condition is is_read__c = true
You can query for Accounts and Attachments in one Parent-Child SOQL Query.
List<Account> accounts = [Select Id, is_read__c, (Select Id from Attachments) from Account where is_read__c = true];
As commentators mentioned above you cannot add custom fields to the Attachment object.