A candidate key chosen by the database designer to act as key is the primary key. An entity type may have more than one candidate keys, in that case the database designer has to designate one of them as primary key, since there is always only a single primary key in an entity type. The primary key can also be defined as the successful candidate key. (Page 82)
A relation is in first normal form if and only if every attribute is single valued for each tuple. This means that each attribute in each row , or each cell of the table, contains only one value. No repeating fields or groups are allowed. An alternative way of describing first normal form is to say that the domains of attributes of a relation are atomic, that is they consist of single units that cannot be broken down further. There is no multivalued (repeating group) in the relation multiple values create problems in performing operations like select or join. For Example there is a relation of Student
it means that when writer is writing something he or she should think who his reader is. What they want to read. The piece of writing should not be too hard or complicated for his readers to understand. It should have verity and interesting elements to keep the reader captivated.
1. Making only minimum wage, I nearly earned $2,000 last summer (Misplaced phrases)2. The bus station was located by a river which was made of red brick (Misplaced words)3. A fish was found in the Pacific Ocean that had been considered extinct (Misplaced clauses)