By Neeraj Sharma, Liviu Perniu, Raul F. Chong, Abhishek Iyer
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Which of the following are relation properties? A. The primary key can’t have null values. B. There aren’t duplicate tuples in a relation. C. Attributes have atomic values. D. There are duplicate tuples in a relation. E. None of the above 8. A relation can have: A. A domain. B. An instance. C. A value. D. A degree. E. None of the above 9. Which of the following statements is true? A. A primary key is also a candidate key. B. Each relation has at least one foreign key. C. Foreign keys can’t have null values.
Relations possess certain important properties. These properties are all of them consequences of the relation definition given above. The four properties are as follows: There are no duplicate tuples in a relation Tuples are unordered (top to bottom) Attributes are unordered (left to right) All attribute values are atomic Informal terms used for relations are table or data file. 5 Schemas A database schema is a formal description of all the database relations and all the relationships existing between them.
Those rules are general, specified at the database schema level, and they must be respected by each schema instance. 2]. If a user attempts to execute an operation that would violate the constraint then the system must then either reject the operation or in more complicated situations, perform some compensating action on some other part of the database. This would ensure that the overall result is still in a correct state. Now, let’s see what the relational data model constraints are. 1 Entity integrity constraint The entity integrity constraint says that no attribute participating in the primary key of a relation is allowed to accept null values.