※ Download: What is a candidate key
There is a fundamental difficulty for efficient algorithms for candidate key computation: Certain sets of functional dependencies lead to exponentially many candidate keys. But still there can be a narrow chance of duplicate row. Every table must have at least one candidate key but at the same time can have several. It is also a great example of a super key with multiple unique identifiers.
The values placed in primary key columns must be unique for each row: no duplicates can be tolerated. Each table of a database can have one or more than one candidate keys. If the primary key is on both RegistrationNo and RollNo then it represents a composite key.
- The set of all candidate keys can be computed e.
This article needs additional citations for. Unsourced material may be challenged and removed. Candidate keys are also variously referred to as primary keys, secondary keys or alternate keys. The constituent attributes are called prime attributes. Conversely, an attribute that does not occur in ANY candidate key is called a non-prime attribute. Since a relation contains no duplicate tuples, the set of all its attributes is a superkey if NULL values are not used. It follows that every relation will have at least one candidate key. The candidate keys of a relation tell us all the possible ways we can identify its tuples. As such they are an important concept for the design of. The definition of candidate keys can be illustrated with the following abstract example. Consider a relation variable relvar R with attributes A, B, C, D that has only the following two legal values r1 and r2: r1 A B C D a1 b1 c1 d1 a1 b2 c2 d1 a2 b1 c2 d1 r2 A B C D a1 b1 c1 d1 a1 b2 c2 d1 a1 b1 c2 d2 Here r2 differs from r1 only in the A and D values of the last tuple. For r1 the following sets have the uniqueness property, i. We have to consider all the relations that might be assigned to a relvar to determine whether a certain set of attributes is a candidate key. For example, if we had considered only r1 then we would have concluded that A,B is a candidate key, which is incorrect. However, we might be able to conclude from such a relation that a certain set is not a candidate key, because that set does not have the uniqueness property example A,D for r1. Note that the existence of a proper subset of a set that has the uniqueness property cannot in general be used as evidence that the superset is not a candidate key. In particular, note that in the case of an empty relation, every subset of the heading has the uniqueness property, including the empty set. The set of all candidate keys can be computed e. It is quite simple to find a single candidate key. If after removing an attribute the attribute closure stays the same, then this attribute is not necessary and we can remove it permanently. Actually we can detect every candidate key with this procedure by simply trying every possible order of removing attributes. However there are many more of attributes n! That is, many attribute orders will lead to the same candidate key. There is a fundamental difficulty for efficient algorithms for candidate key computation: Certain sets of functional dependencies lead to exponentially many candidate keys. That is, the best we can expect is an algorithm that is efficient with respect to the number of candidate keys. It may however be covered by other already known candidate keys. The algorithm checks this case using the 'found' variable. If not, then minimizing the new key yields a new candidate key. The key insight is that all candidate keys can be created this way.
Key not selected as primary key is called ………. In simple words we can understand that such type of keys which full fill all the requirements of primary key which is not null and have unique records is a candidate for primary key. That is, many attribute orders will lead to the same candidate key. Declaring a primary key lets the DBMS help you enforce the data rules. Business will never ask to change the surrogate key because this value is not shared with business. This isn't an absolute rule; it is simply advice. A primary key is always one of the candidate keys. A set of candidate keys can be created by using functional dependencies. This can raise problems.
