How many candidate keys are possible in a table?
Every table must have at least a single candidate key. A table can have multiple candidate keys but only one primary key (the primary key cannot have a NULL value, so the candidate key with NULL value can't be the primary key).
How do I find the number of candidate keys?
A minimal super key is called as a candidate key. For any given relation, It is possible to have multiple candidate keys. There exists no general formula to find the total number of candidate keys of a given relation.
Can a relation have more than one candidate key?
In a table, we can have one or more than one candidate key, but there is one and only one primary key in a relation. However, if there is only one candidate key in a table, then that candidate key is considered as the primary key constraint.
How many candidate keys are there in R?
There are two candidate keys of R: {ABGH} and {GHIJ}.
Can a table have 2 candidate keys?
The value for the Candidate key is unique and non-null for all tuples. And every table has to have at least one Candidate key. But there can be more than one Candidate Key too.
How many types of candidate keys are there?
Eight types of key in DBMS are Super, Primary, Candidate, Alternate, Foreign, Compound, Composite, and Surrogate Key. A super key is a group of single or multiple keys which identifies rows in a table.
Can a candidate key be null?
Any attribute of Primary key can not contain NULL value. While in Candidate key any attribute can contain NULL value.
Can a relation have two foreign keys?
A table may have multiple foreign keys, and each foreign key can have a different parent table. Each foreign key is enforced independently by the database system. Therefore, cascading relationships between tables can be established using foreign keys.
Can Relations have primary keys?
A primary key is a column or a set of columns in a table whose values uniquely identify a row in the table. A relational database is designed to enforce the uniqueness of primary keys by allowing only one row with a given primary key value in a table.
Which are the candidate keys?
A candidate key is a specific type of field in a relational database that can identify each unique record independently of any other data. Experts describe a candidate key of having "no redundant attributes" and being a "minimal representation of a tuple" in a relational database table.
What are the candidate keys of R Mcq?
Keys in Relational Model MCQ Question 6 Detailed Solution Key for R is {E, F, H}. Important Points: In relation algebra, key is primary key or candidate key. {E, F, H, K, L} is super key.
Does every relation have a candidate key?
Every relation without NULL values will have at least one candidate key: Since there cannot be duplicate rows, the set of all columns is a superkey, and if that isn't minimal, some subset of that will be minimal. There is a functional dependency from the candidate key to all the attributes in the relation.
How do you find primary and candidate key?
The difference here is that a primary key acts as a minimal super key. Thus, a relation can only have one primary key. On the other hand, multiple candidate keys (two or more) can take place in any relationship. The attributes in a candidate key may contain a NULL value that opposes the primary key in it.
How do you find the candidate key in normalization?
Algorithm: Verifying That a Set of Attributes Is a Candidate KeyStep 1: Check if the given set is a superkey. Compute the closure of attributes in the set. If the closure is the set of all attributes, the set is a superkey.Step 2: Check if the superkey is minimal. Remove each attribute, one at a time.
Which are the candidate keys?
A candidate key is a specific type of field in a relational database that can identify each unique record independently of any other data. Experts describe a candidate key of having "no redundant attributes" and being a "minimal representation of a tuple" in a relational database table.
How do you find the candidate key for the functional dependency?
The candidate key can be determined from given set of functional dependency in a relation. It is an attribute or minimal set of attributes whose closure is set of all attributes in a relation. = ABCD.
What is the number of candidate keys in a relation?
No of candidate keys in a Relation are nC (floor (n/2)),for example if a Relation have 5 attributes i.e. R (A,B,C,D,E) then total no of candidate keys are 5C (floor (5/2))=10.
What is a candidate key?
Candidate Key: The minimal set of attributes that can uniquely identify a tuple is known as a candidate key. For Example, STUD_NO in STUDENT relation.
What is an alternate key?
Alternate Key: The candidate key other than the primary key is called an alternate key. For Example, STUD_NO, as well as STUD_PHONE both, are candidate keys for relation STUDENT but STUD_PHONE will be alternate key (only one out of many candidate keys).
What is a foreign key?
Foreign Key: If an attribute can only take the values which are present as values of some other attribute, it will be a foreign key to the attribute to which it refers. The relation which is being referenced is called referenced relation and the corresponding attribute is called referenced attribute and the relation which refers to ...
Is a candidate key a super key?
A candidate key is a super key but vice versa is not true. Primary Key: There can be more than one candidate key in relation out of which one can be chosen as the primary key. For Example, STUD_NO, as well as STUD_PHONE both, are candidate keys for relation STUDENT but STUD_NO can be chosen as the primary key (only one out of many candidate keys).
Can there be more than one candidate key in a relation?
There can be more than one candidate key in a relation. For Example, STUD_NO is the candidate key for relation STUDENT.
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What are the criteria for a candidate key?
Basically, a candidate key must satisfy two criteria: 1) A candidate key must be able to determine all other variables. What this basically means is that using the variables in the candidate key, you should be able to find every other variable by following the arrows in the functional dependencies. 2) A candidate key must be minimal.
What is the minimal set of attributes whose attribute closure is a set of all attributes of relation?
The minimal set of attributes whose attribute closure is a set of all attributes of relation is called the candidate key of the relation .
What is the AB key?
First, AB determines both C and D. Also, A and B obviously aren't candidate keys, so we know that AB is minimal. AB is our first candidate key.
Is BC a key?
Using the same logic, since C determines A, if we add B, we'll have AB and be able to find everything missing. Therefore BC is also a key. Again, like the other candidate keys for this exercise, BC is trivially minimal.
Is AD implicit or explicit?
A depends not on D and not on B, neither explicit nor implicit. SO AD and AB is one
Is CD a candidate key?
We could try seeing if CD is a candidate key, but logically, since C doesn't functionally determine anything by itself, D doesn't determine anything by itself, and CD doesn't determine anything, CD can't be a candidate key.
Can you have A and B in the same candidate key?
Next, let's try ABC. Since we know that we can always get B with A (from A => B), it doesn't really make sense to have A and B in the same candidate key. This is because if ABC works, then AC must work, because we can get ABC from AC.
