We can find keys (candidate keys) for a relation by finding the closure of an/set of attributes . Checking each attribute or all subsets of the given set of attributes for a key is time consuming job.
Full Answer
What is the candidate key of a 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. 1. Set K:= R; 2. For each attribute A in K { Compute (K - A)+ with respect to F; If (K - A)+ contains all the attributes in R, then set K := K - {A}; }
How to find all candidate keys of R?
Given the following relation R and the set of functional dependencies F that hold on R, find all candidate keys for R. Let us follow the steps given in the box below to find all the candidate keys of R. Step 1: Let γ = set of attributes not present in the RHS of any FD. The set of attributes in γ must be a part of any candidate key of R.
How to find total number of candidate keys of a relation?
There exists no general formula to find the total number of candidate keys of a given relation. These are candidate keys because each set consists of minimal attributes required to identify each student uniquely in the Student table. We can determine the candidate keys of a given relation using the following steps-
How to find the key of a relation in DBMS?
Given a schema R ( A, B, C, D, E), and the following set of FDs: {A→ E, E → CD, BC → A, D → B} To find the key of a relation, we need to find the closure of attributes. If any attribute’s or set of attributes’ closure gives all the attributes of the relation, then we would say that attribute/set of attributes as the key for that relation.
How do you find the candidate key of a relation?
SHORTCUT TO FIND CANDIDATE KEY. Example 1: Give R(X, Y, Z, W) and Set of Functional Dependency FD = { X → Y, Y → Z, Z → X}. The question is to calculate the candidate key and no. of candidate key in above relation R using a given set of FDs.
What is a candidate key in a relation?
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. It is a minimal super key. It is a super key with no repeated data is called a candidate key.
How many candidate keys in this relation are?
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 do you find the candidate key in a table?
2:413:59Finding Candidate Key | Database Management System - YouTubeYouTubeStart of suggested clipEnd of suggested clipSo we can determine C now from C we can determine D. So we have D. Now from D from this D we canMoreSo we can determine C now from C we can determine D. So we have D. Now from D from this D we can determine B. So B is also there. And from this e we can determine F. So f is also there okay. So if you
Can a relation have no 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.
Is candidate key and primary key same?
No. Primary Key is a unique and non-null key which identify a record uniquely in table. A table can have only one primary key. Candidate key is also a unique key to identify a record uniquely in a table but a table can have multiple candidate keys.
How do I find my candidate key and closure?
we can determine candidate key using below steps:Find out attribute closure for each of the determinant. ... Check which determinant closure derive all the attributes of the relation.Declare all the determinant whose closure set contains all the attributes, as candidate key.More items...
Which is candidate key?
Candidate key is a single key or a group of multiple keys that uniquely identify rows in a table. A Candidate key is a subset of Super keys and is devoid of any unnecessary attributes that are not important for uniquely identifying tuples. The value for the Candidate key is unique and non-null for all tuples.
How many candidate keys can a table have?
One table can only contain a single primary key. It is also a unique type of key that helps in identifying any record on the table uniquely. The difference is that any table can have two or more candidate keys.
How do I find candidate keys and Superkeys?
In general, if we have 'N' attributes with one candidate key then the number of possible superkeys is 2(N – 1). Example-2 : Let a Relation R have attributes {a1, a2, a3,…,an}. Find Super key of R. Maximum Super keys = 2n – 1.
What is candidate key in database design?
What Does Candidate Key Mean? 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 identify a key in DBMS?
A key refers to an attribute/a set of attributes that help us identify a row (or tuple) uniquely in a table (or relation)....Keys are of seven broad types in DBMS:Candidate Key.Primary Key.Foreign Key.Super Key.Alternate Key.Composite Key.Unique Key.
What is candidate key in 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.
What is surrogate key example?
Some examples of Surrogate key are : System date & time stamp. Random alphanumeric string.
What is a candidate key quizlet?
Candidate Key. A candidate key is a column, or set of columns, in a table that can uniquely identify any database record without referring to any other data. Each table may have one or more candidate keys, but one candidate key is unique, and it is called the primary key.
What is super key and candidate key?
Super Key is a set of attributes or columns that uniquely identifies each row table whereas, a candidate key is a set of attributes that recognizes the tuples in a relation, or table.
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 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 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.
Can you find every variable from ABCDE?
For example, we can find every variable from ABCDE, because ACBDE is every variable already. However, we could take some variables off. For example, it can be found that ACD is a candidate key. Since ACD is a valid candidate key, and ACD is a subset of ABCDE, we know that ABCDE can't be a candidate key.
