Hence XYZ, XY, and X are all Super Key, while the only X is a candidate key, Step 1: Let us calculate the closure of XY+ = XYZW (from the method we studied earlier), Step 2: Let us calculate the closure of Z+ = ZYWX (from the method we studied earlier), Since Z closure is determining all the attributes of the table, hence it is Super Key, Step 3: Let us calculate the closure of W+ = WX (from the method we studied earlier), Since X closure is not determining all the attributes of the table, hence it is Not Super Key, since it is not SK it can never be Candidate key. How to reset the primary key of a table in mysql? It is called a minimal superkey because we select a candidate key from a set of super key such that selected candidate key is the minimum attribute required to uniquely identify the table. The question is to calculate the candidate key and no. a) W X + = W X Y Z ( from the method we studied earlier), Since the closure of WX contains all the attributes of R, hence WX is Candidate Key, b) W Y + = W Y Z X (from the method we studied earlier), Since the closure of WX contains all the attributes of R, hence WY is Candidate Key, c) W Z + = W Z X Y (from the method we studied earlier), Since the closure of WX contains all the attributes of R, hence WZ is Candidate Key, From the definition of Candidate Key (Candidate Key is a Super Key whose no proper subset is a Super key). Create a table in MySQL and implement TIMESTAMPDIFF(). All the above keys are able to uniquely identify each row. Example: In the above example, we saw that we have two candidate keys i.e (Roll_no) and (Registration_no). ALTER TABLE to add a composite primary key in MySQL? Now there is confusion as this example is not at all similar to the above three examples. Therefore, there are TWO Candidate key Y X, Y W. Example 3: Give R( P, Q, R, S, T, U) and Set of Functional Dependency FD = { PQ → R, R → S, Q → PT}. QU+ = QUPTRS (from the method we studied earlier). JavaTpoint offers college campus training on Core Java, Advance Java, .Net, Android, Hadoop, PHP, Web Technology and Python. no matter what will be the candidate key, and how many will be the candidate key, but all will have Y compulsory attribute. From the above arrow diagram on R, we can see that an attribute W is not determined by any of the given FD, hence W will be the integral part of the Candidate key, i.e. Each relation may have one or more candidate key. Since the closure of Y contains the only Y, hence it is not a candidate key. all attribute in R is determined by anyone of the FD. The question is to calculate the candidate key and no. Now, if we know the ‘Roll_no’ of the student then there will be no confusion and we can easily select the student from here. Hence for such type of questions, we first check the closure of all attributes individually, then their combination by keeping in mind the definition of Candidate Key. Example: In the above example, we had 6 super keys but all of them cannot become a candidate key. Let us see the definition of Candidate Key again (Candidate Key is a Super Key whose no proper subset is a superkey), From the above definition, XYZ is not a candidate key, as in Step 2 and 3 we found that XY and X are also Super Key (i.e., subset of XYZ are also SK which violate the definition), XY is not a candidate key, as in Step 3 we found that X is also a Super Key (i.e., subset of XY are also SK which violate the definition). QUP, QUR, QUS, QUT, etc. X is the Candidate key: As X cannot be further subdivided, or X cannot have any subset. This is all various types of keys in DBMS. Therefore, candidates for Primary Key is called Candidate Key. As we have talked in the above step only for the super key, not for the candidate key. Difference between Primary Key and Candidate key, Difference between Super Key and Candidate key. JavaTpoint offers too many high quality services. Also, we if know the age of student ‘Andrew’ we can’t distinguish between both the students because both are having the same age. Example: If we have two tables of Student and Course then we can establish a relationship between these two tables using a foreign key. Like super key, a candidate key also identifies each tuple in a table uniquely. How to implement CASCADE rule for the foreign key TRANSACTION_ID defined on the ORDERS table and references to TRANSACTIONS table? The primary key is the minimal set of attributes which uniquely identifies any row of a table. It depends upon our requirement. Hence XY and Z are Super Key, also XY and Z are a candidate key, Step 1: Let us calculate the closure of Y+ = XYZW (from the method we studied earlier), Since Y closure is determining all the attributes of the table, hence it is Super Key, Step 2: Let us calculate the closure of XZW+ = XZWY (from the method we studied earlier), Since a closure is determining all the attributes of the table, hence it is Super Key. Example: In the given Student Table we can have the following keys as the super key. of candidate key in above relation R using a given set of FDs. Here, if we are talking about class then selecting ‘Roll_no’ as the primary key is more logical instead of ‘Registrartion_no’. In the above example, Course_id is not a primary key in the Student table but it is a primary key in the Course table. It also helps in establishing relationship among tables. That’s the reason they are also termed as minimal super key. of candidate key in above relation R using a given set of FDs. Candidate keys are selected from the set of super keys, the only thing we take care while selecting candidate key is that the candidate key should not have any redundant attributes. A table is made up of rows and columns. When we need the data of any column then we can directly access it by the name of the column i.e attribute. From above arrow diagram on R, we can see that an attribute QU is not determined by any of the given FD, hence QU will be the integral part of the Candidate key, i.e. Candidate Key. We will now see how this is done with the help of examples. suppose if ABC is a candidate key then neither A, B, C or any of its combination can be super key, hence we can say candidate key is a minimal set of attributes of an R( Relational Schema) which can be used to identify a tuple of a table uniquely.. OR We have the name of every column so we can easily select any column but rows don't have any specific name. All the candidate key which are not a primary key are called an alternate key. The foreign key of a table is the attribute which establishes the relationship among tables. A super key or simply key is a combination of all possible attribute which can uniquely identify the rows(tuples) in a table. The attributes of Candidate key is called prime attributes. Let us check the combination of Y, i.e. all can act as the nominee of Primary key, hence they all are candidate key. The primary key cannot have a NULL value. The question is to calculate the candidate key and no. We can say that ‘Roll_no’ is the key here. no matter what will be the candidate key, and how many will be the candidate key, but all will have W compulsory attribute. Candidate Key Example. Y+ = Y( from the method we studied earlier). W + = W (from the method we studied earlier). Candidate Key: A candidate key is a set of attributes (or attribute) which uniquely identify the tuples in relation or table. The ‘Roll_no’ attribute will help us in uniquely identifying the rows in a table. So, let's get started. Provided, that the key attribute values must be unique and does not contain NULL. The ‘Course_id’ in the Student table is the foreign key as it establishes the link between the Student and Course Table. Example 4: Give R(A, B, C, D) and Set of Functional Dependency FD = { AB → CD, C → A, D → B}. Therefore, there are THREE Candidate keys WX, WY, WZ. d) Y X + = Y X Z W (from the method we studied earlier), Since the closure of YX contains all the attributes of R, hence YX is Candidate Key, e) Y W + = Y W X W (from the method we studied earlier), Since the closure of YW contains all the attributes of R, hence YW is Candidate Key, f) Y Z + = Y Z (from the method we studied earlier), Since the closure of Y Z contains only Y Z, hence YZ is Not Candidate Key. So, if we need to find the information about any course opted by any student then we can go the Course table using the foreign key. A + = A (from the method we studied earlier), B + = B (from the method we studied earlier), C + = C A (from the method we studied earlier), D + = D B (from the method we studied earlier), A B + = A B C D (from the method we studied earlier), A C + = A C (from the method we studied earlier), A D+ = A D B C (from the method we studied earlier), B C + = B C A D (from the method we studied earlier), B D + = B D (from the method we studied earlier). How to get primary key of a table in MySQL? of candidate key in above relation R using a given set of FDs. From the definition of Candidate Key(Candidate Key is a Super Key whose no proper subset is a Super key). A candidate key is a minimal super key or a super key with no redundant attribute. How to add a specific character to any empty space in MySQL table values? Duration: 1 week to 2 week. From the above definition, Y is candidate key, As Y cannot be further subdivided, or Y cannot have any subset. Each candidate key qualifies for Primary Key. So, from the above discussion, we conclude that we can have only 2 out of above 6 super keys as the candidate key. From the above definition XY is a candidate key, as in Step 2 and 3 none of the subsets of XY i.e. All rights reserved. If we are able to do this then we can say that we can access the table and the hint which will help us in doing so is the key. WX, WY, WZ. Candidate key’s attributes can contain NULL value which oppose to the primary key. These keys would qualify for candidate key as in the below syntax − Therefore, candidates for Primary Key is called Candidate Key. Candidate Keys are not allowed to have NULL values. Given a Relational Schema R(X, Y, Z, W) in the following questions determine Super Key and Candidate key.