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Types of Keys in DBMS

26th Apr, 2024
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    Types of Keys in DBMS

    In the real world, there is a vast amount of data available. Now, many tables are needed for the DBMS to store the data. There could be thousands of duplicates, sorted, and unsorted records in these tables. Now, it is a very challenging process to retrieve any unique or specific record from these tables without any constraints or restrictions.

    A new Keys concept emerged to solve all the problems. Keys can ensure that there are no duplicate records in any rows or tuples. Let's examine the DBMS idea of keys.

    The database management system keys, also known as DBMS keys, represent one or more attributes from any table in the database system that allows for the distinct categorization of a row and/or combination of more than one column, identification of the relationship between the tuple (row) in the table, or identification of the relationship between the two tables. This applies to the tables identified and queried for analysis or report.

    What are the Keys in DBMS?

    In DBMS, KEYS is an attribute or group of characteristics that aid in the identification of a relational row (tuple). You can use them to discover the connection between two tables. By combining one or more of the table's columns, keys enable you to identify each row in the table uniquely. The Key can aid in finding a certain record or row in the table. The database key is useful for locating a specific record or row in a table.

    A set of properties can uniquely identify a tuple (or row) in a relationship called a key in a database management system (DBMS). A relational database's many tables and columns can be related to one another via keys. Key values are the specific values contained within a key. Visit our MongoDB Certification to gain a competitive advantage.

    Consider an example of an educational institution that uses each individual identification number to identify applicants. These can also be used to locate all the candidate-specific information that is kept on the server, such as their address, passport number, or phone number. Keys are essential for classifying and classifying different sorts of data.

    DBMS keys are employed for other purposes as well. We'll soon examine several DBMS key types and their additional functionality.

    Additionally, we can use a single or collection of qualities as a key to identify entries in a certain way. The use of keys is lost even if we designate all the attributes as keys to identify the records uniquely. Because it uses fewer combinations of attributes to achieve the same outcome than other potential keys, it is less effective.

    Why do we Need Keys in DBMS?

    You can even apply this to your surroundings in a real-world application by stating that we can't fit everything into a single table or box. Things get more complicated when we realize that even though they are separated, they still share some common relationships, necessitating their separation into distinct tables. Don't worry; let's use another analogy to comprehend this. 

    Consider a User and a Pizza Shop Ledger as two entities for simplicity. In the user table, we want to keep track of every user, and in the ledger table, we want to keep track of every transaction detail. Each user has a unique username, address, s.no, and phone number. In contrast, the ledger holds data on each user together with the pizza that they purchased from the store.

    Each record in the ledger has all the data pertaining to the user mentioned above as well as all the data pertaining to pizza, such as pizza name, price, toppings, size, and type (vegetarian/non-vegetarian). However, as you may have seen, there is a slight issue. We duplicate all user data into the ledger, which adds unnecessary redundancy. However, we are unable to remove it since we require a means of connecting each item (in this case, pizza) to the user. We cannot know who purchased what, after all.

    User Table


    Pizza Ledger Table


    You can see from the table above how we write the same user data twice without needing to. Even information about pizza is unnecessary.

    In this database, one can search for information unique to users, specific to pizzas, or even information connected to both, such as retrieving a list of all users who have purchased any pizza. We also need keys in the database to retrieve the results.

    Therefore, DBMS keys come to the rescue. Table associations are also established using DBMS keys. To eliminate this repetition, we can utilize a certain kind of Key.

    To summarize the main need for keys in DBMS, here are some reasons:

    • Any row of data in a table can be found using keys. A table could have thousands of records in an actual application. The records can also be duplicates. Despite these difficulties, keys in RDBMS ensure that a table record can be uniquely identified.
    • Allows you to determine the relationship between tables and construct a relationship between them.
    • Assist you in maintaining the relationship's integrity and sense of self.

    Types of Keys in DBMS

    types of keys in DBMS

    The following are the types of keys in the database:

    1. Primary key

    A table's primary Key is a column or group of columns that aids in uniquely identifying each entry in the table. In a table, there can only be one primary Key. Additionally, the primary Key cannot have any row of the same values repeated. The primary Key's values must all be unique and include no duplicates. To grasp what keys are and what is a primary key in DBMS, a primary key is the most important feature among many others.

    A primary key cannot contain duplicates, meaning the table cannot have two instances of the same data. A key can become the Primary Key by following a few guidelines. These are:

    • A value must be present in the Primary Key column, and the Primary Key field cannot be left NULL.
    • No two rows in the table may have the same values in that particular column.
    • No value can be changed or modified in this primary key column if a foreign key in DBMS references the main Key.


    In the following example, StudID is a primary key.

    StudIDRoll NoFirst NameLastNameEmail

    2. Super Key

    A super key is either a single key or a group of keys that aid in distinguishing between different rows in a given table. For distinct identification, a super key in DBMS may include additional qualities that are redundant.


    Let's examine a situation where the EmpId and the mobile number could be regarded as Super Keys.

    Mobile No.EmpIdEmpname

    3. Candidate Key

    A property that identifies a row in a table in a unique way is a candidate key in the database. One of the potential keys is chosen to be a table's primary Key. Therefore, candidate keys share the same characteristics as the primary keys that were described earlier. A table may include multiple candidate keys.

    For any table, there may be more candidate keys than one, but they may never be empty. Each candidate key in DBMS has certain data and values. In addition to these traits, a combination of qualities also functions as a collection of potential keys.


    In the provided table, the candidate keys Stud ID, Roll No., and Email allow us to identify each student record individually.

    StudIDRoll NoFirst NameLastNameEmail

    4. Alternate Key

    There may be more than one possibility for a key to be chosen as the primary Key in a table. An Alternate Key is any key that has the potential to become the Primary Key but is not now the Primary Key. It is a potential primary key that has not yet been chosen.


    StudID, Roll No., and Email are eligible to serve as the main keys in this table. However, because StudID is the main Key, Roll number and Email are now DBMS's secondary Key.

    StudIDRoll NoFirst NameLastNameEmail

    5. Foreign Key

    Relationships between two tables are created using foreign keys. Each value in a column or collection of columns containing a foreign key in the database must match the Primary Key of the referential table. Data and referential integrity are preserved with the use of foreign keys.

    Foreign keys are crucial for two entities that might be connected by the same information but do not share identifiable information to remain distinct. The tables are connected to keep a connection but do not totally replace one another.



    Teacher IDFnameLname

    We have two tables, teacher and department in a school in this Key in the DBMS example. It is impossible to tell which teacher works in which department, though. 

    We may establish a connection between the two tables in this table by adding the Foreign Key in Deptcode to the Teacher name. 

    Teacher IDDeptCodeFnameLname

    6. Composite Key

    A composite key in the database, which unambiguously identifies rows in a table, combines two or more columns. While individual uniqueness is not guaranteed, the combination of columns assures it. In order to uniquely identify records in a table, they are grouped together. 

    Compound keys and composite keys differ in that any component of a compound key is a foreign key, whereas a component of a composite key may or may not be a foreign key. 



    We discovered from the table that there isn't a single property that can serve as a primary key and uniquely identify a record in the database. However, some properties can be combined to provide a key that can be used to locate a tuple in the table. Cust Id and Prod code can work as a primary key in the database in the example above since they can identify a tuple when used in conjunction rather than separately.

    {Cust_Id, Prod_code} is the composite Key for the table.

    7. Unique Key

    Unique and primary keys appear to be very similar, yet they are completely different and have different purposes. A column's value should be unique once an attribute has been specified as a unique key, albeit only ONE value may be null. If you need each user's name to be distinct for any reason, you can use this Key. 

    Additionally, unlike the primary Key, a unique key in DBMS can be used for numerous qualities as opposed to only one. Additionally, we have the ability to change the unique key attribute, which is not available when using the main Key.


    Id (primary Key)NameAge (unique Key)

    We assign a unique key to the Age attribute, preventing any values from duplicating and allowing one value to be null.

    Keys are imperative for analyzing and identifying data types. You can also consider doing our Full Stack Developer Course.  

    Types of Functional Dependencies in DBMS

    types of functional dependencies


    A constraint that describes the relationship between two sets of attributes in which one set reliably predicts the value of the other sets is known as a functional dependency.

    It is represented as X -> Y, where X represents a collection of characteristics that can be used to calculate the value of Y. Determinant refers to the attribute set on the left side of the arrow, X. In contrast, Dependent refers to the attribute set on the right side, Y. 



    1. Trivial Functional Dependency

    A dependent is trivial functional dependence that is invariably a subset of the determinant.

    For example, if X->Y and Y are a subset of X, this is referred to as a trivial functional dependency.

    Here, {roll_no, name} → name is a trivial functional dependency since the dependent name is a subset of determinant set {roll_no, name}.


    2. Non-trivial Functional Dependency

    The Dependent in a non-trivial functional dependency is categorically not a subset of the determinant. 

    If X->Y and Y are not a subset of X, the relationship is said to be non-trivial functional.


    Given {roll_no} → name that the dependant, name, is not a subset of the determinant roll_no, the functional dependency {roll_no} -> name, in this case, is non-trivial.

    Similarly, because age is not a subset of "roll_no, name," it also has a non-trivial functional dependency.

    3. Multivalued Functional Dependency

    Entities in the dependent set are not reliant on one another in multivalued functional dependency. 

    if a-> {b, c} and b and c are not functionally dependent on one another, this is referred to as a multivalued functional dependency. 



    Consider roll_no → {name, age}, since the dependents name & age are not dependent on one another (i.e., name ->age or age -> name doesn't exist!), roll_no-> {name, age} is a multivalued functional dependence in this case. 

    4. Transitive Functional Dependency

    Dependent is indirectly dependent on determinants in transitive functional dependency.

    if a->b & b->c, then a->c according to the transitivity principle. A transitive functional dependency exists here.



    Here, enrol_no ->  building_no is a legitimate functional dependency in this situation since enrol_no -> dept and dept ->  building_no. It is referred to as a transitive functional dependency because it is an indirect functional dependency. 


    We have learned about different types of keys in DBMS and checked out types of functional dependencies in DBMS. 

    All keys in DBMS do serve as a means of uniquely identifying each entry in a table. Discovered the super Key in the database and the method for locating candidate keys from it. A primary key is selected from a group of candidate keys, and all other keys in DBMS are designated as alternate keys in DBMS. A primary key is a composite key with two or more properties, and a unique key makes each attribute's value distinct (accepting null as a value when necessary).

    A composite key in DBMS has numerous properties that uniquely identify rows in a table.

    If you want to improve your DBMS skills quickly, kindly check out Knowledgehut MongoDB Certification.

    Frequently Asked Questions (FAQs)

    1How to find candidate keys and super keys using attribute closure? 

    Only once all the qualities not necessary for its uniqueness are removed from a super key does it become a primary or candidate key. Therefore, while all primary/candidate keys are part of a super key, not all super keys are primary/candidate keys. The tuples of a relation are all known to be unique by the formal definition of a Relation(Table). As a result, the collection of all qualities is a super key. 

    2What is a simple key in DBMS?

    To uniquely identify a record, a basic key just has one field. Furthermore, the field cannot be divided into smaller fields. For instance, studentId is a single field that serves as a simple key because it uniquely identifies a particular student. 

    3What are the 5 types of primary keys?

    There are two different categories of primary keys. A table's explicit primary key only applies to that table. Any table with the same base name, column names, and attribute specifications but different Creator IDs must have a Generic primary key. 

    4What is a unique key DBMS?

    A unique key in relational database management systems is a potential key that is not the relation's primary key. All candidate keys for a relation can each uniquely identify its records, but only one of them is used as the relation's primary key.


    Monica Gupta


    I am Monica Gupta with 19+ years of experience in the field of Training and Development. I have done over 500 Corporate Trainings. I am currently working as a freelancer for several years. My core area of work is Java, C++, Angular, PHP, Python, VBA.

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