3.4 what is the difference between a key and a superkey

Is a primary key a candidate key? Candidate Key — A Candidate Key can be any column or a combination of columns that can qualify as a unique key in the database. Each Candidate Key can qualify as a Primary Key. Primary Key — A Primary Key is a column or a combination of columns that uniquely identify a record. Only one Candidate Key can be Primary Key.

What is a candidate key in DBMS? 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 a super candidate? A superkey is a set of attributes within a table whose values can be used to uniquely identify a tuple. A candidate key is a minimal set of attributes necessary to identify a tuple; this is also called a minimal superkey.

In a real database we do not need values for all of those attributes to identify a tuple. What is foreign key in DBMS? Definition: Foreign keys are the columns of a table that points to the primary key of another table.

They act as a cross-reference between tables. How do you find the primary key? Primary Keys The primary key consists of one or more columns whose data contained within is used to uniquely identify each row in the table.

You can think of them as an address. If the rows in a table were mailboxes, then the primary key would be the listing of street addresses. Can a candidate key be null? If a column part of candidate keys contain one null value, then that column will remain as part of candidate key, but not be considered as primary key.

How does Bcnf decompose? How many Super keys are possible? What is super key in DBMS? Definition of Super Key in DBMS: A super key is a set of one or more attributes columns , which can uniquely identify a row in a table.

What is secondary key? Secondary Key is the key that has not been selected to be the primary key. However, it is considered a candidate key for the primary key.

It is also known s control key. A superkey is a combination of attributes that can be uniquely used to identify a database record. A table might have many superkeys. Candidate keys are a special subset of superkeys that do not have any extraneous information in them. This table has many possible superkeys. Foreign Key A foreign key is an attribute or combination of attribute in a relation whose value match a primary key in another relation. The table in which foreign key is created is called as dependent table.

The table to which foreign key is refers is known as parent table. I have always found it difficult to remember all the keys; so I keep the below notes handy, hope they help someone! Let me know if it can be improved. Foreign Key : FK is a key in one table child that uniquely identifies a row of another table parent.

A FK is not-unique in the child table. It is a candidate key in the parent table. Artificial key generated internally in OLAP. SuperKey : A key that can be uniquely used to identify a database record, that may contain extra attributes that are not necessary to uniquely identify records. Candidate Key : A candidate key can be uniquely used to identify a database record without any extraneous data. They are Not Null and unique. It is a minimal super-key.

Alternate Key : A candidate key that is not the primary key is called an alternate key. So becomes the extraneous data. Below I have tried to use set theory to simplify the representation of the membership of the keys w. Notes: an-overview-of-the-database-keys-primary-key-composite-key-surrogate-key-et-al. Largely based on the accepted answer, but with a few tweaks to fit better the definitions taught in some courses:. Superkey - An attribute or set of attributes that uniquely defines a tuple within a relation.

However, a superkey may contain additional attributes that are not necessary for unique identification. Candidate key - A superkey such that no proper subset is a superkey within the relation. Primary key - The candidate key chosen to identify tuples uniquely within the relation.

Foreign key - An attribute or set of attributes within a relation that matches the candidate key of some relation. Candidate Key : The candidate key can be defined as minimal set of attribute which can uniquely identify a tuple is known as candidate key.

Super Key : The set of attributes which can uniquely identify a tuple is known as Super Key. Adding zero or more attributes to candidate key generates 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 a relation out of which one can be chosen as primary key. Alternate Key : The candidate key other than primary key is called as alternate key. Foreign Key : If an attribute can only take the values which are present as values of some other attribute, it will be foreign key to the attribute to which it refers. The relation which is being referenced is called referenced relation and corresponding attribute is called referenced attribute and the relation which refers to referenced relation is called referencing relation and corresponding attribute is called referencing attribute.

Referenced attribute of referencing attribute should be primary key. Primary key is a subset of super key. Which is uniquely define and other field are depend on it. In a table their can be just one primary key and rest sub set are candidate key or alternate keys. Super Key : Super key is a set of one or more attributes whose values identify tuple in the relation uniquely.

Candidate Key : Candidate key can be defined as a minimal subset of super key. In some cases , candidate key can not alone since there is alone one attribute is the minimal subset. Here Candidate key is id, ssn because we can easily identify the tuple using either id or ssn. Althrough, minimal subset of super key is either id or ssn. Stack Overflow for Teams — Collaborate and share knowledge with a private group.

Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. Differences between key, superkey, minimal superkey, candidate key and primary key Ask Question. Asked 10 years, 3 months ago. Active 2 years, 1 month ago. Viewed k times. Improve this question. Aaron Aaron 1, 6 6 gold badges 27 27 silver badges 43 43 bronze badges.

Add a comment. Active Oldest Votes. Here I copy paste some of the information that I have collected Key A key is a single or combination of multiple fields. Types of Keys Primary Key The attribute or combination of attributes that uniquely identifies a row or record in a relation is known as primary key.

Improve this answer. Informally, a superkey is a set of attributes within a table whose values can be used to uniquely identify a tuple. A candidate key is a minimal set of attributes necessary to identify a tuple, this is also called a minimal superkey. There is no difference between a candidate key and a primary key. They are the same thing. All keys are superkeys because a key AKA "candidate" key or "primary" key is by definition just a minimal superkey.