What are the different types of keys in RDBMS? Please include examples with your answer.
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Candidate keys are those keys which is candidate for primary key of a table. In simple words we can understand that such type of keys which full fill all the requirements of primary key which is not null and have unique records is a candidate for primary key. So thus type of key is known as candidate key. Every table must have at least one candidate key but at the same time can have several.
Such type of candidate key which is chosen as a primary key for table is known as primary key. Primary keys are used to identify tables. There is only one primary key per table. In SQL Server when we create primary key to any table then a clustered index is automatically created to that column.
Foreign key are those keys which is used to define relationship between two tables. When we want to implement relationship between two tables then we use concept of foreign key. It is also known as referential integrity. We can create more than one foreign key per table. Foreign key is generally a primary key from one table that appears as a field in another where the first table has a relationship to the second. In other words, if we had a table A with a primary key X that linked to a table B where X was a field in B, then X would be a foreign key in B.
If any table have more than one candidate key, then after choosing primary key from those candidate key, rest of candidate keys are known as an alternate key of that table. Like here we can take a very simple example to understand the concept of alternate key. Suppose we have a table named Employee which has two columns EmpID and EmpMail, both have not null attributes and unique value. So both columns are treated as candidate key. Now we make EmpID as a primary key to that table then EmpMail is known as alternate key.
When we create keys on more than one column then that key is known as composite key. Like here we can take an example to understand this feature. I have a table Student which has two columns Sid and SrefNo and we make primary key on these two column. Then this key is known as composite key.
A natural key is one or more existing data attributes that are unique to the business concept. For the Customer table there was two candidate keys, in this case CustomerNumber and SocialSecurityNumber. Link http://www.agiledata.org/essays/keys.html
Introduce a new column, called a surrogate key, which is a key that has no business meaning. An example of which is the AddressID column of the Address table in Figure 1. Addresses don't have an "easy" natural key because you would need to use all of the columns of the Address table to form a key for itself (you might be able to get away with just the combination of Street and ZipCode depending on your problem domain), therefore introducing a surrogate key is a much better option in this case. Link http://www.agiledata.org/essays/keys.html
A unique key is a superkey--that is, in the relational model of database organization, a set of attributes of a relation variable for which it holds that in all relations assigned to that variable, there are no two distinct tuples (rows) that have the same values for the attributes in this set
When more than one column is combined to form a unique key, their combined value is used to access each row and maintain uniqueness. These keys are referred to as aggregate or compound keys. Values are not combined, they are compared using their data types.
Simple key made from only one attribute.
A superkey is defined in the relational model as a set of attributes of a relation variable (relvar) for which it holds that in all relations assigned to that variable there are no two distinct tuples (rows) that have the same values for the attributes in this set. Equivalently a super key can also be defined as a set of attributes of a relvar upon which all attributes of the relvar are functionally dependent.
It is a set of attributes that can uniquely identify weak entities and that are related to same owner entity. It is sometime called as Discriminator.
Partial Key:
It is a set of attributes that can uniquely identify weak entities and that are related to same owner entity. It is sometime called as Discriminator.
Alternate Key:
All Candidate Keys excluding the Primary Key are known as Alternate Keys.
Artificial Key:
If no obvious key, either stand alone or compound is available, then the last resort is to simply create a key, by assigning a unique number to each record or occurrence. Then this is known as developing an artificial key.
Compound Key:
If no single data element uniquely identifies occurrences within a construct, then combining multiple elements to create a unique identifier for the construct is known as creating a compound key.
Natural Key:
When one of the data elements stored within a construct is utilized as the primary key, then it is called the natural key.
Ólafur forgot the surrogate key:
A surrogate key in a database is a unique identifier for either an entity in the modeled world or an object in the database. The surrogate key is not derived from application data.
There also exists a UNIQUE KEY. The main difference between PRIMARY KEY and UNIQUE KEY is that the PRIMARY KEY never takes NULL value while a UNIQUE KEY may take NULL value. Also, there can be only one PRIMARY KEY in a table while UNIQUE KEY may be more than one.
There is also a SURROGATE KEY: it occurs if one non prime attribute depends on another non prime attribute. that time you don't now to choose which key as primary key to split up your table. In that case use a surrogate key instead of a primary key. Usually this key is system defined and always have numeric values and its value often automatically incremented for new rows. Eg : ms acces = auto number & my SQL = identity column & oracle = sequence.
(I) Super Key – An attribute or a combination of attribute that is used to identify the records uniquely is known as Super Key. A table can have many Super Keys.
E.g. of Super Key
So on as any combination which can identify the records uniquely will be a Super Key.
(II) Candidate Key – It can be defined as minimal Super Key or irreducible Super Key. In other words an attribute or a combination of attribute that identifies the record uniquely but none of its proper subsets can identify the records uniquely.
E.g. of Candidate Key
For above table we have only two Candidate Keys (i.e. Irreducible Super Key) used to identify the records from the table uniquely. ID Key can identify the record uniquely and similarly combination of Name and Address can identify the record uniquely, but neither Name nor Address can be used to identify the records uniquely as it might be possible that we have two employees with similar name or two employees from the same house.
(III) Primary Key – A Candidate Key that is used by the database designer for unique identification of each row in a table is known as Primary Key. A Primary Key can consist of one or more attributes of a table.
E.g. of Primary Key - Database designer can use one of the Candidate Key as a Primary Key. In this case we have “ID” and “Name, Address” as Candidate Key, we will consider “ID” Key as a Primary Key as the other key is the combination of more than one attribute.
(IV) Foreign Key – A foreign key is an attribute or combination of attribute in one base table that points to the candidate key (generally it is the primary key) of another table. The purpose of the foreign key is to ensure referential integrity of the data i.e. only values that are supposed to appear in the database are permitted.
E.g. of Foreign Key – Let consider we have another table i.e. Department Table with Attributes “Department_ID”, “Department_Name”, “Manager_ID”, ”Location_ID” with Department_ID as an Primary Key. Now the Department_ID attribute of Employee Table (dependent or child table) can be defined as the Foreign Key as it can reference to the Department_ID attribute of the Departments table (the referenced or parent table), a Foreign Key value must match an existing value in the parent table or be NULL.
(V) Composite Key – If we use multiple attributes to create a Primary Key then that Primary Key is called Composite Key (also called a Compound Key or Concatenated Key).
E.g. of Composite Key, if we have used “Name, Address” as a Primary Key then it will be our Composite Key.
(VI) Alternate Key – Alternate Key can be any of the Candidate Keys except for the Primary Key.
E.g. of Alternate Key is “Name, Address” as it is the only other Candidate Key which is not a Primary Key.
(VII) Secondary Key – The attributes that are not even the Super Key but can be still used for identification of records (not unique) are known as Secondary Key.
E.g. of Secondary Key can be Name, Address, Salary, Department_ID etc. as they can identify the records but they might not be unique.
Source: Stackoverflow.com