Keys in DBMS are Super!

keys in dbmsOne critical step in the design process of a relational database is establishing DBMS keys. This is because each key will be utilized for establishing the database’s relationships. Without the keys, the links that join the data are prone to error or at times, not even a possibility. DBMS involves keys in a collection of data or table items which are all organized and described according to the relational model.

In a single table, data represents a relation. A times, a table might have other defined relations with each other. In this type of model, each schema table needs to identify a group of columns or a single column, called primary keys, to identify each row uniquely. Thus between table rows, relationships can be established and in rows of another table through foreign keys. Databases of this type are both queried and created by DBMS.

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In DBMS, a superkey is a relation variable’s attribute set for which it holds that in all assigned relations to that variable, there are no 2 tuples distinct that have the same value for the set attributes. Superkeys can also be defined as an attribute set of a schema or relations upon which all the schema attributes are dependent functionally.

Since in relational algebra, it is not permitted to have duplicate rows, thus the attribute set is a trivial superkey. For example, if set of attributes R is a superkey relation of K, than at all times, you can say that the projection of K over R is the same as the K cardinality itself.

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It can also be said that superkeys in DBMS are attribute sets within a table whose values can be utilized for identifying a tuple uniquely.

Candidate Keys

In DBMS, a relation’s candidate key is the relation’s minimal superkey. It is a set of attributes such that:

  1. For these attributes there is no proper subset for which one holds, indicating that the set is minimal.
  2. The relationship does not have 2 distinctive tuples which are records or rows in the common language of databases, which the same attribute values, which means that the attribute set is also a superkey.

Prime attributes are the term used for constituent attributes. In the same line of thought, non-prime attributes do not occur in any of the candidate keys. Since there are no duplicate rows in a relation, the attribute set is a superkey if you did not use null values. Thus every relation will have a minimum of 1 candidate key.

Candidate keys are minimal attribute sets necessary for identifying rows or tuples. These are also called minimal superkeys. For instance, when you have a schema for employees, consisting of the attributes of name, employee ID, department ID and job, you can combine employeeID with and of this table’s attributes to identify a tuple uniquely in the table.

Superkeys in a schema like this would be {employeeID, Name, job, departmentID}, {employeeID, Name, job} and {employeeID, Name}.The trivial superkey would be the {employeeID, Name, job, departmentID} because it contains all the table attributes for tuple identification.

To identify a tuple in a real database, you won’t really need all these attributes. You will only need the minimal superkey set of {employeeID}. This type of superkey is a minimal attribute set that can be used for identifying single tuples. Thus, this key is also a candidate key.

Foreign Keys

In a DBMS relational table, a foreign key is a field matching another table’s primary key column. The foreign key can be used for cross-referencing tables. There does not have to be unique values for foreign keys in the relation reference. These keys use attributes values effectively in the relation reference to restrict one or more attributes’ domains in the relation reference.

Foreign keys could be formally described as: ‘ For all the referencing relation tuples projected over attributes referenced, there needs to be a tuple existing in the relation reference projected over the same attribute such as each value of the reference attribute matches the value corresponding in the reference attribute.’

Primary or Unique Keys

A tuple within a table is uniquely specified by a primary key. In order for attributes to be primary keys that are considered ‘good,’ these should not be repeated. While good primary keys are sometimes natural attributes, a lot of the time, surrogate keys are used instead.

Artificial attributes assigned to objects which defines it uniquely is the surrogate key. For instance, an information table might assign students ID numbers in school in order for them to be differentiated. There is no inherent or intrinsic meaning to surrogate keys. Instead, it is useful through it abilities to identify a tuple uniquely. Another occurrence that happens a lot especially in the cardinality N:M is the composite key.

Composite Key

Composite keys are made up of equal than or greater than two attributes in a table that together  identify a record uniquely. For example, in databases that related classes, teachers and students, a composite key should identify classes uniquely according to the time slot and room number, since no other class could have the same attribute combination. As a matter of fact, using composite keys such as these can be a kind of verification of data.

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What Good are DBMS Keys Again?

The purpose of DBMS keys is to provide declarative methods of specifying queries and data. Users state directly what information they want from a database and also what the database contains. This lets the software of DBMS take care of describing structures of data for data storage and procedures of retrieval for answering queries.

A Note on Constraints

It is possible to further restrict an attribute’s domains using constraints. For instance, constraints can restrict an integer attribute given to values between one and ten. A constraint provides one method of business rules implementation in DBMS. SQL, for example, implements that functionality of a constraints through check constraints. Data is restricted by constraints and can be stored in a relation.

These are defined usually using expressions resulting in Boolean values, which indicate whether or not the constraint is satisfied by the data. Constraints can apply to one attribute, to an entire relation or to one tuple. There are domain constraints since there is an associated domain in every attribute. For this model, the 2 principal rules are known as referential integrity and entity integrity.

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