Database Keys in Relational DBMS | Studytonight
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. In other words candidate keys are minimal super keys. Primary Key: It is a candidate key that is chosen by the database designer to identify entities with in of attributes that uniquely identifies a row or record in a relation is. Basic Concepts of the Relational Model Database Term Clarification Entity Attributes Entity Set Relationship Relation Tuples Primary Key Candidate Keys Alternate Key Index Composite Key Foreign Key Two synonymous terms for attributes are elements and properties. Each is called a candidate key or super -key.
Well, okay, after you choose a primary key, the other candidate keys are still keys, just none of them are "primary".Relation between Super Key, Candidate Key and Primary Key - Database Management System
After you choose a president, the other candidates tend to disappear into obscurity. How many losing presidential candidates from more than 2 elections ago can you name? In that sense the analogy is imperfect. However, natural primary keys are also often a bad idea because they are unique but not unchanging think of the havoc of changing millions of child rows because a company name changed or because they are less efficient in joins than surrogate keys.
Further, in real life, many potential candidate keys are not nearly stable enough for a true PK and are not as unique as we think. Emails, for instance, can be reused after an account is closed.
You add other indexes to specify uniqueness in order to maintain data integrity.
For instance in your example, you are using a surrogate key to ensure uniqueness of a record. So you would want to create a unique index for any possible candidate key and this can consist of one or more fields in the data if you are using a surrogate key.
This ensures that the items which need to be unique are and allows you to get the performance benefits of a surrogate key. Database Keys Introduction For the purposes of clarity we will refer to keys in terms of RDBMS tables but the same definition, principle and naming applies equally to Entity Modelling and Normalisation.
Keys are, as their name suggests, a key part of a relational database and a vital part of the structure of a table. They ensure each record within a table can be uniquely identified by one or a combination of fields within the table.
They help enforce integrity and help identify the relationship between tables.
ER Model Basic Concepts
There are three main types of keys, candidate keys, primary keys and foreign keys. There is also an alternative key or secondary key that can be used, as the name suggests, as a secondary or alternative key to the primary key Super Key A Super key is any combination of fields within a table that uniquely identifies each record within that table. Candidate Key A candidate is a subset of a super key. A candidate key is a single field or the least combination of fields that uniquely identifies each record in the table.
The least combination of fields distinguishes a candidate key from a super key. Every table must have at least one candidate key but at the same time can have several. This would be a candidate key. These would both be candidate keys.
In order to be eligible for a candidate key it must pass certain criteria.
Foreign key - Wikipedia
It must contain unique values It must not contain null values It contains the minimum number of fields to ensure uniqueness It must uniquely identify each record in the table Once your candidate keys have been identified you can now select one to be your primary key Primary Key A primary key is a candidate key that is most appropriate to be the main reference key for the table.
As its name suggests, it is the primary key of reference for the table and is used throughout the database to help establish relationships with other tables. As with any candidate key the primary key must contain unique values, must never be null and uniquely identify each record in the table.
As an example, a student id might be a primary key in a student table, a department code in a table of all departments in an organisation. When choosing a primary key from the pool of candidate keys always choose a single simple key over a composite key. Foreign Key A 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.