Concept Of Key In SQL


  • In a table, a key is a single field or a set of fields. It’s utilized to get or get records/data rows from a data table based on a condition or requirement.
  • Keys can also be used to link together distinct database tables or views.
  • It’s used to identify any record or row of data in the database in a unique way.
  • It’s also used to construct and identify table relationships.

Types of Keys:

  1. Primary Key
  2. Candidate Key
  3. Super Key
  4. Foreign Key
  5. Alternate Key
  6. Composite Key
  7. Artificial Key

1.Primary Key

  • It is the initial key used to uniquely identify a single instance of an entity.
  • As we saw in the PERSON table, an entity can have many keys.
  • The most appropriate key from those lists becomes the primary key.

2.Candidate Key

  • An characteristic or group of attributes that can uniquely identify a tuple is known as a candidate key.
  • The remaining properties, with the exception of the primary key, are considered candidate keys. Candidates have the same strength as the primary key.

3.Super Key

  • A super key is a collection of attributes that can be used to uniquely identify a tuple. A candidate key is a superset of a super key.

4.Foreign Key

  • The column in a table that points to the primary key of another table is known as a foreign key.
  • Every employee in a corporation is assigned to a specific department, and employee and department are two distinct entities. As a result, the department’s information cannot be stored in the employee table. That’s why the primary key of one table is used to connect these two tables.
  • Department Id, the DEPARTMENT table’s primary key, is added to the EMPLOYEE table as a new attribute.

5.Alternate Key

  • Each tuple in a relation can be uniquely identified by one or more qualities, or by a combination of attributes.
  • The candidate keys are these attributes or combinations of traits.
  • From these candidate keys, one is chosen as the primary key, and the remaining candidate key, if it exists, is referred to as the alternate key.
  • The total number of alternate keys is equal to the total number of candidate keys minus the primary key.
  • It’s possible that an alternate key exists, but it’s also possible that it doesn’t.
  • There is no alternate key in a relation if there is only one candidate key.

6.Composite Key

  • A composite key is defined as a main key that has more than one characteristic.
  • Concatenated Key is another name for this key.

7.Artificial Key

  • Artificial keys are keys that are constructed using data that is randomly assigned.
  • When a primary key is huge and complex and has no relationships with many additional relations, these keys are formed.
  • In most cases, the data values of fake keys are numbered sequentially.

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