Relation (database) - Wikipedia
Once a database is normalized, relationships between the data in the resulting record is dynamic, which means any change made to the. This definition explains the meaning of a relational database and how it stores data in rows and columns in tables. Relation Definition - Relation is sometimes used to refer to a table in a relational database but is more commonly used to describe the relationships that.
So, for example, employee is known only by that name, Yonezawa Akinori, and does not live anywhere else but in Naha, Okinawa. Also, apart from the four employees shown, there is no other employee who has both a name and an address. Under the definition of body, the tuples of a body do not appear in any particular order - one cannot say "The tuple of 'Murata Makoto' is above the tuple of 'Matsumoto Yukihiro'", nor can one say "The tuple of 'Yonezawa Akinori' is the first tuple.
Under the definition of heading, the attributes of an element do not appear in any particular order either, nor, therefore do the elements of a tuple. A similar comment does not apply here to SQL, which does define an ordering to the columns of a table. Relation Variables[ edit ] A relational database consists of named relation variables relvars for the purposes of updating the database in response to changes in the real world.
An update to a single relvar causes the body of the relation assigned to that variable to be replaced by a different set of tuples. Such variables are classified into two classes: A base relation variable is a relation variable which is not derived from any other relation variables.
Foreign key A foreign key is a field in a relational table that matches the primary key column of another table. The foreign key can be used to cross-reference tables. Foreign keys do not need to have unique values in the referencing relation. Foreign keys effectively use the values of attributes in the referenced relation to restrict the domain of one or more attributes in the referencing relation.
A foreign key could be described formally as: Stored procedure A stored procedure is executable code that is associated with, and generally stored in, the database. Stored procedures usually collect and customize common operations, like inserting a tuple into a relationgathering statistical information about usage patterns, or encapsulating complex business logic and calculations.
Frequently they are used as an application programming interface API for security or simplicity. Stored procedures are not part of the relational database model, but all commercial implementations include them. Index database An index is one way of providing quicker access to data.
Indexes can be created on any combination of attributes on a relation. Queries that filter using those attributes can find matching tuples randomly using the index, without having to check each tuple in turn.
This is analogous to using the index of a book to go directly to the page on which the information you are looking for is found, so that you do not have to read the entire book to find what you are looking for.
What is Relation? - Definition from Techopedia
Relational databases typically supply multiple indexing techniques, each of which is optimal for some combination of data distribution, relation size, and typical access pattern. Indices are usually not considered part of the database, as they are considered an implementation detail, though indices are usually maintained by the same group that maintains the other parts of the database. The use of efficient indexes on both primary and foreign keys can dramatically improve query performance.
This is because B-tree indexes result in query times proportional to log n where n is the number of rows in a table and hash indexes result in constant time queries no size dependency as long as the relevant part of the index fits into memory.
Relational algebra Queries made against the relational database, and the derived relvars in the database are expressed in a relational calculus or a relational algebra. In his original relational algebra, Codd introduced eight relational operators in two groups of four operators each.
The first four operators were based on the traditional mathematical set operations: The union operator combines the tuples of two relations and removes all duplicate tuples from the result. The intersection operator produces the set of tuples that two relations share in common. The difference operator acts on two relations and produces the set of tuples from the first relation that do not exist in the second relation.
The cartesian product of two relations is a join that is not restricted by any criteria, resulting in every tuple of the first relation being matched with every tuple of the second relation. The cartesian product is implemented in SQL as the Cross join operator. The remaining operators proposed by Codd involve special operations specific to relational databases: The selection, or restriction, operation retrieves tuples from a relation, limiting the results to only those that meet a specific criterion, i.
It does this by having two fields that reference the primary key of each of the other two tables. The following is an example of a many-to-many relationship: This is the Relationships tab that is displayed when you create a relationship Microsoft Access.
What is a many-to-one relationship?
In this case, a many-to-many relationship has just been created. The Orders table is a junction table that cross-references the Customers table with the Products table.
- Relational database
- relational database
- Relational databases: Defining relationships between database tables
So in order to create a many-to-many relationship between the Customers table and the Products table, we created a new table called Orders. The values that these fields contain should correspond with a value in the corresponding field in the referenced table. So any given value in Orders. CustomerId should also exist in the Customer.
Not good referential integrity.