Editing Relational database (section)

==Keys==
Each row in a table has its own unique key. Rows in a table can be linked to rows in other tables by adding a column for the unique key of the linked row (such columns are known as [[foreign key]]s). Codd showed that data relationships of arbitrary complexity can be represented by a simple set of concepts.<ref name="codd" />

Part of this processing involves consistently being able to select or modify one and only one row in a table. Therefore, most physical implementations have a unique [[primary key]] (PK) for each row in a table. When a new row is written to the table, a new unique value for the primary key is generated; this is the key that the system uses primarily for accessing the table. System performance is optimized for PKs. Other, more [[natural key]]s may also be identified and defined as [[alternate key]]s (AK). Often several columns are needed to form an AK (this is one reason why a single integer column is usually made the PK). Both PKs and AKs have the ability to uniquely identify a row within a table. Additional technology may be applied to ensure a unique ID across the world, a [[globally unique identifier]], when there are broader system requirements.

The primary keys within a database are used to define the relationships among the tables. When a PK migrates to another table, it becomes a foreign key (FK) in the other table. When each cell can contain only one value and the PK migrates into a regular entity table, this design pattern can represent either a [[One-to-one (data model)|one-to-one]] or [[One-to-many (data model)|one-to-many]] relationship. Most relational database designs resolve [[Many-to-many (data model)|many-to-many]] relationships by creating an additional table that contains the PKs from both of the other entity tables{{snd}} the relationship becomes an entity; the resolution table is then named appropriately and the two FKs are combined to form a PK. The migration of PKs to other tables is the second major reason why system-assigned integers are used normally as PKs; there is usually neither efficiency nor clarity in migrating a bunch of other types of columns.

===Relationships===
Relationships are a logical connection between different tables (entities), established on the basis of interaction among these tables. These relationships can be modelled as an [[Entity–relationship model|entity-relationship model]].