One, and only one, record in a table relates to one, and only one, record in a second table.
An association between two tables in which: · The primary key value of each record in the primary table corresponds to the value in the matching field or fields of one and only one record in the related table. · The primary key value of each record in the related table corresponds to the value in the matching field or fields of one and only one record in the primary table. .
a microcosm of these bigger social relations
A relationship between two tables in which a single row in the first table can be related to only one row in the second table, and a row in the second table can be related to only one row in the first table. This type of relationship is unusual. An example is the relationship between an employee table and a mailboxes table used to store information about email addresses in a company. The tables have a one-to-one relationship because each employee has one mailbox, and each mailbox is assigned to only one employee.
A relationship where there is one and only one match for a unique row in two tables. For example, for each video title (identified by a row with a unique key) in a Video Product table, there is one and only one row in the Video Details table that contains the description. Since there is only one description for each product, it could be located directly in the Video Product table; however it could be placed in another table for other processing reasons. In the latter case, the two rows would be uniquely identified by a common key joining them together.
Zero or one instance in entity A exists for each instance in entity B. Notation is 1:1. 1:1 relationships tend to become 1:M over time.
