![unique mean unique mean](https://i.ytimg.com/vi/k4RO_6zYTTE/maxresdefault.jpg)
In the case where the person entity contained the biological father ID, the father ID would not be expected to be unique because a father may have more than one child. In this case, the EmployeeID is both a foreign key and the unique primary key, meaning that the tables have a 1:1 relationship. Foreign keys may be the primary key in another table for example a PersonID may become the EmployeeID in the Employee table. Keys that migrate to another entity may or may not be unique, depending on the design and how they are used in the other table. Keys that are within their originating entity are unique within that entity. Alternate keys may be used like a primary key in a single-table select.Ī key that has migrated to another entity.Īt the most basic definition, "a key is a unique identifier", so unique key is a pleonasm.
![unique mean unique mean](https://media.cheggcdn.com/study/289/2890b272-37e6-4ba1-890c-96adc323e2d2/image.png)
When the data model is instantiated into a physical database, it is the key that the system uses the most when accessing the table, or joining the tables together when selecting data.Ī non-primary key that can be used to identify only one row in a table. This is the key that is allowed to migrate to other entities to define the relationships that exist among the entities. Only one key within an entity is selected to be the primary key. The key that is selected as the primary key. Surrogate keys are usually numeric ID values and often used for performance reasons. In other words, the data is not system generated, such as a social security number imported from another system.Īn artificial key made from data that is system assigned or generated when another candidate key exists. Like a compound key, but the individual attributes need not be simple keys.Ī key made from data that exists outside the current database. There are several types of keys used in database modeling and implementations.Ī key made from more than one attribute joined together as a single key, such as part or whole name with a system generated number appended as often used for E-mail addresses.Ī key made from at least two attributes or simple keys, only simple keys exist in a compound key. For example, a Social Security Number might be a single attribute key for an employee a combination of flight number and date might be a key consisting of two attributes for a scheduled flight. One of those keys per table may be designated the primary key other keys are called alternate keys.Īny key may consist of one or more attributes.
![unique mean unique mean](https://mymomsanerd.com/wp-content/uploads/2020/12/079-names-that-mean-earth.png)
Proper selection of keys when designing a database is therefore an important aspect of database integrity.Ī relational database table may have one or more available unique keys (formally called candidate keys). The DBMS prevents updates that would cause duplicate key values and thereby ensures that tables always comply with the desired rules for uniqueness. a uniqueness constraint) in a table is also a data integrity feature of the database. The enforcement of a key constraint (i.e. There may be several keys inmber and login name are individually unique.
Unique mean update#
For example, in a table of employees, both employee nurs and application software to identify, access and update information in a database table. Keys provide the means for database use any given table.