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Thursday, April 3, 2014

Unique Nail Designs Nail Designs Nail Designs 2014 Tumblr Step By Step For Short Nails With Rhinestones With Bows Tumblr Acrylic Summber Ideas

Unique Nail Designs Biography

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A unique key or primary key is a key that uniquely defines the characteristics of each row. The primary key has to consist of characteristics that cannot collectively be duplicated by any other row. In an entity relationship diagram of a data model, one or more unique keys may be declared for each data entity. Each unique key is composed from one or more data attributes of that data entity. The set of unique keys declared for a data entity is often referred to as the candidate keys for that data entity. From the set of candidate keys, a single unique key is selected and declared the primary key for that data entity. In an entity relationship diagram, each entity relationship uses a unique key, most often the primary key, of one data entity and copies the unique key data attributes to another data entity to which it relates. This inheritance of the unique key data attributes is referred to as a foreign key and is used to provide data access paths between data entities. Once the data model is instantiated into a database, each data entity usually becomes a database table, unique keys become unique indexes associated with their assigned database tables, and entity relationships become foreign key constraints. In integrated data models, commonality relationships do not become foreign key constraints since commonality relationships are a peer-to-peer type of relationship.

The primary key may consist of a single attribute or a multiple attributes in combination. For example, a birthday could be shared by many people and so would not be a prime candidate for the Primary Key, but a social security number or Driver's License number would be ideal since it correlates to one single data value. Another unique characteristic of a Primary Key as it pertains to a relational database, is that a Primary Key must also serve as a Foreign Key on a related table. For example, Here we can see that AUTHOR_ID serves as the Primary Key in AuthorTable but also serves as the Foreign Key on the BookTable. The Foreign Key serves as the link and therefore the connection between the two "related" tables in this sample database.

In a relational database, a unique key index can uniquely identify each row of data values in a database table. A unique key index comprises a single column or a set of columns in a single database table. No two distinct rows or data records in a database table can have the same data value (or combination of data values) in those unique key index columns if NULL values are not used. Depending on its design, a database table may have many unique key indexes but at most one primary key index.A unique key constraint does not imply the NOT NULL constraint in practice. Because NULL is not an actual value (it represents the lack of a value), when two rows are compared, and both rows have NULL in a column, the column values are not considered to be equal. Thus, in order for a unique key to uniquely identify each row in a table, NULL values must not be used. According to the SQL standard and Relational Model theory, a unique key (unique constraint) should accept NULL in several rows/tuples — however not all RDBMS implement this feature correctly. A unique key should uniquely identify all possible rows that exist in a table and not only the currently existing rows Examples of unique keys are Social Security numbers (associated with a specific personor ISBNs (associated with a specific book).Telephone books and dictionaries cannot use names, words, or Dewey Decimal system numbers as candidate keys because they do not uniquely identify telephone numbers or words.

A table can have at most one primary key, but more than one unique key. A primary key is a combination of columns which uniquely specify a row. It is a special case of unique keys. One difference is that primary keys have an implicit constraint while unique keys do not. Thus, the values in unique key columns may or may not be NULL, and in fact such a column may contain at most one NULL fields. Another difference is that primary keys must be defined using another syntax. The relational model, as expressed through relational calculus and relational algebra, does not distinguish between primary keys and other kinds of keys. Primary keys were added to the SQL standard mainly as a convenience to the application programmer.