Monday, February 04, 2013

Primary Foreign and Unique Keys

In a relational database , a "Primary Key" is a key that uniquely defines the characteristics of each row .The primary key has to consist of characteristics that cannot be duplicated by any other row. 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:

Author TABLE Schema:
Book TABLE Schema:
Book TABLE(ISBN,Author_ID,Title,Publisher,Price)
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 columns 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 person) or 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 NOT NULL 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 field. Another difference is that primary keys must be defined using another syntax

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