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DATABASE SYSTEM

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RELATIONAL SYSTEM

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  • pg 1
									Distinguish among these keys Foreign, Candidate, Alternate, and Primary key.

   1. A foreign key is an attribute (or set of attributes) that appears (usually)
      as a non key attribute in one relation and as a primary key attribute in
      another relation. I say usually because it is possible for a foreign key to
      also be the whole or part of a primary key:
         o A many-to-many relationship can only be implemented by
            introducing an intersection or link table which then becomes the
            child in two one-to-many relationships. The intersection table
            therefore has a foreign key for each of its parents, and its primary
            key is a composite of both foreign keys.
         o A one-to-one relationship requires that the child table has no
            more than one occurrence for each parent, which can only be
            enforced by letting the foreign key also, serves as the primary key.
         o
   2. Candidate key:

    Is an attribute (or set of attributes) that uniquely identifies a row. A
   candidate key must possess the following properties:

         o   Unique identification - For every row the value of the key must
             uniquely identify that row.
         o   Non redundancy - No attribute in the key can be discarded
             without destroying the property of unique identification.



Three absolute demands on the candidate key, if it is to be regarded as a
possible primary key.

There are three fundamental demands on the candidate keys that we should
never deviate from, if it is to become the subject for a primary key:

   1. The candidate key must be unique within its domain (the entity it
      represents, and beyond, if you also intend to access external entities,
      with is clearly illustrated in this article).
   2. The candidate key cannot hold NULLs (NULL is not zero. Zero is a
      number. NULL is 'unknown value').
   3. The candidate key should never change. It must hold the same value for
      a given occurrence of an entity for the lifetime of that entity.
3. A primary key is the candidate key which is selected as the principal
   unique identifier. Every relation must contain a primary key. The primary
   key is usually the key selected to identify a row when the database is
   physically implemented. For example, a part number is selected instead
   of a part description.


4. An alternate key is similar to a primary key. It accepts null values; where
   as the primary key does not. The null values can be submitted to the
   attribute in a tuple.

   There can be a key apart from primary key in a table that can also
   be a key. This key may or may not be a unique key. For example,
   in an employee table, empno is a primary key, empname is a
   alternate key that may not be unique but still helps in identifying a
   row of the table.

  Candidate Key is a Key which identifies each row
  of a table
  uniquely. Generally a Candidate Key becomes a
  Primary Key
  of the table. If table has more than one
  Candidate Keys
  then one is called Primary key and rest are
  called Alternative Keys.




 For this relation table shown below.



   Serial num     Name           Department       sex            location
   1              Joseph         Computer         male           South Town
                  Adam           science
   2              Tobbie         Computer         male            Down Town
                  Williams       science
      3              Paul Smith      Computer        female          North side.
                                     science




Generally a candidate key becomes the primary key,so in this case of having a
given relation(table) will have more than one candidate key, when this happen
one of such key is chosen as a primary key while the remaining are alternate
keys.

Primary key for this tuple relation is 1(serial num) while the others 2,3 are
called alternate keys which identify each row record of the table.

A candidate key is a combination of attributes that can be uniquely used to
identify a database record without any extraneous data. Each table may have
one or more candidate keys. One of these candidate keys is selected as the
table primary key.

Any attribute that is uniquely identify a row in a table is
Candidate key for the table. We select one of the candidate key as Primary
key.

								
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