Boyce-Codd NF

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					Boyce-Codd NF
  Takahiko Saito
   Spring 2005
     CS 157A
    Purpose of Normalization

• To reduce the chances for anomalies
  to occur in a database.
• normalization prevents the possible
  corruption of databases stemming
  from what are called "insertion
  anomalies," "deletion anomalies," and
  "update anomalies."
        Insertion Anomaly

• A failure to place a new database
  entry into all the places in the
  database where that new entry needs
  to be stored.
• In a properly normalized database, a
  new entry needs to be inserted into
  only one place in the database
         Deletion Anomaly

• A failure to remove an existing
 database entry when it is time to
 remove that entry.
• In a properly normalized database,
 an old, to-be-gotten-rid-of entry
 needs to be deleted from only one
 place in the database
          Update anomaly

• An update of a database involves
 modifications that may be additions,
 deletions, or both. Thus "update
 anomalies" can be either of the kinds of
 anomalies discussed above.
           1st Normal Form

• A table (relation) is in 1NF if
1. There are no duplicated rows in the
  table.
2. Each cell is single-valued
3. Entries in a column are of the same
  kind.
           2nd Normal Form

• A table is in 2NF if it is in 1NF and if all
  non-key attributes are fully dependent on
  each candidate key.
• A partial dependency occurs when a non-
  key attribute is dependent on only a part
  of the (composite) key
              1NF but not 2NF
• Supplier (supplier#, status, city, part#,
  quantity)
  – (supplier#, part#) -> quantity
  – supplier# -> status
  – supplier# -> city
  – city -> status
• => status and city are dependent on just
  part of the key, namely supplier#.
    1NF but not 2NF (cont’d)

• Decomposition (into 2NF):
  – Supplier (supplier#, status, city)
  – Supplier_Part (supplier#, part#,
    quantity)
       3rd Normal Form (3NF)

• A table is in 3NF if it is in 2NF and if it has
  no transitive dependencies.
• Transitive dependency is a functional
  dependency between non-key attributes.
          2NF but not 3NF

• Supplier (supplier#, status, city)
  – supplier# -> status
  – supplier# -> city
  – city -> status
  => Lacks mutual independence among non-key
    attributes.
    2NF but not 3NF (cont’d)

• Decomposition (into 3NF):
  – SUPPLIER_CITY (supplier#, city)
  – CITY_STATUS (city, status)
             Boyce-Codd NF

• A table is in BCNF if it is in 3NF and if
  every determinant is a candidate key.
• the definition of 3NF does not deal with a
  relation that:
  – has multiple candidate keys, where
  – Those candidate keys are composite, and
  – the candidate keys overlap (i.e., have at least
    one common attribute)
3NF but not boyce-codd NF
• SUPPLIER_PART (supplier#, supplier_name,
 part#, quantity)
  – Two candidate keys:
     • (supplier#, part#) and (supplier_name, part#)
  – (supplier#, part#) -> quantity
  – (supplier#, part#) -> supplier_name
  – (supplier#, part#) -> quantity
  – (supplier#, part#) -> supplier#
  – supplier_name -> supplier#
  – supplier# -> supplier_name
 Another example of boyce-codd NF

    title     year length filmType studioNam   starName
                                        e
  Star Wars   1977 124       color    Fox       Fisher
  Star Wars   1977   124   color      Fox       Hamill
  Star Wars   1977   124   color      Fox        Ford
Mighty Ducks 1991    104   color    Disney     Esteves
Wayne’s World 1992   95    color   Paramount    Carvey
Wayne’s World 1992   95    color   Paramount    Meyers
          Example (cont’d)

• {title, year, starName} as candidate key
• title, year  length, filmType, studioName
• The above FD (Functional Dependency)
 violates the BCNF condition because title
 and year do not determine the sixth
 attribute, starName
            Example (cont’d)
• We solve this BCNF violation by decomposing
  relation Movies into
  1. The schema with all the attributes of the FD
      {title, year, length, filmType,
                    studioName}
  2. The schema with all attributes of Movies
      except the three that appear on the right of
      the FD
      {title, year, starName}
      Summary of Boyce-Codd NF
• When there is more than one candidate key, a relational
  table may be in 3NF and anomalies may still result.
• This occurs when there is a composite primary key, and
  there are two equally valid candidates to make up part
  of this composite primary key. If there is an attribute
  (one or more columns) on which any other attribute is
  fully dependent, and this attribute is NOT itself a
  candidate key, then the table is not in Boyce-Codd
  Normal form (BCNF).
• We fix this by breaking the table up into two tables, both
  in BCNF.

				
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posted:12/15/2011
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