This link has been bookmarked by 128 people . It was first bookmarked on 24 Apr 2006, by Francisca Munoz.
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The goal of database normalization is to ensure that every non-key column in every table is directly dependent on the key, the whole key and nothing but the key and with this goal come benefits in the form of reduced redundancies, fewer anomalies, and improved efficiencies. While normalization is not the be-all and end-all of good design, a normalized schema provides a good starting point for further development.
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insertion anomaly
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deletion anomaly
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update anomaly
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requires that each relation
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has a primary key
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Normalization
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Second,
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Third,
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The first normal form (or 1NF) requires that the values in each column of a table are atomic. By atomic we mean that there are no sets of values within a column.
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From a performance point of view, an integer used as a surrogate primary key can often provide better performance in a join than a composite primary key across several columns.
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To represent a many-to-many relationship in a relational database we need a third table to serve as a link between the two.
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When we have a one-to-many relationship, we place a foreign key in the table representing the �many�, pointing to the primary key of the table representing the �one�. Here is the new Book table:
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Columns in a table that refer to primary keys from another table are known as foreign keys
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A complete normalization of tables is desirable, but you may find that in practice that full normalization can introduce complexity to your design and application. More tables often means more JOIN operations, and in most database management systems (DBMSs) such JOIN operations can be costly, leading to decreased performance. The key lies in finding a balance where the first three normal forms are generally met without creating an exceedingly complicated schema.
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The goal of database normalization is to ensure that every non-key column in every table is directly dependent on the key, the whole key and nothing but the key and with this goal come benefits in the form of reduced redundancies, fewer anomalies, and improved efficiencies
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a deletion anomaly
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an insertion anomaly
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an update anomaly
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each relation (AKA table) has a primary key
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without which a table cannot even be considered to be in first normal form.
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1NF) requires that the values in each column of a table are atomic
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a surrogate primary key
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when a natural primary key is either unavailable or impractical.
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Some developers use surrogate primary keys as a rule, others use them only in the absence of a natural candidate for the primary key
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To represent a many-to-many relationship in a relational database we need a third table to serve as a link between the two
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we place a foreign key in the table representing the �many�
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2NF) any non-key columns must depend on the entire primary key
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cannot depend on only part of the composite key.
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One way to identify transitive dependencies is to look at your table and see if any columns would require updating if another column in the table was updated. If such a column exists, it probably violates 3NF.
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A complete normalization of tables is desirable, but you may find that in practice that full normalization can introduce complexity to your design and application.
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decreased performance
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An INNER JOIN query returns one row for each pair or matching rows in the tables being joined
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LEFT OUTER JOIN, RIGHT OUTER JOIN and FULL OUTER JOIN
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we benefit from decreased redundancy, fewer anomalies and improved efficiency.
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The first normal form (or 1NF) requires that the values in each column of a table are atomic. By atomic we mean that there are no sets of values within a column
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One method for bringing a table into first normal form is to separate the entities contained in the table into separate tables
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surrogate primary key -- an artificial primary key used when a natural primary key is either unavailable or impractical.
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when using a surrogate primary key it is still important to create a UNIQUE key to ensure that duplicate records are not created inadvertently (but some would argue that if you need a UNIQUE key it would be better to stick to a composite primary key).
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- One to (Zero or) One
- One to (Zero or) Many
- Many to Many
There are various types of relationships that can exist between two tables:
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To represent a many-to-many relationship in a relational database we need a third table to serve as a link between the two.
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When we have a one-to-many relationship, we place a foreign key in the table representing the �many�, pointing to the primary key of the table representing the �one�.
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Columns in a table that refer to primary keys from another table are known as foreign keys, and serve the purpose of defining data relationships.
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the Second Normal Form (or 2NF) deals with relationships between composite key columns and non-key columns.
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The second normal form (or 2NF) any non-key columns must depend on the entire primary key. In the case of a composite primary key, this means that a non-key column cannot depend on only part of the composite key.
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To bring the Review table into compliance with 2NF, the Author_URL must be moved to the Author table.
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Third Normal Form (3NF) requires that all columns depend directly on the primary key. Tables violate the Third Normal Form when one column depends on another column, which in turn depends on the primary key (a transitive dependency).
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In the Publisher table the City and State fields are really dependent on the Zip column and not the Publisher_ID. To bring this table into compliance with Third Normal Form, we would need a table based on zip code:
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With our tables now separated by entity, we join the tables together in our SELECT queries and other statements to retrieve and manipulate related data
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05 Feb 09
Charles GnilkaThe most challenging of these to maintain and rework suffer from what one author described as the �Spreadsheet Syndrome�: a tendency for the developer to lump every possible piece of information into as few table as possible, often into a single ta
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05 Jan 09
Rafa BayonaThis article will take a practical look at database normalization, focusing on the first three of seven generally recognized normal forms. Additional resources that look at the theory of database normalization and the additional normal forms can be found
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Nguyen Tien SiOver the years I, like many of you, have had the experience of taking over responsibility for an existing application and its associated schema (sometimes frustratingly woven together as part of a Microsoft Access solution).
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elle mDatabase normalization is a process by which an existing schema is modified to bring its component tables into compliance with a series of progressive normal forms. The goal of database normalization is to ensure that every non-key column in every table
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nnguyen2000compliance with Third Normal Form, we would need a table based on zip code:
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Paul Acquarokey used when a natural primary key is either unavailable or impractical. In the case of author we cannot use the combination of first and last name as a primary key because there is no guarantee that each author's name will be unique, and we cannot assum
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Joe WeinmunsonThe goal of database normalization is to ensure that every non-key column in every table is directly dependent on the key, the whole key and nothing but the key and with this goal come benefits in the form of reduced redundancies, fewer anomalies, and imp
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Lynn YoungThis article is a popular introduction to creating database schema with an eye to efficiency, both in storage and performance.
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Mike RotenbergMySQL - Database Normalization
database normalization mysql design tutorial programming reference software tips toread development optimization
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When users ask for advice about their database applications, one of the first things I try to help them with is the normalization of their table structure. Normalization is the process of removing redundant data from your tables in order to improve storage efficiency, data integrity and scalability. This improvement is balanced against an increase in complexity and potential performance losses from the joining of the normalized tables at query-time.
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29 Oct 04
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24 Sep 04
steve ulricha nice discussion of db normalization, well worth the read even for db nerds
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