Practical database design, Part 2

Normalization, history, and event logging

What are the best choices when designing the schema for a relational database? What is the rationale in deciding in favor of one and against some other alternative? Given the amount of vendor-specific recommendations, it is all too easy to overlook basic relational database fundamentals. In this second of two parts, author Philipp K. Janert presents an introduction to database normalization and to the five Normal Forms. He also discusses other possible uses for a database in a project, for example as a repository for configurational or logging data.

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Philipp Janert (janert@ieee.org), Software Project Consultant, IEEE.org

Philipp K. Janert is a Software Project Consultant, server programmer, and architect. His specific interest is the identification, establishment, and transmission of software engineering's best practices. He maintains the www.BeyondCode.org Web site and his articles have appeared in IEEE Software, Linux Journal, and on the O'Reilly Network site. He holds a Ph.D. in Theoretical Physics from the University of Washington in Seattle. You can contact the author at janert@ieee.org.



24 June 2003

In the first half of this article, I began to discuss some general best practices that I have found to be particularly helpful. Again, none of it is specific to any one vendor's product and everything should, therefore, be applicable, regardless of which database implementation is being used. In this concluding article, I'll attempt to give an approachable introduction to the topic of database normalization and to the five Normal Forms. I'll also discuss other possible uses for a database in a project, such as a repository for configurational or logging data, for example.

Normalization

No discussion of relational database (DB) design is complete without a section on normalization. A normalized DB schema avoids certain anomalies when inserting, updating, or deleting data and, therefore, helps to keep consistent data in the database.

However, the absence of anomalies is only the tangible result of a deeper benefit of normalization -- namely the correct identification and modeling of entities. The insert, update, and delete anomalies I've just referred to are the consequences of the redundancy introduced by improper or inadequate separation between distinct entities. The normalization procedure is, therefore, not just a technical chore to be done out of principle, but it can actively help to improve the understanding of the business domain.

Regrettably, the treatment of normalization is often prohibitively formal, and it suffers from a special, rather non-intuitive terminology. This is unfortunate since the outcome of a normalization procedure often evokes the reaction that it all is nothing more than common sense. I will try to offer explanations of expressions that you are likely to encounter in the literature as they come up in the following discussion.

Overview

Normalization is a process in which an initial DB design is transformed, or decomposed, into a different, but equivalent, design. The resulting schema is equivalent to the original one in the sense that no information is lost when going from one to the other.

The normalization procedure consists of a sequence of projections -- that is, some attributes are extracted from one table to form a new one. In other words, tables are split up vertically. The decomposition is lossless, only if you can restore the original table by joining its projections.

Through such non-loss decompositions it is possible to transform an original schema into a resulting one that satisfies certain conditions, known as Normal Forms:

  • The First Normal Form (1NF) addresses the structure of an isolated table.
  • The Second (2NF), Third (3NF), and Boyce-Codd (BCNF) Normal Forms address one-to-one and one-to-many relationships.
  • The Fourth (4NF) and Fifth (5NF) Normal Forms deal with many-to-many relationships.

These Normal Forms form a hierarchy in such a way that a schema in a higher normal form automatically fulfills all the criteria for all of the lower Normal Forms.

The Fifth Normal Form is the ultimate normal form with respect to projections and joins -- it is guaranteed to be free of anomalies that can be eliminated by taking projections.

In the following discussion, any mention of keys refers to the conceptual keys formed from business data, not to any plainly technical surrogate keys which might have been defined.

First Normal Form

A table is said to be in First Normal Form (1NF), if all entries in it are scalar-valued. Relational database tables are 1NF by construction since vector-valued entries are forbidden. Vector-valued data (that is, entries which have more than one value in each row) are referred to as repeating groups.

The following relation violates 1NF because the SupplierID forms a repeating group (here and in the following examples and text, primary key fields are in bold):

{ PartID, Supplier1ID, Supplier2ID, Supplier3ID }

Repeating groups indicate a one-to-many relationship -- in other words, a relationship which in relational databases is treated using foreign keys. Note that the problem of repeating groups cannot be solved by adding any number of fields to a record; even if the number of elements of the vector-valued data was fixed, finite, and predetermined, searching for a value in all these parallel fields is prohibitively cumbersome.

To achieve 1NF, eliminate repeating groups by creating separate tables for each set of related data.

To demonstrate the typical anomalies that occur in tables that are only 1NF, consider the following example:

{ CustomerID, OrderID, CustomerAddress, OrderDate }

Note the following problems:

  • Insert: It is not possible to add a record for a customer who has never placed an order.
  • Update: To change the address for a customer, this change has to be repeated for all of the customer's existing orders.
  • Delete: Deleting the last order for a customer loses all information about the customer.

Functional dependency

The Second and Third Normal Forms address dependencies among attributes, specifically between key and non-key fields.

By definition, a key uniquely determines a record: Knowing the key determines the values of all the other attributes in the table row, so that given a key, the values of all the other attributes in the row are fixed.

This kind of relationship can be formalized as follows. Let X and Y be attributes (or sets of attributes) of a given relationship. Then Y is functionally dependent on X if, whenever two records agree on their X-values, they must also agree on their Y-values. In this case, X is called the determinant and Y is called the dependent. Since for any X there must be a singleY, this relationship represents a single-valued functional dependency. If the set of attributes in the determinant is the smallest possible (in the sense that after dropping one or more of the attributes from X, the remaining set of attributes does no longer uniquely determine Y), then the dependency is called irreducible.

Note that functional dependency is a semantic relationship: It is the business logic of the problem domain, represented by the relation, which determines whether a certain X determines Y.

Second Normal Form

A table is in Second Normal Form (2NF) if every non-key field is a fact about the entire key. In other words, a table is 2NF if it is 1NF and all non-key attributes are functionally dependent on the entire primary key (that is, the dependency is irreducible).

Clearly, 2NF is only relevant when the key is composite (that is, consisting of several fields). The following example describes a table which is not 2NF since the WarehouseAddress attribute depends only on WarehouseID but not on PartID:

{ PartID, WarehouseID, Quantity, WarehouseAddress }

To achieve 2NF, create separate tables for sets of values that apply to multiple records and relate these tables through foreign keys. The determinants of the initial table become the primary keys of the resulting tables.

Third Normal Form

A relation is in Third Normal Form (3NF) if it is 2NF and none of its attributes is a fact about another non-key field. In other words, no non-key field functionally depends on any other non-key field. (Such indirect dependencies are known as transitive dependencies.)

The following example violates 3NF since the Location is functionally dependent on the DepartmentID:

{ EmployeeID, DepartmentID, Location }

To achieve 3NF, eliminate fields that do not depend on the key from the original table and add them to the table whose primary key is their determinant.

To summarize the normalization procedure up to and including Third Normal Form:

Every field in a record must depend on The Key (1NF), the Whole Key (2NF), and Nothing But The Key (3NF).

Boyce-Codd Normal Form

Boyce-Codd Normal Form (BCNF) is an extension of 3NF in the case with two or more candidate keys which are composite and overlapping (that is, they have at least one field in common). If these conditions are not fulfilled, 3NF and BCNF are equivalent. A table is BCNF if, and only if its only determinants are candidate keys.

In the following table, both {SupplierID, PartID}, as well as {SupplierName, PartID}, are candidate keys. The table is not BCNF since it contains two determinants (SupplierID and SupplierName) which are not candidate keys. (SupplierID and SupplierName are determinants, since they determine each other.)

{ SupplierID, PartID, SupplierName, Quantity }

However, either of the following decompositions is BCNF:

{ SupplierID, SupplierName } 
{ SupplierID, PartID, Quantity }

or

{ SupplierName, SupplierID }
{ SupplierName, PartID, Quantity }

To achieve BCNF, remove the determinants which are not candidate keys.

Many-to-many relationships and higher Normal Forms

Fourth and Fifth Normal Forms apply to situations involving many-to-many relationships. In relational databases, many-to-many relationships are expressed through cross-reference tables.

As an example, consider a case of class enrollment. Each student can be enrolled in one or more classes and each class can contain one or more students. Clearly, there is a many-to-many relationship between classes and students. This relationship can be represented by a Student/Class cross-reference table:

{ StudentID, ClassID }

The key for this table is the combination of StudentID and ClassID. To avoid violation of 2NF, all other information about each student and each class is stored in separate Student and Class tables, respectively.

Note that each StudentID determines not a unique ClassID, but a well-defined, finite set of values. This kind of behavior is referred to as multi-valued dependency of ClassID on StudentID.

Fourth Normal Form

A table is in Fourth Normal Form (4NF) if it is 3NF and it does not represent two or more independent many-to-many relationships.

Consider an example with two many-to-many relationships, between students and classes and between classes and teachers. Also, a many-to-many relationship between students and teachers is implied. However, the business rules do not constrain this relationship in any way -- the combination of StudentID and TeacherID does not contain any additional information beyond the information implied by the student/class and class/teacher relationships. Consequentially, the student/class and class/teacher relationships are independent of each other -- these relationships have no additional constraints. The following table is, then, in violation of 4NF:

{ StudentID, ClassID, TeacherID }

As an example of the anomalies that can occur, realize that it is not possible to add a new class taught by some teacher without adding at least one student who is enrolled in this class.

To achieve 4NF, represent each independent many-to-many relationship through its own cross-reference table.

Fifth Normal Form

A table is in Fifth Normal Form (5NF) if it is 4NF and its information content cannot be reconstructed from several tables containing fewer attributes.

Consider again the student/class/teacher example, but now assume that there is an additional relationship between students and teachers. The previous example table is now 4NF, since all the relationships it describes are interrelated. However, it is not 5NF, since it can be reconstructed from three cross-reference tables, each representing one of the three many-to-many relationships:

{ StudentID, ClassID } 
{ ClassID,   TeacherID } 
{ TeacherID, StudentID }

To achieve 5NF, isolate interrelated many-to-many relationships, introducing the required number of new tables to represent all business domain constraints.

Normalization in context

In practice, many databases are de-normalized to greater or lesser degree. The reason most often stated has to do with performance -- a de-normalized database may require fewer joins and can, therefore, be faster for retrievals.

While this reasoning may be true, the usual caveats against premature optimization apply here as well as everywhere else. First, you should determine sufficiently that a performance problem exists and that the proposed de-normalization improves it before introducing a conceptually suboptimal design.

Furthermore, a de-normalized schema can be harder to update. The additional integrity checks that are necessary in this case may offset the performance gains for queries obtained through denormalization.

Finally, it should be noted that dealing with many-to-many relationships raises some issues that cannot be fully resolved through normalization (Chris Date's article, "Normalization is no Panacea," in Resources covers this topic).


History tables and event logging

Besides holding the data that is necessary to support the primary business purpose of the system under construction, the DB is also a possible location to record information that is useful primarily for internal technical purposes, such as adminstration and maintenance of the system itself.

History tables

In a production system, you may desire to preserve the history of changes to the data in the live database. This can be achieved through the use of history (or backup) tables, and the appropriate INSERT, DELETE, and UPDATE triggers.

Each table in the DB should have a history table, mirroring the entire history of the primary table. If entries in the primary table are to be updated, the old contents of the record are first copied to the history table before the update is made. In the same way, deleted records in the primary table are copied to the history table before being deleted from the primary one. The history tables always have the name of the corresponding primary one, but with _Hist appended.

Entries to the history table are always appended at the end. The history table, therefore, grows strictly monotonically in time. It will become necessary to periodically spool ancient records to tape for archiving. Such records may, as a result, not be immediately available for recall.

The attributes of the history table should agree exactly with the attributes of the primary table. In addition, the history table records the date and type of the change to the primary table. The type is one of the following: Create, Update, or Delete.

Changes to the structure of the primary table affect the history table. When an attribute is added to the primary table, it is added to the history table as well. When an attribute is deleted from the primary table, the corresponding attribute is not deleted from the history table. Instead, this field is left blank (NULL) in all future records. Consequentially, the history table not only grows in length over time, but also in width.

Note that the choice to use such a history mechanism affects neither the overall DB layout, nor applications that access only the primary tables. During development, you can probably dispense with recording changes in this way and leave the creation of the history tables and the necessary triggers until installation time.

Event logging for fun and profit

A database can be used as an event logger. The notion of event is broad, ranging from common debugging and system specific runtime information, to events which are specific to the business domain. Possible candidates for events to be logged to the database include:

  • Transactions making changes to persistent data
  • Transactions crossing component boundaries
  • Errors and exceptions
  • Dispatching of messages to the user
  • Events involving financial transactions
  • State changes to business entities

An EventLog table to log such information contains at least these fields to record:

  • Timestamp
  • EventType (a type code)
  • Details (a descriptive string)

Optionally, it may identify an owner or originator of the event. The owner concept can either identify a logged-in user or admin, but it may as well describe a part or module of the system itself. In applications dealing with financial transactions, additional (optional) fields identifying the from- and to-accounts can be useful.

System config tables

Finally, it is possible to use the database as centralized storage for configurational data. Usually this information is kept distributed in miscellaneous plain-text files, such as start-up scripts or property files. The database can provide a single, managed storage facility for such information.

Besides start-up parameters, which are usually supplied to the system at boot-time, one may also think of properties that are required at runtime, such as localized strings and messages.

Lastly, the database is a possible place to keep system documentation. This is most useful, of course, for information that is naturally in tabular form (rather than free text), such as lists of assigned port numbers or shared memory keys, for instance. But this approach is not limited to codes. A data dictionary, defining the permissible values for each field, is a necessity on any non-trivial project. This also can be made accessible to all developers and administrators by storing it in the database.

In any case, the data is stored in simple key/value pairs. Additional table attributes can contain comments or pointers (URLs) to relevant offline documentation.

The primary advantage to keeping such information in the database is that the database provides a central repository for all relevant information, as opposed to the typical approach in which data is scattered over miscellaneous files.


Summary

In this article I've covered database normalization and the five Normal Forms. In the normalization process, an original database design is transformed into an equivalent one, which avoids certain anomalies when inserting, updating, or deleting records. Proper normalization also helps to identify entities correctly. I also discussed the possible use of a database as a central repository for logging information or configurational data.

Resources

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