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Table 2-5. Skills Table in Second Normal Form Skill ID Skill Name
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Table 2-6. Employee Skills Table in Second Normal Form Employee Number Skill ID Skill Level
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Now skills can exist in your database without any corresponding employees having that skill, and you can reclassify a skill in a single operation just look up the Skill ID in the Skills table and
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C HA PTE R 2 RELATIONA L DATABA SE M ODELING A ND DATABA SE DES IGN
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change its name. You can also delete any information about employees without losing information about the skills themselves.
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Third Normal Form (3NF)
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A table is said to be in 3NF if it is already in 2NF and every non-key attribute is fully and directly dependent on the primary key. To enforce 3NF, you must eliminate the columns that aren t dependent on the key. If an attribute doesn t contribute to a description of the key, remove it to a separate table. The Employees table (Table 2-2) satisfies 1NF, since it contains no repeating groups. It satisfies 2NF, since it doesn t have a composite key. However, the table s key is Employee Number, and you can see that the Department Name and Department Location columns aren t dependent on the Employee Number (the primary key for the table) they are dependent on Department Number column values. To achieve 3NF, you must now move the department information into a separate table. You can make Department Number the key for your new Departments table. The motivation for the decomposition of the Employees table is straightforward you want to avoid delete and update anomalies. Suppose there is no employee hired for a new department yet. Under the present setup, you can t have a record of the department in the Employees table. Table 2-7 shows your tables in the third normal form.
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Table 2-7. Tables in the Third Normal Form
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Employees Table
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Employee Number Employee Name Department Number
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Departments Table
Department Number Department Name Department Location
Skills Table
Skill ID Skill Name
Employee Skills Table
Employee Number Skill ID Skill Level
If all of the preceding information seems a bit confusing to you initially, don t lose heart. The following is an easier way to remember and understand this whole process of putting a relation in 3NF: A relation is said to be in the third normal form if all the non-key attributes are fully dependent on the primary key, the whole primary key, and nothing but the primary key. Although there are more advanced forms of normalization, it is commonly accepted that normalization up to the 3NF is adequate for most business needs. For completeness, though, the other popular normal forms are outlined briefly in the next sections.
Boyce-Codd Normal Form (BCNF)
The Boyce-Codd normal form (BCNF) is based on the functional dependencies that exist in the relation. The BCNF is based on candidate keys. A relation is said to be in BCNF if, and only if, every determinant is a primary key. BCNF is a more strongly defined relationship than the 3NF. BCNF requires that if A determines B, then A must be a candidate key.
CHAPTER 2 RELA TION AL DA TA BAS E MODE LING AND DATA BASE DESIGN
Fourth Normal Form (4NF)
The 4NF is designed to take care of a special type of dependency called the multivalued dependency. A multivalued dependency exists among attributes X, Y, and Z if X determines more than one value of both Y and Z, and the values of Y and Z are independent of each other. A relationship is defined as being in the 4NF if it is in the BCNF and contains no nontrivial, multivalued dependencies.
Fifth Normal Form (5NF)
When a relation is decomposed into several relations, and then the subrelations are joined back again, you are not supposed to lose any tuples. This property is defined as a lossless-join dependency. 5NF is defined as a relation that has no join dependency. Even if you don t know much about the concept of normalizing data, by following a set of simple rules, and with the help of ER modeling tools, you can design sound databases.
ER Modeling Tools
Although you can design the basics of a system without the help of any tools per se, for most realworld systems it is better to use a modeling and designing tool. There are several excellent tools that can help you in your data-modeling efforts. Oracle provides the Oracle Designer as part of the Oracle Developer Suite. ERWin, PowerDesigner, and ER/Studio (from Embarcadero Technologies) are wellknown ER modeling tools. Quest Software (http://www.quest.com/) produces many useful tools, including the well-known TOAD software, both for Oracle developers and DBAs.
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