Database Modeling & Design Fourth Edition- P30: Database technology has evolved rapidly in the three decades since the rise and eventual dominance of relational database systems. While many specialized database systems (object-oriented, spatial, multimedia, etc.) have found substantial user communities in the science and engineering fields, relational systems remain the dominant database technology for business enterprises. | 132 CHAPTER 6 Normalization skill_required is decomposed into skill_req1 and skill_req3. In general but not always decomposition of a table into 4NF tables results in less data redundancy. Decomposing Tables to 4NF Algorithms to decompose tables into 4NF are difficult to develop. Let s look at some straightforward approaches to 4NF from BCNF and lower normal forms. First if a table is BCNF it either has no FDs or each FD is characterized by its left side being a superkey. Thus if the only MVDs in this table are derived from its FDs they have only superkeys as their left sides and the table is 4NF by definition. If however there are other nontrivial MVDs whose left sides are not superkeys the table is only in BCNF and must be decomposed to achieve higher normalization. The basic decomposition process from a BCNF table is defined by selecting the most important MVD or if that is not possible then by selecting one arbitrarily defining its complement MVD and decompose the table into two tables containing the attributes on the left and right sides of that MVD and its complement. This type of decomposition is lossless because each new table is based on the same attribute which is the left side of both MVDs. The same MVDs in these new tables are now trivial because they contain every attribute in the table. However other MVDs may be still present and more decompositions by MVDs and their complements may be necessary. This process of arbitrary selection of MVDs for decomposition is continued until only trivial MVDs exist leaving the final tables in 4NF. As an example let R A B C D E F with no FDs and with MVDs A - B and CD - EF. The first decomposition of R is into two tables R1 A B and R2 A C D E F by applying the MVD A - B and its complement A - CDEF. Table R1 is now 4NF because A - B is trivial and is the only MVD in the table. Table R2 however is still only BCNF because of the nontrivial MVD CD - EF. We then decompose R2 into R21 C D E F and R22 C D A by applying
