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Higher Forms of Normalization

Higher Forms of Normalization. Chapter 13.1-13.3 (skim). "Lossless" Joins. The main idea: if you decompose a relation schema, then join the parts of an instance via a natural join, you might get more rows than you started with, i.e., spurious tuples This is bad! Called a "lossy join".

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Higher Forms of Normalization

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  1. Higher Forms of Normalization Chapter 13.1-13.3 (skim)

  2. "Lossless" Joins • The main idea: if you decompose a relation schema, then join the parts of an instance via a natural join, you might get more rows than you started with, i.e., spurious tuples • This is bad! • Called a "lossy join". • Goal: decompositions which produce only "lossless" joins • "non-additive" join is more descriptive

  3. Preserving FDs • What if, when a relation is decomposed, the X of an XY ends up only in one of the new relations and the Y ends up only in another? • Such a decomposition is not dependency-preserving. • Goal: Always have FD-preserving decompositions

  4. Fact of life... Finding a decomposition which is both lossless and dependency-preserving is not always possible.

  5. Multivalued Dependencies (MVDs) • XY means that given X, there is a unique set of possible Y values (which do not depend on other attributes of the relation) • Classic example: PARENTNAMECHILDNAME • An FD is also a MVD • MVD problems arise if there are two independent 1:N relationships in a relation.

  6. Fourth Normal Form • A relation R is in 4NF if for every nontrivial XY, X is a superkey of R. • Decomposition into 4NF: If there is a non-trivial XY, form one relation with only X and Y, and another with R-Y. • This will be lossless, but not necessarily FD-preserving. • Achieving 4NF is a trade-off

  7. Fifth Normal Form • Sometimes a relation cannot be losslessly decomposed into two relations, but can be into three or more. • 5NF captures the idea that a relation scheme must have some particular lossless decomposition ("join dependency"). • Finding actual 5NF cases is difficult.

  8. Normalization Summary • 1NF: usually part of the woodwork • even so, know how to decompose • 2NF: usually skipped • but lots of defs. that make great exam Q's! • 3NF: a biggie • Always aim for this • BCNF and 4NF: tradeoffs start here • in re: d-preserving and losslessness • 5NF: You can say you've heard of it...

  9. Caveat • Normalization is not the be-all and end-all of DB design • Example: suppose attributes A and B are always used together, but normalization theory says they should be in different tables. • Normalization might produce unacceptable performance loss (extra disk reads)

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