清华大学数据库原理课件第七章(2)

1. compute + (the attribute closure of ) 2. verify that it includes all attributes of R, that is, it is a superkey of R
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Simplified Testing for BCNF

– R1 and R2 in BCNF – Lossless-join decomposition – Dependency preserving
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Testing for BCNF

To check if a non-trivial dependency in F+ causes a violation of BCNF
– Testing for JK L requires a join
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Redundancy in 3NF
There is some redundancy in this schema Equivalent to example in book:

Banker-schema = (branch-name, customer-name, banker-name) banker-name branch-name branch-name customer-name banker-name b1 b1 b1 c1 c2 c1 k1 k1 k2

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3NF Decomposition Algorithm

Fourth Normal Form
Overall Database Design Process
2
Boyce-Codd Normal Form
A relation schema R is in BCNF with respect to a set F of functional dependencies if for all functional dependencies in F+ of the form , where R and R, at least one of the following holds:
– Test Ri for BCNF with respect to the
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Testing Decomposition with F
A
relation Ri in a decomposition of R
– for every set of attributes Ri, check that + (the
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3NF and BCNF

If a relation is in BCNF it is in 3NF (since in BCNF one of the first two conditions above must hold)

Third condition is a minimal relaxation of BCNF to ensure dependency preservation

Final decomposition R1, R3, R4
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Testing Decomposition with
A
+ F
relation Ri in a decomposition of R
restriction of F to Ri (that is, all FDs in F+ that contain only attributes from Ri)
数据库原理
Chapter 7 Relational Database Design(c1)
王建民 清华大学软件学院 2003年/秋
Main Contents

Boyce-Codd Normal Form Third Normal Form Multivalued Dependencies


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Example of BCNF Decomposition
R = (branch-name, branch-city, assets, customer-name, loan-number, amount) F = {branch-name assets branch-city loan-number amount branch-name} Key = {loan-number, customer-name} Decomposition
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Example of 3NF

R = (J, K, L) Two candidate keys, JK and JL
– R is in 3NF
JK L JK is a superkey L K K is contained in a candidate key

BCNF decomposition has (JL) and (LK) F = {JK L, L K}
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BCNF Decomposition Algorithm
result := {R}; done := false; compute F+; while (not done) do if (there is a schema Ri in result that is not in BCNF) then begin let be a nontrivial functional dependency that holds on Ri such that Ri is not in F+, and ? = ; result := (result – Ri) (Ri – ) (, ); end else done := true; Ri Ri--
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BCNF and DP
It is not always possible to get a BCNF decomposition that is dependency preserving, ’overnormalize’ R = (J, K, L) F = {JK L, L K} Two candidate keys = JK and JL R is not in BCNF Any decomposition of R will fail to preserve JK L
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Testing for 3NF
Testing for 3NF has been shown to be NP-hard Need to check only FDs in F, need not check all FDs in F+ Use attribute closure to check, for each dependency , if is a superkey If is not a superkey, we have to verify if each attribute in is contained in a candidate key of R
attribute closure of ) either includes no attribute of Ri- , or includes all attributes of Ri – If the condition is violated by some in F, the dependency (+ - ) Ri can be shown to hold on Ri, and Ri violates BCNF – We use above dependency to decompose Ri

There are some situations where
– BCNF is not dependency preserving, and – Efficient checking for FD violation on updates is
important

Solution
– define a weaker normal form, called Third Normal

To check if a relation schema R with a given set of functional dependencies F is in BCNF, it suffices to check only the dependencies in the given set F for violation of BCNF, rather than checking all dependencies in F+
Form(3NF)
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Third Normal Form

A relation schema R is in third normal form if for all: in F+ at least one of the following holds:
– is trivial (i.e., )
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No Test on a Decomposition of R

Consider R (A, B, C, D), with F = { A B, B C}
– Decompose R into R1(A,B) and R2(A,C,D) – Neither of the dependencies in F contain only
attributes from (A,C,D) so we might be mislead into thinking R2 satisfies BCNF – In fact, dependency A C in F+ shows R2 is not in BCNF

However, using only F is incorrect when testing a relation in a decomposition of R

– R1 = (branch-name, branch-city, assets) – R2 = (branch-name, customer-name, loan-number, amount) – R3 = (branch-name, loan-number, amount) – R4 = (customer-name, loan-number)
– is a superkey for R – Each attribute A in – is contained in a
candidate key for R

Can the candidate key be replaced by superkey? Should each attribute be in the same candidate key?
– We can show that if none of the dependencies in F
causes a violation of BCNF, then none of the dependencies in F+ will cause a violation of BCNF either

is trivial (i.e., ) is a superkey for R
3
Example
R = (A, B, C) F = {A B B C} Key = {A} R is not in BCNF Decomposition R1 = (A, B), R2 = (B, C)
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Main Contents

Boyce-Codd Normal Form Third Normal Form Multivalued Dependencies


Fourth Normal Form
Overall Database Design Process
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Motivation of 3NF