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Normalization CMSC 461 Michael Wilson. Anomalies  Poor relational database design can lead to the occurrence of anomalies  Anomalies that we tend to.

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Presentation on theme: "Normalization CMSC 461 Michael Wilson. Anomalies  Poor relational database design can lead to the occurrence of anomalies  Anomalies that we tend to."— Presentation transcript:

1 Normalization CMSC 461 Michael Wilson

2 Anomalies  Poor relational database design can lead to the occurrence of anomalies  Anomalies that we tend to encounter  Redundancy  Data is repeated unnecessarily in several tuples  Update anomalies  Data is updated in one place but not another  Deletion anomalies  Data is deleted from a relation and additional data is removed along with it unintentionally

3 Redundancy titleyearlengthgenrestudioStar Star Wars SciFiFoxCarrie Fisher Star Wars SciFiFoxMark Hamill Star Wars SciFiFoxHarrison Ford Gone With the Wind DramaMGMVivien Leigh Wayne’s World ComedyParamountDana Carvey Wayne’s World ComedyParamountMike Meyers

4 Delete anomaly

5 Redundancy  We have the movie title, movie year, film length, genre, and studio name repeated each and every time we want to add a new star to the relation

6 Update anomaly titleyearlengthgenrestudioStar Star Wars SciFiFoxCarrie Fisher Star Wars SciFiFoxMark Hamill Star Wars SciFiFoxHarrison Ford Gone With the Wind DramaMGMVivien Leigh Wayne’s World ComedyParamountDana Carvey Wayne’s World ComedyParamountMike Meyers

7 Update anomaly  We changed the film length for one of the entries  Forgot to update it for the rest of the entries  Now the data is inconsistent

8 Delete anomaly titleyearlengthgenrestudioStar Star Wars SciFiFoxCarrie Fisher Star Wars SciFiFoxMark Hamill Star Wars SciFiFoxHarrison Ford Gone With the Wind DramaMGMVivien Leigh Wayne’s World ComedyParamountDana Carvey Wayne’s World ComedyParamountMike Meyers

9 Delete anomaly titleyearlengthgenrestudioStar Star Wars SciFiFoxCarrie Fisher Star Wars SciFiFoxMark Hamill Star Wars SciFiFoxHarrison Ford Wayne’s World ComedyParamountDana Carvey Wayne’s World ComedyParamountMike Meyers

10 Delete anomaly  Say we wanted to remove Vivien Leigh as a star  We unintentionally removed all information about Gone With the Wind from our relation

11 How do we address these anomalies?  Same answer for all three  Decomposing relations  Split the attributes of a relation to make two new relations  Relatively simple operation

12 Decomposing a relation  With a relation R(A 1, A 2,…,A n ), you can decompose into relations S(B 1,B 2,…,B m ) and T(C 1,C 2,…,C k )  {A 1,A 2,…A n }={B 1,B 2,…,B m }U{C 1,C 2,…,C k }  S = π B1,B2,…,Bm (R)  T = π C1,C2,…,Ck (R)  In other words, take a subset of R’s attributes and stuff them into S, and take the rest of R’s attributes and stuff them into T

13 Decomposing our movies relation – movies2 titleyearlengthgenreStudio Star Wars sciFiFox Gone With the Wind DramaMGM Wayne’s World199295comedyParamount

14 Decomposing our movies relation – movies3 titleyearstar Star Wars1977Carrie Fisher Star Wars1977Mark Hamill Star Wars1977Harrison Ford Gone With the Wind 1939Vivien Leigh Wayne’s World1992Dana Carvey Wayne’s World1992Mike Meyers

15 Further decomposition?  There are still some anomaly possibilities in these resulting relations  Also, we would like to be able to handle movies named the same thing in the same year  How could we decompose them further to avoid this?

16 Normalization  There are normal forms that can be applied to relation schemas and databases  Organizing the attributes to reduce redundancy and dependency on one another  Done primarily through decomposing of relations  Many different normal forms

17 Normalization forms  You’ll see NF to mean “normal form”  1NF  Attributes are atomic. That is, they cannot be decomposed any further  2NF  Satisfied 1NF, and also all non-prime attributes are dependent wholly on a candidate key (no subsets)  Non-prime attributes are attributes not part of a candidate key

18 Third normal form (3NF)  The relation satisifies 2NF  Every non-prime attribute is directly dependent on every superkey of the relation  Directly dependent means that it is not transitive  If you have two FDs:  A→B  B→C  A→C is a transitive dependency  In other words, relations that satisfied A→B must only have the key A or attributes directly related to A

19 Bill Kent’s pledge  3NF: “Every non-key attribute must provide a fact about the key, the whole key, and nothing but the key.”  1NF – The key exists  2NF – Non-key attributes dependent on the whole key  3NF – Non-key attributes must be dependent on nothing but the key


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