Normalization CMSC 461 Michael Wilson

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

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

Delete anomaly

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

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

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

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

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

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

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

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

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

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

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?

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

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

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

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