Anomalies—Design Problems

Anomalies—Design Problems • Redundancy • Update Anomalies (modification, insertion, deletion) HasReservationFor Redundancy: e.g. data value computable from other data values & constraints—e.g., Kennedy Modification Anomaly: e.g., change Kennedy room to have 3 beds instead of 2—must update all redundant values consistently. Insertion Anomaly: e.g., add a new room (the Gold room)—necessarily yields a null. Deletion Anomaly: e.g., G4 cancels reservation—the fact that the Green room is room 5 is lost. Guest RoomNr RName NrBeds G1 1 Kennedy 2 G2 1 Kennedy 2 G3 1 Kennedy 2 G4 5 Green 1 G5 3 Carter 2 G5 2 Nixon 2 G5 4 Blue 1 RoomNr  RName, NrBeds RName  RoomNr, NrBeds

Decomposition Can Resolve Anomalies Guest RoomNr G1 1 G2 1 G3 1 G4 5 G5 3 G5 2 G5 4 RoomNr RName NrBeds 1 Kennedy 2 2 Nixon 2 3 Carter 2 4 Blue 1 5 Green 1 RoomNr  RName, NrBeds RName  RoomNr, NrBeds • Modification Anomaly—2 beds to 3 in Kennedy room, only one change • Insertion Anomaly—Adding Gold room, no null values • Deletion Anomaly—G4 cancels, room 5 is still there • Redundancy—None

Decomposition in General Relation R decomposed into {X1, X2, …, Xn} - X1 R, X2 R, …, Xn R and X1U X2 U … U Xn = R - r1(X1) = πX1R, …, rn(Xn) = πXnR - e.g. R = ABCDE OK: {AB, BCD, AE} NOT OK: {ABCD, DEF} NOT OK: {AB, BCD} Cannot just decompose arbitrarily - May lose information - May not remove redundancy & update anomalies

Lossy Decomposition Guest RoomNr G1 1 G2 1 G3 1 G4 5 G5 3 G5 2 G5 4 RName NrBeds Kennedy 2 Nixon 2 Carter 2 Blue 1 Green 1 Applying |X| = full cross product = original e.g. Asserts (for example): <G1, 1, Blue, 1>, which is false

Overlapping Attributes is Not Enough Guest NrBeds G1 2 G2 2 G3 2 G4 1 G5 2 G5 1 RoomNr RName NrBeds 1 Kennedy 2 2 Nixon 2 3 Carter 2 4 Blue 1 5 Green 1 Applying |X| = Natural Join = original e.g. Asserts (for example): <G1, 3, Carter, 2>, but G1 does not have a reservation for room 3 If overlapping attributes include a superkey of one (or both) of the decomposed relations, the join is lossless. e.g., see original decomposition

Boyce-Codd Normal Form (BCNF) A condition that guarantees no redundancy and no update anomalies (wrt the functional dependencies). Let U be a set of attributes and let F be a set of FDs over U. Let R U. R is in Boyce-Codd Normal Form (BCNF) if for every nontrivial FD XY F+ such that XY R, X is a superkey of R. • Example: for U = Guest RoomNr RName NrBeds • and F = {RoomNr  RName, NrBeds; RName  RoomNr, NrBeds} • Guest RoomNr RName NrBeds is not in BCNF • Guest RoomNr & RoomNr RName NrBeds are both in BCNF

Boyce-Codd Normal Form (BCNF) A condition that guarantees no redundancy and no update anomalies (wrt the functional dependencies). Let U be a set of attributes and let F be a set of FDs over U. Let R U. If for every nontrivial FD XY F+ that applies to R, X is a superkey of R, then R is in BCNF. BCNF tells us both: 1. When we need to decompose 2. How to decompose

How do we decompose? • Answer: Make the attributes XY of the offending FD a schema and remove the Y attributes from the rest, R–Y (this assumes X ∩ Y = Ø; if not, XY–X will be an offending FD) • Since (R–Y) ∩ XY = X when X ∩ Y = Ø, if X is a superkey in XY, we have the condition we need for the decomposition to be lossless. • We are notguaranteed that either R–Y or XY is in BCNF, so we may have to repeat the process. • Hint: To arrive at the end quickly, make Y have as many attributes as possible (but still maintain X ∩ Y = Ø ) • When do we decompose? • Answer: When there is a nontrivial FD, XY  F+ such that XY R and X is not a superkey

Example Consider: Reservations(Guest, RoomNr, RName, NrBeds) With FDs: RoomNrRName, NrBeds RNameRoomNr, NrBeds • Are there any FDs (implied or given) that violate BCNF? • RoomNrRName, NrBeds violates BCNF • RoomNr is not a superkey because RoomNrGuest • Split the relation in two (XY, and R–Y): • 1. XY: The violating FD: {RoomNr, RName, NrBeds} • 2. R–Y: {Guest, RoomNr} And thus minimal keys: Guest RoomNr and Guest RName

Example (cont.) • Consider the new relational schemas: • RoomNr, RName, NrBeds • F+= { RoomNrRName, NrBeds, RNameRoomNr, NrBeds, … NrBedsNrBeds, RoomNr, NrBedsRName} • Keys: RoomNr and RName • All FDs satisfy BCNF • Guest, RoomNr • Any two-attribute relation R(AB) is in BCNF • Proof: • Case1: There are no nontrivial FDs  only nontrivial FDs can violate BCNF. • Case2: AB holds, but BA does not. A is the key, and the only nontrivial FDs are AB (& AAB); no BCNF violation. • Case 3: BA holds, but AB does not. Symmetric to Case 2. • Case 4: Both AB and BA hold. A and B are both keys; no BCNF violation.

Example—Multiple Decompositions F = { AB  C, A  C, C  DE, D  EF } • R = ABCDEF is not in BCNF • e.g., C+ = CDEF so CDEF violates BCNF since CDEF applies and is non-trivial and since C is not a superkey • R–Y = ABC and XY = CDEF • ABC is not in BCNF • e.g., A+ = ACDEF so AC and A is not a superkey • R–Y = AB and XY = AC -- both are in BCNF (2 attributes) • CDEF is not in BCNF • e.g., D+ = DEF so DEF and D is not a superkey • R–Y = CD and XY = DEF -- both are in BCNF • We end up with {AB, AC, CD, DEF}

Dependency Preserving If we decompose R into X1, X2, … then if enforcing the FDs that hold on X1, X2, … is sufficient to guarantee that all FDs on R hold, the decomposition is dependency preserving. • Not Dependency Preserving: F = {ABC, CA} • ABC not in BCNF • {AC, BC} in BCNF • {CA}  F (Since AB+ = AB, ABC is not implied by {CA}.) • Dependency Preserving: F = {ABC, AC} • ABC not in BCNF • {AC, AB} in BCNF • {AC} F (Since AB+ = ABC, ABC is implied by {AC}.)

Is Our Previous Example Dependency Preserving? F = { AB  C, A  C, C  DE, D  EF } • We ended up with {AB, AC, CD, DEF} • {AC, CD, DEF}  F • since AC, ABC holds by augmentation & projection • since CD and DEF, CDE holds by accumulation & projection • {AB, AC, CD, DEF} is dependency preserving

3NF • BCNF can guarantee no redundancy or update anomalies, but cannot guarantee dependency preserving. • 3NF relaxes the requirements and can guarantee dependency preserving at the cost of not always removing all redundancy. • Let U be a set of attributes and let F be a set of FDs over U. Let R U. R is in Third Normal Form (3NF) if for every nontrivial FD XY  F+ such that XY R, X is a superkey of R or each attribute in Y–X appears in some candidate key of R.

3NF Examples • ABC {ABC, CA} (the earlier problematic example) • In 3NF (but not in BCNF) • C is not a superkey, but A is in a candidate key • ABC {AB, BC} • Not in 3NF • B is not a superkey, and C is not in a candidate key • ABCD {ABC, BD} • Not in 3NF • B is not a superkey, and D is not in a candidate key

3NF Motivation PickyGuest makes reservations only for one room Room PickyGuest Date {Room, Date  PickyGuest; PickyGuest  Room} • {Room, Date, PickyGuest}—Not in BCNF: PickyGuest+ {Room, Date, PickyGuest} • So Decompose to: {Date, PickyGuest}, {PickyGuest, Room} • Both in BCNF—no redundancy or update anomolies. • BUT can’t directly enforce: Room,Date  PickyGuest • Options: • Don’t decompose and live with redundancy (i.e., if pickyGuest changes favorite room, must change all pickyGuest reservations) • Decompose and write code to join the relations to check violations whenever there is a modification or insert • Always consider the business implications when making this decision. Maybe you really won’t want to keep the offending constraint (or even have code that checks it).

3 3 3 3 2 2 2 1 1 Basic Patterns of Violations A B C D E G • BCNF: None of these is OK • 3NF: OK i.e., GA, A is part of a candidate key • 2NF: & OK • 1NF: & & OK • 1NF requires atomic values atomic • i.e., the DB cannot address subatomic elements Address: “12 Maple, Boston, MA” vs. StreetNr, City, State “I do solemnly swear to make every attribute functionally depend on the key, the whole key, and nothing but the key.”

URA: Universal Relation Assumption • A “hidden” assumption about FD theory and normalization—the URA must hold. • URA • Informal: All relationships over any set of attributes of the database have the same meaning (semantic equivalence). Further, there are no nulls and no dangling tuples. • Formal: For any valid database instance, r1(R1), …, rn(Rn), Ri(r1 || … || rn) = ri for 1  i  n.

Semantics & Semantic Equivalence q = Room, Name(r || s) ?? Case 1: q is name of guest staying in room. Room  Guest and Guest  Name implies Room  Name. But the meaning of the implied relationship set, Room  Name, is the composition of the meanings of the relationship sets, Room  Guest and Guest  Name, which may or may not be the meaning of the relationship set q. Case 2: q is name of room.

FD Theory & Normalizationwith and without URA • With URA (Case 1) • q = Room,Name(r || s) • Room  Guest and Guest  Name and Guest is not a key • {Room, Guest, Name} is not in BCNF. • Decompose to: {Room, Guest} and {Guest, Name} • The DB has these two schemas. • Another way to see this is to note that Guest  Name is redundant • Delete it. • Map the two remaining relationships to schemas. • Without URA (Case 2) • q Room,Name(r || s) • Here Guest  Name is not redundant • Keep it. • Map the three relationships to three schemas.

Anomalies—Design Problems