First page Back Continue Last page Graphics
Dependencies in a decomposition
Which dependencies hold in the R1 and R2?
- R = {A,B,C} and F = { A B, B C} and we replace R by { R1 , R2 } where R1 {A,B} and R2 = {B,C}.
- R = {A,B,C} and F = { A B, C B} and we replace R by { R1 , R2 } where R1 {A,B} and R2 = {A,C}.
- R = {A,B,C} and F = { A B, B C} and we replace R by { R1 , R2 } where R1 {A,B} and R2 = {A,C}
Notes:
The first example is trivial.
The second example shows that we lose the ability to express C->B without taking a join.
The third example shows that we lose B->C but we do have A->C in R2. This illustrates that we must take F+ before projecting it on the subrelations.