Tentamen Flashcards
What does it mean for a natural join to not be lossless?
There may be tuples in the natural join of R1 and R2 that are not found in R.
What does it mean to be lossless?
All relations in the natural join of R1 and R2 are also in the original relation R.
What are the differences between relations and relationships?
Relations: bags of tuples
Relationships: Concept in ER, where it is about a connection between two concepts
What is a virtual view?
A view contains values from tables, a virtual view does not store these values, thus they change when the underlying data changes.
What is a materialized view?
A view contains values from tables, a material view stores the values of the view. This can be updated periodically or every time the underlying tables are updated.
What are indices?
An index is a tree-like sturcture, called a B-tree, in which you can easiliy find nodes that correspond to the tuples with a certaint value for the attributes. This makes it vary low cost to search in a B-tree.
However it is very “expensive” to create, and update. An index ofter is created for something for which we create a where statement.
How to find the BCNF violations?
Check all FD’s (also implied) to find all the FD’s that are not a super key for the entire relation.
How to find all FD’s that hold in decomposition?
Find all FD’s (also implied) that are in the relation
How to find the FD’s in a join?
Just all FD’s that you had before.
What is a weak entity set? How to find it?
A weak entity set is a set that hasn’t enought entities to identify itself. They have a double line.
What are referential-integrity constraints?
Any one that is a weak ES. (I believe just every many-one relationship).
