voc midterm 2 Flashcards
many 2 many
The maximum multiplicity (i.e. the cardinality) in each direction is “many”
association class
association that specifies a many cardinality in the multiplicity constraints of both ends of the association.
derived attributes
a value that can be computed by using values in other attributes within a table
junction table
for every many to many association in the database that will store all the relationship instances
junction table keys
pk is fk from the 2 parents
many to many with history
design pattern that allows a many to many associtation to broken up by using an association class that contains a desicrimnator attribute. this allows us to tell the difference between two identical instances with something like a date time stamp.
discriminator attribute
allows us to discriminate between the muliple pairing of the same attributes between classes.
functional dependencies
an attribute that is dependent on another atribute.
functional determines
an attribute that affects the state of another attribute
subkeys
an attribute that functionally determines only a subset of the attributes in the table
4 part process to fix subkeys
Replace R by two schema, R1 and R2 as described in the following steps.
Assign R1 the attributes in the union of the attributes in the subkey FD. That is R1={W ∪ Z}. Since W→Z, by definition, W is a superkey of R1.
Assign R2 the set of attributes {R - Z}, that is, all the attributes in R except those in Z, the attributes on the right-hand-side of the subkey FD
Both R1 and R2 share W in common. In R1, W is a superkey and in R2 it becomes a foreign key.
lossless join property of a decomposition
This property, whereby the information modeled in one database is not lost when the database is redesigned
normalization
a procedure or set of procedures to insure a database is well designed. mostly by eliminating duplicate information ie: subkeys
third normal form
a well designed db.
null
a special constant value in the database represents an absent value
repeated attributes
attributes that repeat in the same table ie: phone number, cell phone, home phone, work phone, second phone…
weak entity
an class that can not exist on its own. a class within a class. ie: phone class for contact.
multivalued attribute
storing mutiple values in an attribute.
an plural attribute