Chapter 6 Flashcards
three types of knowledge
- Entity: organisation or a person
- organisational knowledge: retain or increase knowledge when people leave or join an organisation
- databases: permanent knowledge
attributes/ fields
columns
records/ tuples
rows
unique key
makes it possible to link different tables to each other
candidate keys
when multiple keys are possible
primary key
the most suitable key of the candidate keys
join-operation
action where several related tables are linked in complex SQL queries to provide as much information as possible
First Normal Form (1NF)
- only atomic values: value cannot be divided into separate forms
- only 1 value in the domain: not several values per box
Second Normal Form (2NF)
- meets 1NF
- if there is a composite key, there are no partial dependencies: one of the attributes cannot depend on one of the two columns instead of both
third normal form (3NF)
- meets 2NF
2. there are no transitive dependencies: there are no attributes that are only indirectly linked to the primary key
entities
things that are important to an organisation and can be both immaterial and physical
attributes
describe an entity and thus represent the properties of a particular entity
relationships
links between different entities
cardinalities
indicate how many numbers of a given entity are related to numbers from another entity
one to one
one element of entity A is related to one element of entity B (|)
one to many
one element of entity A is associated with multiple elements in entity B
many to many
multiple A entities are related to multiple B entities
optional entity
relationship may not occur for certain entities (O)
lower cardinalities
minimum number of occurences
upper cardinalities
maximum number of events
unary/recursive relationships
relationships that an entity has with itself (film and sequel)
tenary relationships
relationships between 3 entities
N-ary relationships
number of entities that are connected through relationships can in theory be endless
Data definition language
used to create, delete and modify databases and tables –> structure!
data manipulation language
used to receive, insert or change and remove rows –> content!
data control language
users- and content-management –> access (not important for the course)
relational database
data is organised in one (or more) related tables
entity relationship model
describes the relationship between physical or conceptual matters that are important for the preparation of the database
structured query language
default language to communicate with database systems
