Parcial 2 Flashcards
Fundamental, Additional, Union Compatibility
Relational Algebra Operators
Selection, Projection, Union, Set, Cartessian
Fundamental
Intersection, 0-join, Natural join, semijoin, division
Additional
Same degree, corresponding attributes
Union compatibility
Produces a horizontal subset of the operand relation
Selection
Produces vertical slice if a relation
Projection
Result contains tuples that are in R or in S, but not both (duplicates removed)
R, S should be union-compatible
Union
Result contains all tuples that are in R, but not in S
Set Difference
The result of R × S is a relation of degree (k1+ k2) and consists
of all (n1* n2)-tuples where each tuple is a concatenation of one
tuple of R with one tuple of S.
Cartesian
Typical set intersection, R, S union-compatible
Intersection
There are various forms of join. The
primary classification is between inner join and outer join.
0-Join
defined over the set of attributes A, by relation
S, defined over the set of attributes B, is the subset of tuples of R that
participate in the join of R with S.
Semijoin
T = R ÷ S gives T of degree k1-k2 [i.e., T(Y) where Y = A-B] such that for a
tuple t to appear in T, the values in t must appear in R in combination with every
tuple in S
Division
*Specify the properties that the result should hold
*Tuple relational calculus
*Domain relational calculus
Relational Calculus
Query of the form {t|F{t}} where
t is a tuple variable
F is a well-formed formula
Tuple Relational Calculus
Query of the form x1, x2, …, xn|F(x1, x2, …, xn) where
F is a well-formed formula in which x1, x2, …, xn are the free variables
Domain Relational Calculus
An interconnected
collection of autonomous
computers that are
capable of exchanging
information among
themselves.
Computer network
Hosts (nodes, end
systems)
Switches
Communication link
Components
Network of networks
Internet
WAN, LAN, MAN
Types of networks according to scale
✦ Distance between any two nodes > 20km and can go as high as thousands of kms
✦ Long delays due to distance traveled
✦ Heterogeneity of transmission media
✦ Speeds of 150Mbps to 10Gbps (OC192 on the backbone)
WAN
✦ Limited in geographic scope (usually < 2km)
✦ Speeds 10-1000 Mbps
✦ Short delays and low noise
LAN
In between LAN and WAN
MAN
Irregular, Bus, Star, Ring, MESH
Types of networks according to Topology
Unicast, multi-point
Communication Schemes
One or more (direct or indirect) links between each pair of nodes
Communication always between two nodes
Receiver and sender are identified by their addresses included in the message
header
Message may follow one of many links between the sender and receiver using
switching or routing
Unicast (Point-to-point)
Messages are transmitted over a shared channel and received by all the nodes
Each node checks the address and if it not the intended recipient, ignores
Multi-cast: special case
✦ Message is sent to a subset of the nodes
Multi-point(Broadcast)
Twisted pair
*Coaxial
*Fiber optic cable
*Satellite
*Microwave
*Wireless
Communication alternatives
Header, text. Block Error Check
Packet Format
Source addres, Destination addres, Message number, Packet number, Acknowledgment, Control Information
Parts of header
Software that ensures error-free, reliable and efficient communication
between hosts
Communication Protocols
