Joke: No money this week

Two friends meet on a Miami street. One looked forlorn, and
almost on the verge of tears. The other man said, "Hey, how
come you look like the whole world caved in?"

The sad fellow said, "Let me tell you. Three weeks ago, an
uncle died and left me forty thousand dollars."

"That's not bad."

"Hold on, I'm just getting started. Two weeks ago, a cousin
I never knew kicked the bucket , and left me eighty-five
thousand free and clear."

"Sounds like you should be grateful."

"Last week my great aunt passed away. I inherited almost a
quarter of a million."

"Then how come you look so glum?"

"This week........ nothing!"

Relationships And Relationship Sets

A relationship is an association between several entities.
A relationship set is a set of relationships of the same type.
A relationship type R among n entity types E1, E2,....., En defines a set of associations or a relationship set among entities from these types.
In ER diagrams, relationship types are displayed as diamond-shaped boxes, which are connected by straight lines to the rectangular boxes representing the participating entity types. The relationship name is displayed in the diamond-shaped box.

RELATIONSHIP DEGREE : The degree of a relationship type is the number of participating entity types. A relationship type of degree two is called binary, and one with degree three is called ternary.




ROLE NAMES and RECURSIVE RELATIONSHIPS :
Role names signifies the role that a participating entity from the entity type plays in each relationship instance, and helps to explain what the relationship means.
Role names are not necessary in relationship types where all the participating entity types are distinct, since each entity type name can be used as a role name. However, in some cases, the same entity type participates more than once in a relationship type in different roles. In such cases the role name becomes essential for distinguishing the meaning of each participation. Such relationships are called recursive relationships.


MAPPING CONSTRAINTS :
An E-R scheme may define certain constraints to which the contents of a database must conform.
* Mapping Cardinality : It expresses the number of entities to which another entity can be associated via a relationship. For binary relationship sets between entity sets A and B, the mapping cardinality must be one of:
1. One-to-one: An entity in A is associated with at most one entity in B, and an entity in B is associated with at most one entity in A.
2. One-to-many: An entity in A is associated with any number in B. An entity in B is associated with at most one entity in A.
3. Many-to-one: An entity in A is associated with at most one entity in B. An entity in B is associated with any number in A.
4. Many-to-many: Entities in A and B are associated with any number from each other.
The appropriate mapping cardinality for a particular relationship set depends on the real world being modeled.
* Existence Dependencies: if the existence of entity X depends on the existence of entity Y, then X is said to be existence dependent on Y. (Or we say that Y is the dominant entity and X is the subordinate entity.)
For example,
o Consider account and transaction entity sets, and a relationship log between them.
o This is one-to-many from account to transaction.
o If an account entity is deleted, its associated transaction entities must also be deleted.
o Thus account is dominant and transaction is subordinate.

Mathematics with a twist

ROMANCE MATHEMATICS
~~~~~~~~~~~~

Smart man + smart woman = romance
Smart man + dumb woman = affair
Dumb man + smart woman = marriage
Dumb man + dumb woman = pregnancy

OFFICE ARITHMETIC

Smart boss + smart employee = profit
Smart boss + dumb employee = production
Dumb boss + smart employee = promotion
Dumb boss + dumb employee = overtime

GENERAL EQUATIONS & STATISTICS

A woman worries about the future until she gets a husband.
A man never worries about the future until he gets a wife.
A successful man is one who makes more money than his wife can spend.
A successful woman is one who can find such a man.

HAPPINESS

To be happy with a man, you must understand him a lot and love him a little.
To be happy with a woman, you must love her a lot and not try to understand her at all.

LONGEVITY

Married men live longer than single men do, but married men are a lot more willing to die.

PROPENSITY TO CHANGE

A woman marries a man expecting he will change, but he doesn't.
A man marries a woman expecting that she won't change, and she does.

DISCUSSION TECHNIQUE

A woman has the last word in any argument.
Anything a man says after that is the beginning of a new argument.