DBMS_An_Interactive_Tutorial.pdf

# Note that a label 1 h on an edge is equivalent to a

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Note that a label 1.. h on an edge is equivalent to a double line. For example, consider the following figure. The edge between loan and borrower has a cardinality constraint of 1..1, meaning the minimum and the maximum cardinality are both 1. That is, each loan must have exactly one associated customer. The limit 0.. on the edge from customer to borrower indicates that a customer can have zero or more loans. Thus, the relationship borrower is one to many from customer to loan, and further the participation of loan in borrower is total. Figure: Cardinality limits on relationship sets. Weak Entity Sets An entity set that do not have sufficient attributes to form a primary key is called a weak entity set . An entity set that has a primary key is termed a strong entity set . As an illustration, consider the entity set player , which has two attributes – name and number . Now, name cannot be a primary key as there might be more than one player with the same name. However, number is unique for a particular team, but different teams have the same player number . For example, there E R Figure: E-R notation for total participation of entity set in relationship.

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12 might be player A with number 1 of team X , and player A with number 1 of team Y . So, the entity set player cannot be complete without being somehow associated with the entity set team . As another illustration, consider the entity set payment , which has the three attributes: payment-number , payment-date , and payment-amount . Payment numbers are typically sequential numbers, starting from 1, generated separately for each loan payment. Thus, although each payment entity is distinct, payments for different loans may share the same payment number. Thus, this entity set does not have a primary key; it is a weak entity set. Figure: E-R diagram with a weak entity set. head2right For a weak entity set to be meaningful, it must be associated with another entity set, called the identifying or owner entity set . head2right Every weak entity must be associated with an identifying entity; that is, the weak entity set is said to be existence dependent on the identifying entity set. The identifying entity set is said to own the weak entity set that it identifies. head2right The relationship associating the weak entity set with the identifying entity set is called the identifying relationship . head2right The identifying relationship is many to one from the weak entity set to the identifying entity set, and the participation of the weak entity set in the relationship is total . head2right In our example, the identifying entity set for payment is loan , and a relationship loan-payment that associates payment entities with their corresponding loan entities is the identifying relationship.
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