# 1) List the summary of the notations for ER diagrams. Include symbols used in ER diagram and their meaning. 8 Marks (Jun / July2014)

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1 Question Paper Solutions UNIT-2 1) List the summary of the notations for ER diagrams. Include symbols used in ER diagram and their meaning. 8 Marks (Jun / July2014)

2 2) With respect to ER model explain with example. 4 Marks (Dec/Jan 2013) i) Strong entity: Regular entity types that do have a key attribute are sometimes called strong entity types. ii) Weak entity: entity types that do not have key attributes of their own are called weak entity types iii) Participation constraints: It specifies whether the existence of an entity depends on its being related to another entity via the relationship type. This constraint specifies the minimum number of relationship instances that each entity can participate in and is sometimes called the minimum cardinality constraint. iv) Cardinality ratio: The cardinality ratio for a binary relationship specifies the maximum number of relationship instances that an entity can participate in. v) Recurring relationships: 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 relationship types are called recursive relationship Atomic Attribute: Simple. Attribute that are not divisible are called simple or atomic attribute. For example, Street_name or Door_number. i.e. the division of composite attribute. Recursive Relationship Type An relationship type whose with the same participating entity type in distinct roles Example: In the SUPERVISION relationship EMPLOYEE participates twice in two distinct roles: supervisor (or boss) role supervisee (or subordinate) role, each relationship instance relates two distinct EMPLOYEE entities: One employee in supervisor role One employee in supervisee role. Weak Entity Types An entity that does not have a key attribute. A weak entity must participate in an identifying relationship type with an owner or identifying entity type. Entities are identified by the combination of: A partial key of the weak entity type The particular entity they are related to in the identifying entity type. 3. Explain hoe role names are assigned in case of recursive relationships? Illustrate this concept with an example.. 8 Marks (Jun / July 2013) A relationship can relate two entities of the same entity type; for example, a SUPERVISION relationship type relates one EMPLOYEE (in the role of supervisee) to another EMPLOYEE (in the role of supervisor). This is called a recursive relationship type.

3 4) What is meant by partial key? Explain. 4 Marks (Jun / July2014) A weak entity type must participate in an identifying relationship type with an owner or identifying entity type Entities are identified by the combination of: A partial key of the weak entity type The particular entity they are related to in the identifying entity type A weak entity type usually has a partial key, which is the set of attributes that can uniquely identify weak entities that are related to the same owner entity. For example, Name of DEPENDENT is the partial key 5. Define an entity and an attribute, explain the different types of attributes that occur in an ERdiagram model, with an example. 8 Marks (Dec /Jan 2013, Dec/Jan 2014) Our focus now is on the second phase, conceptual design, for which The Entity Relationship (ER) Model is a popular high-level conceptual data model. In the ER model, the main concepts are entity, attribute, and relationship. Entities and Attributes Entity: An entity represents some "thing" (in the miniworld) that is of interest to us, i.e., about which we want to maintain some data. An entity could represent a physical object (e.g., house, person, automobile, and widget) or a less tangible concept (e.g., company, job, academic course). Attribute: An entity is described by its attributes, which are properties characterizing it. Each attribute has a value drawn from some domain (set of meaningful values). Example: A PERSON entity might be described by Name, BirthDate, Sex, etc., attributes, each having a particular value. What distinguishes an entity from an attribute is that the latter is strictly for the purpose of describing the former and is not, in and of itself, of interest to us. It is sometimes said that an entity has an independent existence, whereas an attribute does not. In performing data

5 cases a particular entity might not have an applicable value for a particular attribute. Or that value may be unknown. Or, in the case of a multi-valued attribute, the appropriate value might be the empty set. Example: The attribute DateOfDeath is not applicable to a living person and its correct value may be unknown for some persons who have died. In such cases, we use a special attribute value (non-value?), called null. There has been some argument in the database literature about whether a different approach (such as having distinct values for not applicable and unknown) would be superior. 6. Define the following with an example. 20 Marks (Jun/July 2013 & June/July 2105) i) Weak Entity Types: An entity type that has no set of attributes that qualify as a key is called weak. (Ones that do are strong.). An entity of a weak identity type is uniquely identified by the specific entity to which it is related (by a so-called identifying relationship that relates the weak entity type with its so-called identifying or owner entity type) in combination with some set of its own attributes (called a partial key). Example: A DEPENDENT entity is identified by its first name together with the EMPLOYEE entity to which it is related via DEPENDS_ON. (Note that this wouldn't work for former heavyweight boxing champion George Foreman's sons, as they all have the name "George"!) Because an entity of a weak entity type cannot be identified otherwise, that type has a total participation constraint (i.e., existence dependency) with respect to the identifying relationship. This should not be taken to mean that any entity type on which a total participation constraint exists is weak. For example, DEPARTMENT has a total participation constraint with respect to MANAGES, but it is not weak. In an ER diagram, a weak entity type is depicted with a double rectangle and an identifying relationship type is depicted with a double diamond. ii) Participation constraint Participation: specifies whether or not the existence of an entity depends upon its being related to another entity via the relationship. Total participation (or existence dependency): To say that entity type A is constrained to participate totally in relationship R is to say that if (at some moment in time) R's instance set is { (a1, b1), (a2, b2),... (am, bm) }, then (at that same moment) A's instance set must be { a1, a2,..., am }. In other words, there can be no member of A's instance set that does not participate in at least one instance of R. According to our informal description of COMPANY, every employee must be assigned to some department. That is, every employee instance must participate in at least one instance of WORKS_FOR, which is to say that EMPLOYEE satisfies the total participation constraint with respect to the WORKS_FOR relationship. In an ER diagram, if entity type A must participate totally in relationship type R, the two are connected by a double line. Partial participation: the absence of the total participation constraint! (E.g., not every employee has to participate in MANAGES; hence we say that, with respect to MANAGES, EMPLOYEE participates partially. This is not to say that for all employees to be managers is not allowed; it only says that it need not be the case that all employees are managers. iii) Cardinality ratio: often in order to make a relationship type be an accurate model of the miniworld concepts that it is intended to represent, we impose certain constraints that limit the possible corresponding relationship sets. (That is, a constraint may make "invalid" a particular set of instances for a relationship type.)

6 There are two main kinds of relationship constraints (on binary relationships). For illustration, let R be a relationship set consisting of ordered pairs of instances of entity types A and B, respectively. Cardinality ratio: 1:1 (one-to-one): Under this constraint, no instance of A may participate in more than one instance of R; similarly for instances of B. In other words, if (a1, b1) and (a2, b2) are (distinct) instances of R, then neither a1 = a2 nor b1 = b2. Example: Our informal description of COMPANY says that every department has one employee who manages it. If we also stipulate that an employee may not (simultaneously) play the role of manager for more than one department, it follows that MANAGES is 1:1. 1: N (one-to-many): Under this constraint, no instance of B may participate in more than one instance of R, but instances of A are under no such restriction. In other words, if (a1, b1) and (a2, b2) are (distinct) instances of R, then it cannot be the case that b1 = b2. Example: CONTROLS is 1:N because no project may be controlled by more than one department. On the other hand, a department may control any number of projects, so there is no restriction on the number of relationship instances in which a particular department instance may participate. For similar reasons, is also 1:N. N:1 (many-to-one): This is just the same as 1:N but with roles of the two entity types reversed. Example: WORKS_FOR and DEPENDS_ON are N: 1. M: N (many-to-many): Under this constraint, there are no restrictions. (Hence, the term applies to the absence of a constraint!) Example: WORKS_ON is M:N, because an employee may work on any number of projects and a project may have any number of employees who work on it. Suppose that, in designing a database, we decide to include a binary relationship R as described above (which relates entity types A and B, respectively). To determine how R should be constrained, with respect to cardinality ratio, the questions you should ask are these: May a given entity of type B be related to multiple entities of type A? May a given entity of type A be related to multiple entities of type B? The pair of answers you get maps into the four possible cardinality ratios as follows: (yes, yes) --> M:N (yes, no) --> N:1 (no, yes) --> 1:N (no, no) --> 1:1 iv) Ternary relationship Also note that, in our COMPANY example, all relationship instances will be ordered pairs, as each relationship associates an instance from one entity type with an instance of another (or the same, in the case of SUPERVISES) relationship type. Such relationships are said to be binary, or to have degree two. Relationships with degree three (called ternary) or more are also possible, although not as common. SUPPLY (perhaps not the best choice for a name) has as instances ordered triples of suppliers, parts, and projects, with the intent being that inclusion of the ordered triple (s2, p4, j1), for example, indicates that supplier s2 supplied part p4 to project j1). v) Recursive relationship An exception to this rule occurs when the same entity type plays two (or more) roles in the same relationship. (Such relationships are said to be recursive, which I find to be a misleading use of that term. A better term might be self-referential.) For example, in each instance of a SUPERVISES relationship set, one employee plays the role of supervisor and the other plays the role of supervisee.

7 7) Design an ER Diagram for keeping track of Information about Bank Database, Taking into account 4 entities? 8 Marks (Dec /Jan 2013)

8 8) Describe how to map the following Scenario s in ER Model to schema, with suitable example: 8 Marks (Dec/Jan 2014) i) Strong Entity:- For each regular (strong) entity type E in the ER schema, create a relation R that includes all the simple attributes. For composite attributes in E, convert them to simple attributes in R by converting each component to an attribute. For multi-value attributes in E, leave them to step 6 Choose one of the key attributes of E as the primary key for R If the chosen key of E is composite, the set of simple attributes that form it will together form the primary key of R ii) One-to-one Relationship Mapping of Binary Relation Types with 1:1 cardinality ratio (no new relation is Data Base created) For each binary 1:1 relationship type R in the ER schema, identify the relations S and T that correspond to the entity types participating in R. There are three possible approaches: Foreign Key approach: Choose one of the relations say S and include a foreign key in S the primary key of T. It is better to choose an entity type with total participation in R in the role of S. Include in S all the attributes from R Example: 1:1 relation MANAGES is mapped by choosing the Participating entity type DEPARTMENT to serve in the role of S, because Its participation in the MANAGES relationship type is total. 9) Define and explain Partial Key, with example? 4 Marks (Dec/Jan 2014, Jun / July2013) An attribute is a Partial Key if a Key from a related entity type must be used in conjunction with the attribute in question to uniquely identify instances of a corresponding entity set.

9 10) What is meant by recursive relationship? Bring out the importance of role names in recursive relationship, with an example? 8 Marks (Jun / July 2013) A relationship can relate two entities of the same entity type; for example, a SUPERVISION relationship type relates one EMPLOYEE (in the role of supervisee) to another EMPLOYEE (in the role of supervisor). This is called a recursive relationship type. A relationship type can have attributes; for example, HoursPerWeek of WORKS_ON; its value for each relationship instance describes the number of hours per week that an EMPLOYEE works on a PROJECT. Structural constraints on relationships: Cardinality ratio (of a binary relationship): 1:1, 1: N, N: 1, or M: N. Participation constraint (on each participating entity type): total (called existence dependency) or partial. Alternative (min, max) notation for relationship structural constraints: Specified on each participation of an entity type E in a relationship type R. Specifies that each entity e in E participates in at least min and at most max relationship instances in R. Default(no constraint): min=0, max=n.

10 11) Design an ER diagram for an insurance company. Assume suitable entity types like CUSTOMER, AGENT, BRANCH, POLICY, PAYMENT and the relation between them. 10 marks (June/ July 2015) (Dec 14/Jan 15)

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12 13) What are structural constraints on relationship types? Explain with an example? 10 Marks (Dec 14/Jan 15) Participation: specifies whether or not the existence of an entity depends upon its being related to another entity via the relationship. Total participation (or existence dependency): To say that entity type A is constrained to participate totally in relationship R is to say that if (at some moment in time) R's instance set is {(a1, b1), (a2, b2)... (am, bm)}, then (at that same moment) A's instance set must be {a1, a2... am}. In other words, here can be no member of A's instance set that does not participate in at least one instance of R. According to our informal description of COMPANY, every employee must be assigned to some department. That is, every employee instance must participate in at least one instance of WORKS_FOR, which is to say that EMPLOYEE satisfies the total participation constraint with respect to the WORKS_FOR relationship. In an ER diagram, if entity type A must participate totally in relationship type R, the two are connected by a double line. Partial participation: the absence of the total participation constraint! (E.g., not every employee has to participate in MANAGES; hence we say that, with respect to MANAGES, EMPLOYEE participates partially. This is not to say that for all employees to be managers is not allowed; it only says that it need not be the case that all employees are managers. 14) What are weak entity type? Explain the role of partial key in design of weak entity type? 5 Marks (Dec 14/Jan 15) An entity type that has no set of attributes that qualify as a key is called weak. (Ones that do are strong.) An entity of a weak identity type is uniquely identified by the specific entity to which it is related (by a so-called identifying relationship that relates the weak entity type with its socalled identifying or owner entity type) in combination with some set of its own attributes (called a partial key). Example: A DEPENDENT entity is identified by its first name together with the EMPLOYEE entity to which it is related via DEPENDS_ON. (Note that this wouldn't work for former heavyweight boxing champion George Foreman's sons, as they all have the name "George"!) Because an entity of a weak entity type cannot be identified otherwise, that type has a total participation constraint (i.e., existence dependency) with respect to the identifying relationship. This should not be taken to mean that any entity type on which a total participation constraint exists is weak. For example, DEPARTMENT has a total participation constraint with respect to MANAGES, but it is not weak. In an ER diagram, a weak entity type is depicted with a double rectangle and an identifying relationship type is depicted with a double diamond.

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