Lecture2.ppt

Entity-Relationship Model  Chapter 7: Entity-Relationship Model Design Process Modeling Constraints E-R Diagram Design Issues Weak Entity Sets Extended E-R Features Design of the Bank Database Reduction to Relation Schemas Database Design UML ...

Entity-Relationship Model  Chapter 7: Entity-Relationship Model Design Process Modeling Constraints E-R Diagram Design Issues Weak Entity Sets Extended E-R Features Design of the Bank Database Reduction to Relation Schemas Database Design UML  Design Phases The initial phase of database design is to characterize fully the data needs of the prospective database users. Next, the designer chooses a data model and, by applying the concepts of the chosen data model, translates these requirements into a conceptual schema of the database. A fully developed conceptual schema also indicates the functional requirements of the enterprise. In a “specification of functional requirements”, users describe the kinds of operations (or transactions) that will be performed on the data.  Design Phases (Cont.) The process of moving from an abstract data model to the implementation of the database proceeds in two final design phases. Logical Design – Deciding on the database schema. Database design requires that we find a “good” collection of relation schemas.  Business decision – What attributes should we record in the database?  Computer Science decision – What relation schemas should we have and how should the attributes be distributed among the various relation schemas? Physical Design – Deciding on the physical layout of the database  Design Approaches Entity Relationship Model (covered in this chapter)  Models an enterprise as a collection of entities and relationships  Entity: a “thing” or “object” in the enterprise that is distinguishable from other objects – Described by a set of attributes  Relationship: an association among several entities  Represented diagrammatically by an entity-relationship diagram: Normalization Theory (Later in this course)  Formalize what designs are bad, and test for them Outline of the ER Model  ER model -- Database Modeling The ER data mode was developed to facilitate database design by allowing specification of an enterprise schema that represents the overall logical structure of a database. The ER model is very useful in mapping the meanings and interactions of real-world enterprises onto a conceptual schema. Because of this usefulness, many database-design tools draw on concepts from the ER model. The ER data model employs three basic concepts:  entity sets,  relationship sets,  attributes. The ER model also has an associated diagrammatic representation, the ER diagram, which can express the overall logical structure of a database graphically.  Entity Sets An entity is an object that exists and is distinguishable from other objects.  Example: specific person, company, event, plant An entity set is a set of entities of the same type that share the same properties.  Example: set of all persons, companies, trees, holidays An entity is represented by a set of attributes; i.e., descriptive properties possessed by all members of an entity set.  Example: instructor = (ID, name, street, city, salary ) course= (course_id, title, credits) A subset of the attributes form a primary key of the entity set; i.e., uniquely identifiying each member of the set. Entity Sets -- instructor and student instructor_ID instructor_name student-ID student_name  Relationship Sets A relationship is an association among several entities Example: 44553 (Peltier) advisor 22222 (Einstein) student entity relationship set instructor entity A relationship set is a mathematical relation among n  2 entities, each taken from entity sets {(e1, e2, … en) | e1  E1, e2  E2, …, en  En} where (e1, e2, …, en) is a relationship  Example: (44553,22222)  advisor Relationship Set advisor  Relationship Sets (Cont.) An attribute can also be associated with a relationship set. For instance, the advisor relationship set between entity sets instructor and student may have the attribute date which tracks when the student started being associated with the advisor  Degree of a Relationship Set binary relationship  involve two entity sets (or degree two).  most relationship sets in a database system are binary. Relationships between more than two entity sets are rare. Most relationships are binary. (More on this later.)  Example: students work on research projects under the guidance of an instructor.  relationship proj_guide is a ternary relationship between instructor, student, and project  Mapping Cardinality Constraints Express the number of entities to which another entity can be associated via a relationship set. Most useful in describing binary relationship sets. For a binary relationship set the mapping cardinality must be one of the following types:  One to one  One to many  Many to one  Many to many  Mapping Cardinalities One to one One to many Note: Some elements in A and B may not be mapped to any elements in the other set  Mapping Cardinalities Many to one Many to many Note: Some elements in A and B may not be mapped to any elements in the other set  Complex Attributes Attribute types:  Simple and composite attributes.  Single-valued and multivalued attributes  Example: multivalued attribute: phone_numbers  Derived attributes  Can be computed from other attributes  Example: age, given date_of_birth Domain – the set of permitted values for each attribute Composite Attributes  Redundant Attributes Suppose we have entity sets:  instructor, with attributes: ID, name, dept_name, salary  department, with attributes: dept_name, building, budget We model the fact that each instructor has an associated department using a relationship set inst_dept The attribute dept_name appears in both entity sets. Since it is the primary key for the entity set department, it replicates information present in the relationship and is therefore redundant in the entity set instructor and needs to be removed. BUT: when converting back to tables, in some cases the attribute gets reintroduced, as we will see later.  Weak Entity Sets Consider a section entity, which is uniquely identified by a course_id, semester, year, and sec_id. Clearly, section entities are related to course entities. Suppose we create a relationship set sec_course between entity sets section and course. Note that the information in sec_course is redundant, since section already has an attribute course_id, which identifies the course with which the section is related. One option to deal with this redundancy is to get rid of the relationship sec_course; however, by doing so the relationship between section and course becomes implicit in an attribute, which is not desirable.  Weak Entity Sets (Cont.) An alternative way to deal with this redundancy is to not store the attribute course_id in the section entity and to only store the remaining attributes section_id, year, and semester. However, the entity set section then does not have enough attributes to identify a particular section entity uniquely; although each section entity is distinct, sections for different courses may share the same section_id, year, and semester. To deal with this problem, we treat the relationship sec_course as a special relationship that provides extra information, in this case, the course_id, required to identify section entities uniquely. The notion of weak entity set formalizes the above intuition. A weak entity set is one whose existence is dependent on another entity, called its identifying entity; instead of associating a primary key with a weak entity, we use the identifying entity, along with extra attributes called discriminator to uniquely identify a weak entity. An entity set that is not a weak entity set is termed a strong entity set.  Weak Entity Sets (Cont.) 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. The relationship associating the weak entity set with the identifying entity set is called the identifying relationship. Note that the relational schema we eventually create from the entity set section does have the attribute course_id, for reasons that will become clear later, even though we have dropped the attribute course_id from the entity set section. E-R Diagrams  Entity Sets Entities can be represented graphically as follows: Rectangles represent entity sets. Attributes listed inside entity rectangle Underline indicates primary key attributes  Relationship Sets Diamonds represent relationship sets. Relationship Sets with Attributes  Roles Entity sets of a relationship need not be distinct  Each occurrence of an entity set plays a “role” in the relationship The labels “course_id” and “prereq_id” are called roles.  Cardinality Constraints We express cardinality constraints by drawing either a directed line (), signifying “one,” or an undirected line (—), signifying “many,” between the relationship set and the entity set. One-to-one relationship between an instructor and a student :  A student is associated with at most one instructor via the relationship advisor  A student is associated with at most one department via stud_dept  One-to-Many Relationship one-to-many relationship between an instructor and a student  an instructor is associated with several (including 0) students via advisor  a student is associated with at most one instructor via advisor,  Many-to-One Relationships In a many-to-one relationship between an instructor and a student,  an instructor is associated with at most one student via advisor,  and a student is associated with several (including 0) instructors via advisor  Many-to-Many Relationship An instructor is associated with several (possibly 0) students via advisor A student is associated with several (possibly 0) instructors via advisor  Total and Partial Participation Total participation (indicated by double line): every entity in the entity set participates in at least one relationship in the relationship set participation of student in advisor relation is total  every student must have an associated instructor Partial participation: some entities may not participate in any relationship in the relationship set  Example: participation of instructor in advisor is partial  Notation for Expressing More Complex Constraints A line may have an associated minimum and maximum cardinality, shown in the form l..h, where l is the minimum and h the maximum cardinality A minimum value of 1 indicates total participation. A maximum value of 1 indicates that the entity participates in at most one relationship A maximum value of * indicates no limit. Instructor can advise 0 or more students. A student must have 1 advisor; cannot have multiple advisors Notation to Express Entity with Complex Attributes  Expressing Weak Entity Sets In E-R diagrams, a weak entity set is depicted via a double rectangle. We underline the discriminator of a weak entity set with a dashed line. The relationship set connecting the weak entity set to the identifying strong entity set is depicted by a double diamond. Primary key for section – (course_id, sec_id, semester, year) E-R Diagram for a University Enterprise Reduction to Relation Schemas  Reduction to Relation Schemas Entity sets and relationship sets can be expressed uniformly as relation schemas that represent the contents of the database. A database which conforms to an E-R diagram can be represented by a collection of schemas. For each entity set and relationship set there is a unique schema that is assigned the name of the corresponding entity set or relationship set. Each schema has a number of columns (generally corresponding to attributes), which have unique names.  Representing Entity Sets A strong entity set reduces to a schema with the same attributes student(ID, name, tot_cred) A weak entity set becomes a table that includes a column for the primary key of the identifying strong entity set section ( course_id, sec_id, sem, year )  Representing Relationship Sets A many-to-many relationship set is represented as a schema with attributes for the primary keys of the two participating entity sets, and any descriptive attributes of the relationship set. Example: schema for relationship set advisor advisor = (s_id, i_id) Representation of Entity Sets with Composite Attributes Composite attributes are flattened out by creating a separate attribute for each component attribute  Example: given entity set instructor with composite attribute name with component attributes first_name and last_name the schema corresponding to the entity set has two attributes name_first_name and name_last_name  Prefix omitted if there is no ambiguity (name_first_name could be first_name) Ignoring multivalued attributes, extended instructor schema is  instructor(ID, first_name, middle_initial, last_name, street_number, street_name, apt_number, city, state, zip_code, date_of_birth) Representation of Entity Sets with Multivalued Attributes A multivalued attribute M of an entity E is represented by a separate schema EM Schema EM has attributes corresponding to the primary key of E and an attribute corresponding to multivalued attribute M Example: Multivalued attribute phone_number of instructor is represented by a schema: inst_phone= ( ID, phone_number) Each value of the multivalued attribute maps to a separate tuple of the relation on schema EM  For example, an instructor entity with primary key 22222 and phone numbers 456-7890 and 123-4567 maps to two tuples: (22222, 456-7890) and (22222, 123-4567)  Redundancy of Schemas Many-to-one and one-to-many relationship sets that are total on the many-side can be represented by adding an extra attribute to the “many” side, containing the primary key of the “one” side Example: Instead of creating a schema for relationship set inst_dept, add an attribute dept_name to the schema arising from entity set instructor  Redundancy of Schemas (Cont.) For one-to-one relationship sets, either side can be chosen to act as the “many” side  That is, an extra attribute can be added to either of the tables corresponding to the two entity sets If participation is partial on the “many” side, replacing a schema by an extra attribute in the schema corresponding to the “many” side could result in null values  Redundancy of Schemas (Cont.) The schema corresponding to a relationship set linking a weak entity set to its identifying strong entity set is redundant. Example: The section schema already contains the attributes that would appear in the sec_course schema Advanced Topics  Non-binary Relationship Sets Most relationship sets are binary There are occasions when it is more convenient to represent relationships as non-binary. E-R Diagram with a Ternary Relationship Cardinality Constraints on Ternary Relationship We allow at most one arrow out of a ternary (or greater degree) relationship to indicate a cardinality constraint For example, an arrow from proj_guide to instructor indicates each student has at most one guide for a project If there is more than one arrow, there are two ways of defining the meaning.  For example, a ternary relationship R between A, B and C with arrows to B and C could mean 1. Each A entity is associated with a unique entity from B and C or 2. Each pair of entities from (A, B) is associated with a unique C entity, and each pair (A, C) is associated with a unique B  Each alternative has been used in different formalisms  To avoid confusion we outlaw more than one arrow  Specialization Top-down design process; we designate sub-groupings within an entity set that are distinctive from other entities in the set. These sub-groupings become lower-level entity sets that have attributes or participate in relationships that do not apply to the higher-level entity set. Depicted by a triangle component labeled ISA (e.g., instructor “is a” person). Attribute inheritance – a lower-level entity set inherits all the attributes and relationship participation of the higher-level entity set to which it is linked.  Specialization Example Overlapping – employee and student Disjoint – instructor and secretary Total and partial  Representing Specialization via Schemas Method 1:  Form a schema for the higher-level entity  Form a schema for each lower-level entity set, include primary key of higher-level entity set and local attributes schema attributes person ID, name, street, city student ID, tot_cred employee ID, salary  Drawback: getting information about, an employee requires accessing two relations, the one corresponding to the low-level schema and the one corresponding to the high-level schema Representing Specialization as Schemas (Cont.) Method 2:  Form a schema for each entity set with all local and inherited attributes schema attributes person ID, name, street, city student ID, name, street, city, tot_cred employee ID, name, street, city, salary  Drawback: name, street and city may be stored redundantly for people who are both students and employees  Generalization A bottom-up design process – combine a number of entity sets that share the same features into a higher-level entity set. Specialization and generalization are simple inversions of each other; they are represented in an E-R diagram in the same way. The terms specialization and generalization are used interchangeably. Design Constraints on a Specialization/Generalization Completeness constraint -- specifies whether or not an entity in the higher-level entity set must belong to at least one of the lower- level entity sets within a generalization.  total: an entity must belong to one of the lower-level entity sets  partial: an entity need not belong to one of the lower-level entity sets Partial generalization is the default. We can specify total generalization in an ER diagram by adding the keyword total in the diagram and drawing a dashed line from the keyword to the corresponding hollow arrow-head to which it applies (for a total generalization), or to the set of hollow arrow- heads to which it applies (for an overlapping generalization). The student generalization is total: All student entities must be either graduate or undergraduate. Because the higher-level entity set arrived at through generalization is generally composed of only those entities in the lower-level entity sets, the completeness constraint for a generalized higher-level entity set is usually total  Aggregation Consider the ternary relationship proj_guide, which we saw earlier Suppose we want to record evaluations of a student by a guide on a project  Aggregation (Cont.) Relationship sets eval_for and proj_guide represent overlapping information  Every eval_for relationship corresponds to a proj_guide relationship  However, some proj_guide relationships may not correspond to any eval_for relationships  So we can’t discard the proj_guide relationship Eliminate this redundancy via aggregation  Treat relationship as an abstract entity  Allows relationships between relationships  Abstraction of relationship into new entity  Aggregation (Cont.) Eliminate this redundancy via aggregation without introducing redundancy, the following diagram represents:  A student is guided by a particular instructor on a particular project  A student, instructor, project combination may have an associated evaluation  Representing Aggregation via Schemas To represent aggregation, create a schema containing  Primary key of the aggregated relationship,  The primary key of the associated entity set  Any descriptive attributes In our example:  The schema eval_for is: eval_for (s_ID, project_id, i_ID, evaluation_id)  The schema proj_guide is redundant. Design Issues  Entities vs. Attributes Use of entity sets vs. attributes Use of phone as an entity allows extra information about phone numbers (plus multiple phone numbers)  Entities vs. Relationship sets Use of entity sets vs. relationship sets Possible guideline is to designate a relationship set to describe an action that occurs between entities Placement of relationship attributes For example, attribute date as attribute of advisor or as attribute of student  Binary Vs. Non-Binary Relationships Although it is possible to replace any non-binary (n-ary, for n > 2) relationship set by a number of distinct binary relationship sets, a n-ary relationship set shows more clearly that several entities participate in a single relationship. Some relationships that appear to be non-binary may be better represented using binary relationships  For example, a ternary relationship parents, relating a child to his/her father and mother, is best replaced by two binary relationships, father and mother  Using two binary relationships allows partial information (e.g., only mother being known)  But there are some relationships that are naturally non-binary  Example: proj_guide Converting Non-Binary Relationships to Binary Form In general, any non-binary relationship can be represented using binary relationships by creating an artificial entity set.  Replace R between entity sets A, B and C by an entity set E, and three relationship sets: 1. RA, relating E and A 2. RB, relating E and B 3. RC, relating E and C  Create an identifying attribute for E and add any attributes of R to E  For each relationship (ai , bi , ci) in R, create 1. a new entity ei in the entity set E 2. add (ei , ai ) to RA 3. add (ei , bi ) to RB 4. add (ei , ci ) to RC  Converting Non-Binary Relationships (Cont.) Also need to translate constraints  Translating all constraints may not be possible  There may be instances in the translated schema that cannot correspond to any instance of R  We can avoid creating an identifying attribute by making E a weak entity set (described shortly) identified by the three relationship sets  E-R Design Decisions The use of an attribute or entity set to represent an object. Whether a real-world concept is best expressed by an entity set or a relationship set. The use of a ternary relationship versus a pair of binary relationships. The use of a strong or weak entity set. The use of specialization/generalization – contributes to modularity in the design. The use of aggregation – can treat the aggregate entity set as a single unit without concern for the details of its internal structure. Summary of Symbols Used in E-R Notation Symbols Used in E-R Notation (Cont.)  Alternative ER Notations Chen, IDE1FX, … Alternative ER Notations Chen IDE1FX (Crows feet notation)  UML UML: Unified Modeling Language UML has many components to graphically model different aspects of an entire software system UML Class Diagrams correspond to E-R Diagram, but several differences.  ER vs. UML Class Diagrams *Note reversal of position in cardinality constraint depiction  ER vs. UML Class Diagrams ER Diagram Notation Equivalent in UML *Generalization can use merged or separate arrows independent of disjoint/overlapping  UML Class Diagrams (Cont.) Binary relationship sets are represented in UML by just drawing a line connecting the entity sets. The relationship set name is written adjacent to the line. The role played by an entity set in a relationship set may also be specified by writing the role name on the line, adjacent to the entity set. The relationship set name may alternatively be written in a box, along with attributes of the relationship set, and the box is connected, using a dotted line, to the line depicting the relationship set.