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Conceptual Design Using the Entity-Relationship (ER) Model Module 2, Lectures 3 Database Management Systems, 1 Database Design Process Requirements analysis – What data, what applications, what most frequent operations,… Conceptual database design – High level description of the data and the constraint – This step can use ER or similar high level models Logical database design – Convert database design into a database schema, e.g. relational db schema Database Management Systems, 2 Database Design Process (cont.) Schema refinement – Analyze the the collection of the data for potential problems and refine it Physical database design – Ensure that the design meets the performance requirements, based on used indexation, etc. Security design – Identify different user groups with different roles, so that data protection is enforced accordingly. Database Management Systems, 3 ER Model Basics ssn Entity: Real-world object distinguishable from other objects. – An entity is described (in DB) using a set of attributes. Entity Set: A collection of similar entities. E.g., all employees. name lot Employees ssn name lot 123-22-3666 Attishoo 48 231-31-5368 Smiley 22 131-24-3650 Smethurst 35 – All entities in an entity set have the same set of attributes. CREATE TABLE Employees – Each entity set has a key, uniquely (ssn CHAR(11), identifies it. name CHAR(20), – Each attribute has a domain. lot INTEGER, – Can map entity set to a relation easily. PRIMARY KEY (ssn)) 4 Database Management Systems, ER Model Basics (Contd.) name ssn lot since name ssn dname lot did Employees budget supervisor Employees Works_In Departments subordinate Reports_To Relationship: Association among 2 or more entities. E.g., Attishoo works in Pharmacy department. Relationship Set: Collection of similar relationships. – An n-ary relationship set R relates n entity sets E1 ... En; each relationship in R involves entities e1 E1, ..., en En Same entity set could participate in different relationship sets, or in different “roles” in same set. Database Management Systems, 5 ER Model Basics (Contd.) CREATE TABLE Works_In( Relationship sets can also have ssn CHAR(1), descriptive attributes (e.g., the did INTEGER, since attribute of Works_In). since DATE, In translating a relationship set PRIMARY KEY (ssn, did), to a relation, attributes of the FOREIGN KEY (ssn) relation must include: REFERENCES Employees, FOREIGN KEY (did) – Keys for each participating REFERENCES Departments) entity set (as foreign keys). This set of attributes forms superkey for the relation. – All descriptive attributes. Database Management Systems, ssn 123-22-3666 123-22-3666 231-31-5368 did 51 56 51 since 1/1/91 3/3/93 2/2/92 6 Key Constraints ssn since name dname lot did Consider Works_In: Manages Employees An employee can work in many departments; a dept can have many employees. In contrast, each dept has at most one manager, according to the 1-to Many Many-to-1 1-to-1 key constraint on Manages. * Translation to relational model? Database Management Systems, budget Departments Many-to-Many 7 Translating ER Diagrams with Key Constraints CREATE TABLE Manages( Map relationship to a table: ssn CHAR(11), did INTEGER, – Note that did is the key now! since DATE, PRIMARY KEY (did), – Separate tables for Employees and FOREIGN KEY (ssn) REFERENCES Employees, Departments. FOREIGN KEY (did) REFERENCES Departments) Since each department CREATE TABLE Dept_Mgr( has a unique manager, did INTEGER, we could instead combine Manages and dname CHAR(20), Departments. budget REAL, Every dept. may not have ssn CHAR(11), a manager, null values since DATE, allowed PRIMARY KEY (did), FOREIGN KEY (ssn) REFERENCES Employees) Database Management Systems, 8 Participation Constraints Every department have a manager – If so, this is a participation constraint: the participation of Departments in Manages is said to be total (vs. partial). Every did value in Departments table must appear in a row of the Manages table (with a non-null ssn value!) In the following query the Null value for the ssn in Dept_Mgr is not allowed… NO ACTION specification is actually the default case, if not mentioned – It ensures that an Employee tuple cannot be deleted while it is pointed to by a Dept_Mgr Database Management Systems, 9 Participation Constraints in SQL We can capture participation constraints involving one entity set in a binary relationship, but little else (without resorting to CHECK constraints). CREATE TABLE Dept_Mgr( did INTEGER, dname CHAR(20), budget REAL, ssn CHAR(11) NOT NULL, since DATE, PRIMARY KEY (did), FOREIGN KEY (ssn) REFERENCES Employees, ON DELETE NO ACTION) Database Management Systems, 10 Participation Constraints (cont.) Many participations cannot be captured by SQL-92 For example, take the following two relationship set (Manages and Works_In) ER diagram. How to ensure total participation in SQL relation corresponding Works_In relationship? We have to guarantee that every did value in Departments appears in a tuple of Works_In This tuple must also have non null values in the foreign key fields This cannot be achieved similar to Manages relationship, because did cannot be taken as key for Works_In relationship This situation needs assertions… Database Management Systems, 11 Participation Constraints (cont.) Another constraint that cannot be express in SQL is the requirement that each employee must manage at least one department. Such cases requires constraints that involve more than one table. In SQL, this is achieved using Assertions, which are constraints that associated to multiple relations or tables. Assertions are imposed by the CHECK clause in SQL, although its implementation is cumbersome. All the entities and the relationships can be mapped to one single relation! Database Management Systems, 12 Participation constraints (cont.) Two relationship set ER since name ssn dname did lot Employees Manages budget Departments Works_In since Database Management Systems, 13 Weak Entities A weak entity can be identified uniquely only by considering the primary key of another (owner) entity. Owner entity set and weak entity set must participate in a one-to-many relationship set. Weak entity set must have total participation in this identifying relationship set. A weak entity always has a partial key, it can only be uniquely defined if we take its key together with the key of the owner entity. Database Management Systems, 14 Weak Entities (cont.) Employee is owner entity, Dependent is weak entity, Policy is the relationship set, in this ER diagram. name ssn lot Employees Database Management Systems, cost Policy pname age Dependents 15 Translating Weak Entity Sets Weak entity set and identifying relationship set are translated into a single table. – When the owner entity is deleted, all owned weak entities must also be deleted. CREATE TABLE Dep_Policy ( pname CHAR(20), age INTEGER, cost REAL, ssn CHAR(11) NOT NULL, PRIMARY KEY (pname, ssn), FOREIGN KEY (ssn) REFERENCES Employees, ON DELETE CASCADE) Database Management Systems, 16 name ISA (`is a’) Hierarchies ssn lot Employees As in C++, or other PLs, hours_worked hourly_wages attributes are inherited. ISA contractid If we declare A ISA B, every A entity is also considered to be a B Contract_Emps Hourly_Emps entity. (Query answers should reflect this: unlike C++!) Overlap constraints: Can Joe be an Hourly_Emps as well as a Contract_Emps entity? (Allowed/disallowed) Covering constraints: Does every Employees entity also have to be an Hourly_Emps or a Contract_Emps entity? (Yes/no) Reasons for using ISA: – To add descriptive attributes specific to a subclass. – To identify entities that participate in a relationship. Database Management Systems, 17 Translating ISA Hierarchies to Relations General approach: – 3 relations: Employees, Hourly_Emps and Contract_Emps. Hourly_Emps: Every employee is recorded in Employees. For hourly emps, extra info recorded in Hourly_Emps (hourly_wages, hours_worked, ssn); must delete Hourly_Emps tuple if referenced Employees tuple is deleted). Queries involving all employees easy, those involving just Hourly_Emps require a join to get some attributes. Alternative: Just Hourly_Emps and Contract_Emps. – Hourly_Emps: ssn, name, lot, hourly_wages, hours_worked. – Each employee must be in one of these two subclasses. Database Management Systems, 18 Translating ISA Hierarchies to Relations(cont.) The second alternative is not applicable if there are employees who are neither of the subclasses. Also, with the second method there will be more redundant fields, repeated in the subs. Also, overlap and covering constraints can only be expressed using assertions, in SQL-92. Database Management Systems, 19 Aggregation Monitors has three attributes: ssn, (did, pid), until Used when we have to Sponsors has three attributes (did, pid), model a relationship since involving (entitity sets Obviously, not every sponsorship appears and) a relationship set. Aggregation allows us to in Monitors treat a relationship set as If the Sponsors has total participation in an entity set for Monitors, no separate relation is required purposes of for it. participation in (other) Monitors is a distinct relationship, relationships. with a descriptive attribute. Monitors mapped to Also, can say that each sponsorship table like any other relationship set. is monitored by at most one employee. Database Management Systems, 20 Aggregation Example name ssn lot Employees Monitors until started_on pid dname pbudget Projects Database Management Systems, did Sponsors budget Departments 21 Conceptual Design Using the ER Model Design choices: – Should a concept be modelled as an entity or an attribute? – Should a concept be modelled as an entity or a relationship? – Identifying relationships: Binary or ternary? Aggregation? Constraints in the ER Model: – A lot of data semantics can (and should) be captured. – But some constraints cannot be captured in ER diagrams. Need for further refining the schema: – Relational schema obtained from ER diagram is a good first step. But ER design subjective & can’t express certain constraints; so this relational schema may need refinement. Database Management Systems, 22 Entity vs. Attribute Should address be an attribute of Employees or an entity (connected to Employees by a relationship)? Depends upon the use we want to make of address information, and the semantics of the data: If we have several addresses per employee, address must be an entity (since attributes cannot be set-valued). If the structure (city, street, etc.) is important, e.g., we want to retrieve employees in a given city, address must be modelled as an entity (since attribute values are atomic). Database Management Systems, 23 Entity vs. Attribute (Contd.) name from to dname Works_In2 does not ssn lot did budget allow an employee to Departments Works_In2 Employees work in a department for two or more periods. Similar to the problem of wanting to record several addresses for an name dname employee: we want to ssn lot did budget record several values of the Works_In3 Departments Employees descriptive attributes for each instance of this Duration to from relationship. Database Management Systems, 24 Entity vs. Relationship First ER diagram OK if since dbudget name a manager gets a ssn lot did separate discretionary budget for each dept. Employees Manages2 What if a manager gets a discretionary budget that covers all name managed depts? ssn lot did – Redundancy of dbudget, Employees which is stored for each dept managed by the manager. Misleading: suggests dbudget apptnum tied to managed dept. Database Management Systems, Manages3 dname budget Departments dname budget Departments since Mgr_Appts dbudget 25 Binary vs. Ternary Relationships name ssn If each policy is owned by just 1 employee: – Key constraint on Policies would mean policy can only cover 1 dependent! What are the additional constraints in the 2nd diagram? Database Management Systems, pname lot Employees Dependents Covers Bad design Policies policyid cost name ssn age pname lot age Dependents Employees Purchaser Better design policyid Beneficiary Policies cost 26 Binary vs. Ternary Relationships (Contd.) CREATE TABLE Policies ( The key policyid INTEGER, constraints allow cost REAL, us to combine ssn CHAR(11) NOT NULL, Purchaser with PRIMARY KEY (policyid). Policies and FOREIGN KEY (ssn) REFERENCES Employees, Beneficiary with ON DELETE CASCADE) Dependents. Participation CREATE TABLE Dependents ( constraints lead to pname CHAR(20), NOT NULL age INTEGER, constraints. policyid INTEGER, What if Policies is PRIMARY KEY (pname, policyid). a weak entity set? FOREIGN KEY (policyid) REFERENCES Policies, ON DELETE CASCADE) Database Management Systems, 27 Binary vs. Ternary Relationships (Contd.) Previous example illustrated a case when 2 binary relationships were better than a ternary relationship. An example in the other direction: a ternary relation Contracts relates entity sets Parts, Departments and Suppliers, and has descriptive attribute qty. No combination of binary relationships is an adequate substitute: – S ``can-supply’’ P, D ``needs’’ P, and D ``deals-with’’ S does not imply that D has agreed to buy P from S. – How do we record qty? Database Management Systems, 28 Constraints Beyond the ER Model Functional dependencies: – e.g., A dept can’t order two distinct parts from the same supplier. Can’t express this wrt ternary Contracts relationship. – Normalization refines ER design by considering FDs. Inclusion dependencies: – Special case: Foreign keys (ER model can express these). – e.g., At least 1 person must report to each manager. (Set of ssn values in Manages must be subset of supervisor_ssn values in Reports_To.) Foreign key? Expressible in ER model? General constraints: – e.g., Manager’s discretionary budget less than 10% of the combined budget of all departments he or she manages. Database Management Systems, 29 Summary of Conceptual Design Conceptual design follows requirements analysis, – Yields a high-level description of data to be stored ER model popular for conceptual design – Constructs are expressive, close to the way people think about their applications. Basic constructs: entities, relationships, and attributes (of entities and relationships). Some additional constructs: weak entities, ISA hierarchies, and aggregation. Note: There are many variations on ER model. Database Management Systems, 30 Summary of ER (Contd.) Several kinds of integrity constraints can be expressed in the ER model: key constraints, participation constraints, and overlap/covering constraints for ISA hierarchies. Some foreign key constraints are also implicit in the definition of a relationship set. – Some of these constraints can be expressed in SQL only if we use general CHECK constraints or assertions. – Some constraints (notably, functional dependencies) cannot be expressed in the ER model. – Constraints play an important role in determining the best database design for an enterprise. Database Management Systems, 31 Summary of ER (Contd.) ER design is subjective. There are often many ways to model a given scenario! Analyzing alternatives can be tricky, especially for a large enterprise. Common choices include: – Entity vs. attribute, entity vs. relationship, binary or n-ary relationship, whether or not to use ISA hierarchies, and whether or not to use aggregation. Ensuring good database design: resulting relational schema should be analyzed and refined further. FD information and normalization techniques are especially useful. Database Management Systems, 32 Views Views are computed as needed using view definition Example CREATE VIEW B-Students(name, sid, course) AS SELECT S.name, S.sid, E.cid FROM Students S, Enrolled E WHERE S.sid=E.sid AND E.grade=‘B’ Whenever B-Students is used in a query, the view definition is first evaluated, before using B-Students in any other query operation. View concept provides logical data independence, as it can be used to mask the changes in the conceptual schema. Views can be defined taken security aspect into consideration, e.g., dbadmin, user, group, etc.. Database Management Systems, 33 Updates on Views The updateable views are the one defined on single base tables, without aggregate operations. Update on an updateable table implies update of the corresponding base table as well. A view can be dropped by DROP VIEW command. Deleting base tables is more restrictive, as they have to have RESTRICT or CASCADE options, concerning integrity… With RESTRICT, drop of a base is possible if no reference to it exist; with CASCADE, drop of base causes drop of all references recursively… Database Management Systems, 34