Transcript Document
Conceptual Design Using the Entity-Relationship (ER) Model 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., Zeynep works in Sales department. Relationship Set: Collection of similar relationships. Same entity set could participate in different relationship sets, or in different “roles” in the same set. Database Management Systems, 5 ER Model Basics (Contd.) CREATE TABLE Works_In( Relationship sets can also have ssn CHAR(11), 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 Consider Works_In: An employee can work in many departments; a dept can have many employees, manyto-many, as in the previous figure In contrast, each dept has at most one manager, according to the key constraint on Manages, one-tomany… Database Management Systems, 7 Key Constraints since name ssn dname lot Employees Database Management Systems, did Manages budget Departments 8 Types of relationship sets 1-to-1 Database Management Systems, 1-to Many Many-to-1 Many-to-Many 9 Translating ER Diagrams with Key Constraints-1 Map relationship to a table: Example: manages relationship – Note that did is the key now! – Separate tables for Employees and Departments. CREATE TABLE Manages( ssn CHAR(11), did INTEGER, since DATE, PRIMARY KEY (did), FOREIGN KEY (ssn) REFERENCES Employees, FOREIGN KEY (did) REFERENCES Departments) THIS MAPPING CAN NOT INCLUDE TOTAL PARTICIPATION! Database Management Systems, 10 Translating ER Diagrams with Key Constraints-2 Since each department has a unique manager, we could instead combine Manages and Departments, create relationship Dept_Mgr. Every dept. may not have a manager, null values allowed for ssn. CREATE TABLE Dept_Mgr( did INTEGER, dname CHAR(20), budget REAL, ssn CHAR(11), since DATE, PRIMARY KEY (did), FOREIGN KEY (ssn) REFERENCES Employees) This shows key constraint but not total participation because of absence of NOT NULL on ssn Database Management Systems, 11 Participation Constraints:Total vs. Partial 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). For total participation, every did value in Departments table must appear in a row of the Manages table, with a non-null ssn value, indicating that every department has a manager. Database Management Systems, 12 Participation constraints (cont.) since name ssn dname did lot Employees Database Management Systems, Manages budget Departments 13 Participation Constraints(Cont.) In the corresponding SQL mapping, the Null value for the ssn in Dept_Mgr is not allowed… ON DELETE NO ACTION specification is actually the default case: – It ensures that an Employee tuple cannot be deleted while it is pointed to by a Dept_Mgr Database Management Systems, 14 Participation Constraints in SQL Capture of participation constraints using one entity set, in a binary relationship. 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, 15 Participation Constraints (cont.) Many participations cannot be captured by SQL easily For example, take the two relationship set as in the following Manages and Works_In ER diagram. since name ssn dname did lot Employees Manages budget Departments Works_In since Database Management Systems, 16 Participation Constraints (cont.) How to ensure total participation in SQL relation corresponding to Works_In relationship? Note that ssn and did alone cannot be keys for ths relationship…. We have to guarantee that every did value in Departments appears in a tuple of Works_In – This tuple must also have NOT NULL values in the foreign key fields – This cannot be achieved similar to Manages relationship, because did cannot be taken as the key for Works_In relationship – This situation needs assertions in SQL, which exist in the current SQL releases… Database Management Systems, 17 Participation Constraints (cont.) Another constraint that cannot be express in SQL easily is the requirement that each employee must manage at least one department. – Again, 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… this is left out of scope of this lecture Database Management Systems, 18 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: one owner many weak entities 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, 19 Weak Entities (cont.) Employee is owner entity, Dependent is weak entity, Policy is the relationship set: name ssn lot Employees Database Management Systems, cost Policy pname age Dependents 20 Translating Weak Entity Sets Weak entity set and identifying relationship set are translated into a single table. ssn cannot be null. – 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, 21 ISA (`is a’) Hierarchies As in C++, or other PLs, attributes are inherited. If we declare A ISA B, every A entity is also considered to be a B entity. Database Management Systems, 22 name ISA (`is a’) Hierarchies-2 ssn lot Employees Subclasses are defined first! hourly_wages hours_worked ISA contractid Hourly_Emps Contract_Emps Overlap constraints: Can Zeynep be an Hourly_Emps as well as a Contract_Emps entity? (posibility:Allowed using OVERLAP) Covering constraints: Does every Employees entity also have to be an Hourly_Emps or a Contract_Emps entity? (allowed using COVER) Database Management Systems, 23 ISA (`is a’) Hierarchies-3 Reasons for using ISA: – To add descriptive attributes specific to a subclass. – To identify the set of entities that participate in a particular relationship, e.g. only Senior Employees can be allowed to be Managers of Departments. Database Management Systems, 24 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 are easy, those involving just Hourly_Emps require a join to get some attributes. Alternative: two subentities exist: Hourly_Emps and Contract_Emps: – Each employee must be in one of these two subclasses. Database Management Systems, 25 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 may be more redundant fields, repeated in the subs. Overlap and covering constraints can only be expressed using assertions. Database Management Systems, 26 name ssn Aggregation-1 Suppose: entity set Projects each Projects is sponsored by at least one Departments each Departments that sponsors a Projects might assign employees to monitor sponsorship Intuitively… Monitors should be a relationship set that associates a Sponsors relation (versus a Projects or Departments) with an Employees entity. Database Management Systems, lot Employees since started_on pid pbudget Projects did Sponsors dname budget Departments 27 name ssn Aggregation-2 Used when we have to model a relationship involving (entity sets and) a relationship set. Aggregation allows us to treat a relationship set as an entity set for purposes of participation in (other) relationships. Database Management Systems, lot Employees Monitors since started_on pid pbudget Projects until did Sponsors dname budget Departments 28 Aggregation vs. Ternary relation name ssn lot Employees Each relationship in an aggregation may have a different descriptive attribute like since and until. Suppose: No need to record until attribute of Monitors one can use ternary relationship Sponsors2 Then… since started_on pid pbudget Projects did Sponsors2 dname budget Departments But suppose we have constraint that: Each sponsorship (of a project by a department) be monitored by at most one employee? Then… one can’t do it with Sponsors2 Need aggregated relationship Sponsors Database Management Systems, 29 Aggregation vs. Ternary relation (Contd.) name ssn lot Employees Monitors Monitors is a distinct relationship, with a descriptive attribute (until). until since started_on pid pbudget Projects did Sponsors dname budget Departments Also, can say that each sponsorship is monitored by at most one employee. Database Management Systems, 30 Conceptual Design Using the ER Model Design choices: – Should a concept be modeled as an entity or an attribute? – Should a concept be modeled 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 refinement of the schema: – Relational schema obtained from ER diagram is a good first step. But ER design is subjective & can’t express certain constraints; so this relational schema may need refinement. Database Management Systems, 31 Entity vs. Attribute Should address be an attribute of Employees or an entity (connected to Employees by a relationship)? Depending upon the purpose 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 modeled as an entity (since attribute values are atomic). Database Management Systems, 32 Entity vs. Attribute (Contd.) Works_In2 does not allow an employee to work in a department for two or more periods. from name ssn dname lot Employees to did Works_In2 budget Departments Similar to the problem of wanting to record several addresses for an employee: we want to record several values of the descriptive attributes for each instance of this relationship. Database Management Systems, 33 Entity vs. Attribute (Contd.) Use Duration entity to allow more than one duration per relationship… name dname ssn lot Employees from Database Management Systems, did Works_In3 Duration budget Departments to 34 Entity vs. Relationship Following ER diagram OK if a manager gets a separate discretionary budget for each dept. since dbudget name ssn dname lot Employees Database Management Systems, did Manages2 budget Departments 35 Entity vs. Relationship What if a manager gets a discretionary budget is sum that covers by that employee? Then the previous ER diagram will cause – – all depts managed Redundancy of dbudget, where the same value is stored for each dept managed by the same manager. Misleading as it suggests dbudget tied to managed dept rather than emloyee. Instead a separate entity Mgr_Appts can be used with apptnum as key. This way the redundancy is eliminated! name ssn dname lot Employees did Manages3 budget Departments since apptnum Mgr_Appts dbudget Database Management Systems, 36 Binary vs. Ternary Relationships If each policy is owned by just 1 employee: – Key constraint on Policies would mean policy can only cover 1 dependent! Every policy must be owned by some employee Dependents is a weak entity set name ssn pname lot Employees Bad design: with or without dark arrow Database Management Systems, age Dependents Covers Policies policyid cost 37 Binary vs. Ternary Relationships What are the additional constraints in the 2nd diagram? A policy cannot be owned by more than one employee, every policy must be owned by some employee an cover at least one dependent, dependent is a weak entity set. Beneficiary is an identifying relationship for the weak entity Dependents, ie. Each dependent will have one policy only… name ssn pname lot Dependents Employees Purchaser Better design policyid Database Management Systems, age Beneficiary Policies With two binary relationships cost 38 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. CREATE TABLE Dependents ( pname CHAR(20), Participation constraints lead to age INTEGER, policyid INTEGER, NOT NULL constraints. PRIMARY KEY (pname, policyid). FOREIGN KEY (policyid) REFERENCES Policies, ON DELETE CASCADE) Database Management Systems, 39 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 with descriptive attribute qty relates entity sets Parts, Departments and Suppliers. No combination of binary relationships is an adequate substitute for ternary relationship: – 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, 40 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, 41 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, 42 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, 43 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, 44 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, 45 Example name addr Bookshops name Books Sells license Note: license = Books, yes, no Attend name Database Management Systems, Likes Reader manf Bookshops sell som Books. readers like to read some Books. Readers attend some Bookshops. addr 46 Relationship Set The current “value” of an entity set is the set of entities that belong to it. Example: the set of all Bakkals in our database. The “value” of a relationship is a set of lists of currently related entities, one from each of the related entity sets. Database Management Systems, 47 Examples A University database contains information about professors (identified by social security number, or SSN) and courses (identified by courseid). Professors teach courses. Each of the following situations concerns the Teaches relationship set. For each situation, draw an ER diagram that describes it (assuming that no further constraints hold). Database Management Systems, 48 Examples (cont’) Professors can teach the same course in several semesters, and each offering must be recorded. 2) Professors can teach the same course in several semesters, and only the most recent such offering needs to be recorded. (Assume this condition applies to all subsequent questions.) 3) Every professor must teach some course 4) Every professor teaches exactly one course (no more, no less). 5) Every professor teaches exactly one course (no more no less), and every course must be taught by some professor. 6) Now assume that certain courses can be taught by a team of professors jointly, but it is possible that no one professor in a team can teach the course. Model this situation introducing additional entity sets and relationship sets if necessary. Database Management Systems, 49 1) Example (cont’) 1) Professors can teach the same course in several semesters, and each offering must be recorded. Database Management Systems, 50 Example (cont’) 2) Professors can teach the same course in several semesters, and only the most recent such offering needs to be recorded. (Assume this condition applies to all subsequent questions.) Database Management Systems, 51 Example (cont’) 3) Every professor must teach some course Database Management Systems, 52 Example (cont’) 4) Every professor teaches exactly one course (no more, no less). Database Management Systems, 53 Example (cont’) 5) Every professor teaches exactly one course (no more no less), and every course must be taught by some professor. Database Management Systems, 54 Example (cont’) 6) Now assume that certain courses can be taught by a team of professors jointly, but it is possible that no one professor in a team can teach the course. Model this situation introducing additional entity sets and relationship sets if necessary. Database Management Systems, 55 Database Management Systems, 56 Example A company database needs to store information about employees (identified by ssn, with salary and phone as attributes); departments (identified by dno, with dname and budget as attributes); and children of employees (with name and age as attributes). Employees work in departments; each department is managed by an employee; a child must be identified uniquely by name when the parent (who is an employee; assume that only one parent works for the company) is known. We are not interested in information about a child once the parent leaves the company. Database Management Systems, 57 Database Management Systems, 58