Database Management Systems. Chapter 3 Part 2

Transcription

1 Database Management Systems Chapter 3 Part 2 The Relational Model Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 1 Logical DB Design: ER to Relational Entity sets to tables: CREATE TABLE ( CHAR(11), CHAR(20), INTEGER, CONSTRAINT EmKey PRIMARY KEY ()); Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 2

2 Relationship Sets to Tables In translating a relationship set to a relation, attributes of the relation must include: Primary keys for each participating entity set (as foreign keys). This set of attributes forms a superkey for the relation. The primary key is decided by the key constraint of the relationship. All descriptive attributes. Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 3 Binary Relationship d Each dept has at least one employee, and each employee works for at least one department, according to the key constraint on Works_In. Works_In Translation to relational model? Many-to-Many Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 4

3 d Works_In CREATE TABLE Works_In( CHAR(11), INTEGER, DATE, CONSTRAINT WIKey PRIMARY KEY (, ), CONSTRAINT SSNFK FOREIGN KEY () REFERENCES () ON DELETE CASCADE ON UPDATE NO ACTION, CONSTRAINT DIDFK FOREIGN KEY () REFERENCES () ON DELETE CASCADE ON UPDATE NO ACTION); Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 5 Binary Relationship d Each dept has at most one manager, according to the key constraint on Manages. Manages Translation to relational model? 1-to Many Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 6

4 Translating ER Diagrams with Key Constraints Map relationship to a table: Since each department has most one manager, there are no two tuples with the same but differ on the value, is the key now! Separate tables for and. Manages d CREATE TABLE Manages( CHAR(11), INTEGER, DATE, PRIMARY KEY (), FOREIGN KEY () REFERENCES (), FOREIGN KEY () REFERENCES ()); Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 7 Translating ER Diagrams with Key Constraints Since each department has a unique manager, we could instead combine Manages and. can take null values several departments have no managers. Manages d CREATE TABLE Dept_Mgr( INTEGER, d CHAR(20), REAL, CHAR(11), DATE, PRIMARY KEY (), FOREIGN KEY () REFERENCES ); Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 8

5 Review: Participation Constraints Does every department have a manager? If so, this is a participation constraint: the participation of in Manages is said to be total (vs. partial). Every value in table must appear in a row of the Manages table (with a non-null value!) d Manages Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 9 Participation Constraints in SQL This approach is good for one-to-many relationships, when entity set with key constraint also has a total participation constraint. (Two tables are combined) CREATE TABLE Dept_Mgr( INTEGER, d CHAR(20), REAL, CHAR(11) NOT NULL, DATE, PRIMARY KEY (), FOREIGN KEY () REFERENCES, ON DELETE NO ACTION); An tuple cannot be deleted while it is pointed to by a Dept_Mgr tuple Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 10

6 Translating ER Diagrams with Key Constraints Every department has at most one manager, an employee could only manage at most one department. should be unique. Manages d CREATE TABLE Dept_Mgr( INTEGER, d CHAR(20), REAL, chair_ CHAR(11), DATE, PRIMARY KEY (), UNIQUE (chair_), FOREIGN KEY (chair_) REFERENCES ()); Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 11 Binary Relationship to RM Many-to-many: The primary key of R includes all the attributes in the primary keys of A and B. One-to-many: The primary key of R is the same as B (i.e., the entity set on the many side, e.g. An employee could manage many dept., each dept. has at most one manager). One-to-one: R has two canate keys. The first (second) one is the same as A (B). One is primary key and the other is unique. Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 12

7 Review: 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 (1 owner, many weak entities). Weak entity set must have total participation in this identifying relationship set. cost p age Policy Dependents Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 13 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 ( p CHAR(20) NOT NULL, age INTEGER, cost REAL, CHAR(11) NOT NULL, PRIMARY KEY (p, ), FOREIGN KEY () REFERENCES () ON DELETE CASCADE); Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 14

8 Multiple-way Relationship Multi-way relationship set R: Create a table that includes the canate keys of the participating entity sets the attributes of R (if any). The primary key of the table includes all the attributes of the primary keys of the participating entity sets. Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 15 d Works_In2 address Locations capacity CREATE TABLE Works_In2( CHAR(11), INTEGER, address CHAR(20), DATE, PRIMARY KEY (,, address), FOREIGN KEY () REFERENCES (), FOREIGN KEY () REFERENCES (), FOREIGN KEY (address) REFERENCES Locations (address)); Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 16

9 supervisor subordinate Reports_To CREATE TABLE Reports_To( supervisor_ CHAR(11), subordinate_ CHAR(11), PRIMARY KEY (supervisor_, subordinate_), FOREIGN KEY (supervisor_) REFERENCES (), FOREIGN KEY (subordinate_) REFERENCES ()); Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 17 Review: ISA Hierarchies As in C++, or other PLs, attributes are inherited. hourly_wages If we declare A ISA B, every A entity is also considered to be a B entity. hours_worked ISA contractid Hourly_Emps Contract_Emps Overlap constraints: Can Joe be an Hourly_Emps as well as a Contract_Emps entity? (Allowed/disallowed) Covering constraints: Does every entity also have to be an Hourly_Emps or a Contract_Emps entity? (Yes/no) Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 18

10 Translating ISA Hierarchies to Relations General approach: 3 relations:, Hourly_Emps and Contract_Emps. Hourly_Emps: Every employee is recorded in. For hourly emps, extra info recorded in Hourly_Emps (hourly_wages, hours_worked, ); must delete Hourly_Emps tuple if referenced tuple is deleted). Queries involving all employees easy, those involving just Hourly_Emps require a join to get some attributes. Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 19 hourly_wages hours_worked ISA contractid Hourly_Emps Contract_Emps CREAT TABLE ( CHAR(10), CHAR(20), INTEGER, CONSTRAINT Key PRIMARY KEY ()); CREATE TABLE Hourly_Emps ( CHAR(10), hourly_wages REAL, hours_worked INTEGER, CONSTRAINT HourlyEmplsKey PRIMARY KEY (), FOREIGN KEY () REFERENCES ON DELETE CASCADE); Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 20

11 Translating ISA Hierarchies to Relations Alternative: Just Hourly_Emps and Contract_Emps. Hourly_Emps:,,, hourly_wages, hours_worked. Contract_Emps:,,, contractid. Each employee must be in one of these two subclasses. If an employee is both an Hourly_Emps and a Contract_Emps entity, then the same and values are stored in two tables. Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 21 Translating ER Diagram with Aggregation Monitors until pid started_on p d Projects Sponsors Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 22

12 CREATE TABLE ( CHAR(11), CHAR(20), INTEGER, PRIMARY KEY ()); CREATE TABLE ( INTEGER, d CHAR(20), REAL, PRIMARY KEY ()); CREATE TABLE Projects (pid INTEGER, started_on DATE, p REAL, PRIMARY KEY (pid)); CREATE TABLE Sponsors ( INTEGER, pid INTEGER, DATE, PRIMARY KEY (, pid), FOREIGN KEY () REFERENCES, FOREIGN KEY (pid) REFERENCES Projects); CREATE TABLE Monitors ( CHAR(11), INTEGER, pid INTEGER, until DATE, PRIMARY KEY (,, pid), FOREIGN KEY () REFERENCES, FOREIGN KEY () REFERENCES, FOREIGN KEY (pid) REFERENCES Projects); Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 23 Review: Binary vs. Ternary Relationships p age What are the additional constraints in the 2nd diagram? Bad design Covers Policies policyid cost Dependents p age Dependents Purchaser Better design Policies Beneficiary policyid cost Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 24

13 Binary vs. Ternary Relationships (Contd.) The key constraints allow us to combine Purchaser with Policies and Beneficiary with Dependents. Participation constraints lead to NOT NULL constraints. What if Policies is a weak entity set? CREATE TABLE Policies ( policyid INTEGER, cost REAL, CHAR(11) NOT NULL, PRIMARY KEY (policyid). FOREIGN KEY () REFERENCES, ON DELETE CASCADE); CREATE TABLE Dependents ( p CHAR(20), age INTEGER, policyid INTEGER, PRIMARY KEY (p, policyid). FOREIGN KEY (policyid) REFERENCES Policies, ON DELETE CASCADE); Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 25 Policies is a weak entity set CREATE TABLE Dependents ( p CHAR(20), CHAR(11), age INTEGER, policyid INTEGER NOT NULL, PRIMARY KEY (p, policyid, ), FOREIGN KEY (policyid, ) REFERENCES Policies, ON DELETE CASCADE); Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 26

14 Relational Model: Summary A tabular representation of data. Simple and intuitive, currently the most widely used. Integrity constraints can be specified by the DBA, based on application semantics. DBMS checks for violations. Two important ICs: primary and foreign keys In addition, we always have domain constraints. Powerful and natural query languages exist. Rules to translate ER to relational model Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 27