1 Theory, Practice & Methodology of Relational Database Design and Programming Copyright © Ellis Cohen The Relational Model & Relational Mapping These slides are licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 2.5 License. For more information on how you may use them, please see

2 © Ellis Cohen Overview of Lecture Representing Relationships as Bridge Tables Relational Models & Referential Integrity Foreign Keys Relational Mapping of 1:M Relationships Relational Mapping Exercises Mapping Reflexive Relationships Conceptual & Relational Models Defining and Changing Attributes Assigning Sequential & Unique Values to Attributes Metadata & System Data

3 © Ellis Cohen Representing Relationships as Bridge Tables

4 © Ellis Cohen Implementing Conceptual Models A conceptual model abstractly describes the information we want to persistently store in a database. But, how do we actually represent the model using the tables provided by a relational database?

5 © Ellis Cohen Entity Classes  Tables Make each entity class a table Each attribute of the entity class becomes an attribute (i.e. a column) of the table Define a primary key for the entity class if necessary The primary key of the entity class becomes the primary key of the table Emps 30SALES 10ACCOUNTING 50SUPPORT deptno dname Depts empno ename addr 7499ALLEN10 Lehigh Way, … 7654MARTIN22 Gleason St, … 7844TURNER… 7212LAVICH… 7698BLAKE… 7986STERN…

6 © Ellis Cohen Linking Instances 7698BLAKE12 Rara Road 7499ALLEN15 Pogo Lane 7654MARTIN… 7986STERN… 7844TURNER… 30SALES 10ACCOUNTING 7212LAVICH… Dept works for Employee deptno empno Child Entity ClassParent Entity Class Relational databases don't actually have "relationships". How do we implement them?

7 © Ellis Cohen Represent Links by a Bridge Table Emps 30SALES 10ACCOUNTING 50SUPPORT deptno dname Depts empno ename addr empno deptno WorksFor ALLEN… 7654MARTIN… 7844TURNER… 7212LAVICH… 7698BLAKE… 7986STERN… Dept works for Employee deptno empno Child Entity ClassParent Entity Class Suppose an employee does not have a department? Suppose an a department does not have any employees? What's the primary key of WorksFor?

8 © Ellis Cohen Table-Based Mapping 1.Make each entity class a table 2.Make each relationship a table 3.Combine tables with the same primary keys to make queries of the related tables more efficient (usually)

9 © Ellis Cohen Combine WorksFor with Emps 30SALES 10ACCOUNTING 50SUPPORT deptno dname Depts empno ename addr deptno 7499ALLEN MARTIN… TURNER… LAVICH… 7698BLAKE… STERN…10 Emps 30SALES 10ACCOUNTING 50SUPPORT deptno dname Depts empno ename addr empno deptno WorksFor ALLEN… 7654MARTIN… 7844TURNER… 7212LAVICH… 7698BLAKE… 7986STERN… Suppose an employee does not have a department?

10 © Ellis Cohen Reference Columns Help Match Information for Querying 30SALES 10ACCOUNTING 50SUPPORT deptno dname Depts empno ename addr deptno 7499ALLEN MARTIN… TURNER… LAVICH… 7698BLAKE… STERN…10 Emps Suppose we want to find out the name of the Department that BLAKE works in A query can 1.Determine BLAKE's department #  10 2.Determine the name of department #10 We find the tuple in Depts where Depts.deptno matches Emps.deptno We will see shortly how SQL Joins can obtain this information in a single query!

11 © Ellis Cohen Relational Models & Referential Integrity

12 © Ellis Cohen Associated Columns empno ename addr deptno 7499ALLEN MARTIN… TURNER… LAVICH… 7698BLAKE… STERN…10 Emps Every employee has a single empno, ename, addr, and a single deptno (which may be NULL if the employee is unassigned) The value of the deptno column identifies another tuple – the tuple which represents the employee's department

13 © Ellis Cohen Referential Integrity 30SALES 10ACCOUNTING 50SUPPORT deptno dname Depts empno ename addr deptno 7499ALLEN MARTIN… TURNER… LAVICH… 7698BLAKE… STERN…10 Emps In fact, we want to check that every value of deptno in Emps identifies a legal department – that is, it must match a deptno value in Depts. This is called "referential integrity"

14 © Ellis Cohen SQL Representation CREATE TABLE Depts( deptno number(3) primary key, dname varchar(20) ) CREATE TABLE Emps( empno number(4) primary key, ename varchar(30), addr varchar(80), deptno number(3) references Depts(deptno) ) referential integrity constraint

15 © Ellis Cohen Referential Integrity Example Emps( empno, ename, addr, deptno ) deptno references Depts( deptno ) Every value in Emps.deptno is in Depts.deptno  NULLs in Emps.deptno are OK  Not every value in Depts.deptno needs to be in Emps.deptno (e.g. 50 isn't) 30SALES 10ACCOUNTING 50SUPPORT deptno dname Depts empno ename addr deptno 7499ALLEN MARTIN… TURNER… LAVICH… 7698BLAKE… STERN…10 Emps

16 © Ellis Cohen Referential Integrity Violation Emps( empno, ename, addr, deptno ) deptno references Depts( deptno ) is violated, since Emps.deptno contains a value (70), which is not in Depts.deptno The database will NOT allow this! 30SALES 10ACCOUNTING 50SUPPORT deptno dname Depts empno ename addr deptno 7499ALLEN MARTIN… TURNER… LAVICH… BLAKE… STERN…10 Emps

17 © Ellis Cohen Relational Models A Relational Model describes –The characteristics of each relation (i.e. table) –Including any referential integrity constraints SQL is a textual description of a relational model Relational Schema Diagrams are a visual description of a relational model

18 © Ellis Cohen Relational Schema Diagrams Depts( deptno, dname ) Emps( empno, ename, addr, deptno ) deptno references Depts( deptno ) VISUAL Relational Model (Relational Schema) BRIEF TEXTUAL Relational Model (TRex) Emps empno ename addr deptno Depts deptno dname referential integrity constraint

19 © Ellis Cohen Relational Schemas as 1:M Relationships Emps empno ename addr deptno Depts deptno dname The primary key of Emps is empno  The values of empno are all unique and non-null  Every employee has a single empno, a single ename, and a single addr, but moreover … 2. Every employee has a single deptno However, multiple employees could have the same deptno  There's a 1:M relationship between employees and whatever deptno represents 3. deptno represents a department (but multiple empno's could have the same value for deptno), so there's 1:M relationship between employees and department * EmployeeDept works for

20 © Ellis Cohen What's Wrong? Emps empno ename addr Depts deptno dname empno X What's wrong with this representation? WRONG! Don't do this!

21 © Ellis Cohen Only One Employee per Dept! Emps empno ename addr Depts deptno dname empno X This relation has one tuple per department So, each department has a single empno value associated with it! That means that a department can have at most a single employee!

22 © Ellis Cohen Foreign Keys

23 © Ellis Cohen Possible Referential Integrity Emps empno ename addr hiredate Projs pno pname budget startdate What might this mean, based on referential integrity? Is it legal? ?

24 © Ellis Cohen empno ename address hiredate pno pname budget startdate Illegal Referential Integrity X What might this mean? If a value for a hiredate is in Emps, then the same value for startdate MUST be in Projs: Employees could only be hired on the day some project started! Is it legal? No. We only allow foreign key constraints, where the attributes referenced (e.g. startdate) MUST BE unique (preferably a primary key). Projs.startdate is almost certainly not unique. Why only foreign key constraints? Commercial databases only support foreign key constraints; some only allow references to primary keys EmpsProjs

25 © Ellis Cohen Foreign Key Constraint A foreign key constraint is a referential integrity constraint, where the referenced column (s) must have unique values. The standard relational model only uses foreign key constraints referenced column has UNIQUE values Foreign Key Constraint Foreign Key The database continually checks that a foreign key value matches a value of the referenced attribute –on insert/update of the foreign key, or –on update/delete of the referenced attribute(s) Changes that violate the constraint are disallowed (there are other ways of handling this as well …) Emps( empno, ename, addr, deptno ) deptno references Depts( deptno )

26 © Ellis Cohen Foreign Primary Key Constraint A foreign primary key constraint is a foreign key constraint, where the referenced column (s) is a primary key. Emps( empno, ename, addr, deptno ) deptno references Depts Foreign Primary Key Constraint Foreign Key The referenced column does not need to be explicitly listed because it is the primary key of Depts

27 © Ellis Cohen SQL Representation CREATE TABLE Depts( deptno number(3) primary key, dname varchar(20) ) CREATE TABLE Emps( empno number(4) primary key, ename varchar(30), addr varchar(80), deptno number(3) references Depts)

28 © Ellis Cohen Relational Mapping of 1:M Relationships

29 © Ellis Cohen Conceptual & Relational Models A conceptual model Abstractly represents the database design Based on entity classes & relationships A relational model Concretely represents the details of the database design using relations (tables) Relationships are not built-in to the relational model. They are implemented referential integrity constraints. A relation is the mathematical term for a table. So the relational model is about how the tables are modeled in the actual database. The relational model is NOT about relationships

30 © Ellis Cohen Database Design Process Requirements Physical Model using DDL & DCL Relational Model Conceptual Model Conceptual Design & Conceptual Normalization Relational Mapping & Relational Normalization Physical Mapping

31 © Ellis Cohen Linking Instances 7698BLAKE12 Rara Road 7499ALLEN15 Pogo Lane 7654MARTIN… 7986STERN… 7844TURNER… 30SALES 10ACCOUNTING 7212LAVICH… Dept works for Employee deptno empno Child Entity ClassParent Entity Class Map to a relational model to implement the links between instances

32 © Ellis Cohen Table-Based Mapping 1.Make each entity class a table 2.Make each relationship a table (with foreign keys) 3.Combine tables with the same primary keys to make queries of the related tables more efficient (usually)

33 © Ellis Cohen Column Based Mapping 1.Make each entity class a table 2.Represent a 1:M relationship by adding a foreign key (no need to consider bridge tables)

34 © Ellis Cohen Representing Links by Foreign Keys 30SALES 10ACCOUNTING 50SUPPORT deptno dname Depts empno ename addr deptno 7499ALLEN MARTIN… TURNER… LAVICH… 7698BLAKE… STERN…10 Emps An explicit reference to a tuple in the table for the Parent Entity Class Dept works for Employee Child Entity ClassParent Entity Class deptno empno At most a single dept is associated with an employee. Identify that department

35 © Ellis Cohen Crow Magnum & Relational Schemas DeptEmployee works for deptno dname empno ename addr Emps empno ename addr deptno Depts deptno dname foreign key constraint Visual CONCEPTUAL Model (Crow Magnum) Visual RELATIONAL Model (Relational Schema) Does NOT include deptno deptno * Child Entity ClassParent Entity Class

36 © Ellis Cohen Chen & Relational Schemas Emps empno ename addr deptno Depts deptno dname Visual CONCEPTUAL Model (Chen) Visual RELATIONAL Model (Relational Schema) Chen Employee Dept works for Note the arrows point in the same direction!

37 © Ellis Cohen The Relational Schema Arrow Hint DeptEmployee works for deptno dname empno ename addr Emps empno ename addr deptno Depts deptno dname Child Entity ClassParent Entity Class In a 1:M relationship, the Crow's Foot symbol points to the parent entity class The reference arrow points in the same direction

38 © Ellis Cohen Relational Mapping Exercises

39 © Ellis Cohen Reverse Engineering Exercise Design Corresponding ER Models for each of these HealthPlans Players planid plannam plantyp playid pnam teamid planid Teams teamid tnam Emps Depts empno ename sal deptno hirediv deptno dname divid Divs divid divnam

40 © Ellis Cohen Interpreting Exercise 1 HealthPlans Players planid plannam plantyp playid pnam teamid planid Teams teamid tnam Each player (identified by playid) has a single pnam a single teamid (but many players could reference the same team) a single planid (but many players could reference the same plan)

41 © Ellis Cohen Answer to Exercise 1 Visual CONCEPTUAL Model (Easy Crow Magnum) TeamPlayerHealth Plan has enrolled in teamid tnam playid pnam planid plannam plantyp Visual RELATIONAL Model: Relational Schema HealthPlans Players planid plannam plantyp playid pnam teamid planid Teams teamid tnam Note direction of Crow Magnum indicators & reference arrows I use singular for entity class names, plural for table names

42 © Ellis Cohen Answer to Exercise 2 Visual CONCEPTUAL Model (Easy Crow Magnum) DivisionDeptEmployee part of works for initially hired divid divnam deptno dname empno ename sal Visual RELATIONAL Model (Relational Schema) Note direction of Crow Magnum indicators & reference arrows Emps Depts empno ename sal deptno hirediv deptno dname divid Divs divid divnam I use singular for entity class names, plural for table names

43 © Ellis Cohen Merging Foreign Key Lines Emps Depts empno ename sal deptno hirediv deptno dname divid Divs divid divnam Emps Depts empno ename sal deptno hirediv deptno dname divid Divs divid divnam When two foreign keys reference the same attribute, you can merge the lines (Don't do this in ER diagrams though!) Could two different Emps attributes both reference divid?

44 © Ellis Cohen Mapping Exercise Design Corresponding Relational Schemas for each of these StyleItem has itemsku size color stylecode stylenam styledate Category has catid catnam PlayerChild has childid cname playid pnam Team has teamid tnam gives scholarship to

45 © Ellis Cohen Interpreting Mapping Exercise 1 StyleItem has itemsku size color stylecode stylenam styledate Category has catid catnam Items Styles itemsku size color stylecode stylenam styledate catid Each style is associated with a single category Each item is associated with a single style

46 © Ellis Cohen Answer to Mapping Exercise 1 StyleItem has itemsku size color stylecode stylenam styledate Category has catid catnam Visual CONCEPTUAL Model (Easy Crow Magnum) Visual RELATIONAL Model (Relational Schema) Items Styles itemsku size color stylecode stylenam styledate catid Categories catid catnam Arrowheads are REQUIRED! Note direction of Crow Magnum indicators & reference arrows

47 © Ellis Cohen Answer to Mapping Exercise 2 Visual CONCEPTUAL Model: Easy Crow Magnum ER Diagram Visual RELATIONAL Model: Relational Schema Children Players childid cname playid schteamid playid pnam teamid Teams teamid tnam PlayerChild has childid cname playid pnam Team has teamid tnam gives scholarship to

48 © Ellis Cohen Keeping the Bridge Table Emps 30SALES 10ACCOUNTING 50SUPPORT deptno dname Depts empno ename addr empno deptno WorksFor ALLEN… 7654MARTIN… 7844TURNER… 7212LAVICH… 7698BLAKE… 7986STERN… Dept works for Employee deptno empno Child Entity ClassParent Entity Class Occasionally, relational designs keep bridge tables for 1:M relationships. If so, what would be the corresponding relational schema?

49 © Ellis Cohen Relational Schema with Bridge Table Emps 30SALES 10ACCOUNTING 50SUPPORT deptno dname Depts empno ename addr empno deptno WorksFor ALLEN… 7654MARTIN… 7844TURNER… 7212LAVICH… 7698BLAKE… 7986STERN… Depts WorksFor deptno dname empno deptno Emps empno ename addr

50 © Ellis Cohen Mapping Reflexive Relationships

51 © Ellis Cohen Mapping Reflexive 1:M Relationships Emps empno ename mgr 7499ALLEN… MARTIN7698 BLAKE7839 KING 7844TURNER STERN7839 Add mgr to Emps, referencing empno Employee manages

52 © Ellis Cohen Schema for Reflexive Relationships Dept works for deptno dname empno ename addr Emps empno ename addr deptno mgr Depts deptno dname Visual CONCEPTUAL Model (Crow Magnum ER Diagram) Visual RELATIONAL Model (Relational Schema) Does NOT include deptno or mgr Employee manages *

53 © Ellis Cohen Visual & Brief Textual Relational Models Depts( deptno, dname ) Emps( empno, ename, addr, deptno, mgr ) deptno references Depts mgr references Emps VISUAL Relational Model (Relational Schema) BRIEF TEXTUAL Relational Model (TRex) Emps empno ename addr deptno mgr Depts deptno dname

54 © Ellis Cohen SQL Representation CREATE TABLE Depts( deptno number(3) primary key, dname varchar(20) ) CREATE TABLE Emps( empno number(4) primary key, ename varchar(30), addr varchar(80), deptno number(3) references Depts, mgr number(4) references Emps )

55 © Ellis Cohen Conceptual and Relational Models

56 © Ellis Cohen Database Design Process Requirements Physical Model using DDL & DCL Relational Model Conceptual Model Conceptual Design & Conceptual Normalization Relational Mapping & Relational Normalization Physical Mapping Why not just design the relational model directly from the requirements? Why design an intermediate conceptual model?

57 © Ellis Cohen Why Do Conceptual Design? Faster to create & draw a conceptual model Clearer semantic intent Easier to understand and reason about Many ways to design a relational model for a given conceptual model Focus on essential design decisions Allows secondary design decisions to be deferred

58 © Ellis Cohen Combining the Conceptual & Relational Models Emps empno ename addr deptno mgr Depts deptno dname works for manages Some modeling tools and technologies combine the conceptual and the relational model. We think it is better to keep the two distinct

59 © Ellis Cohen UML Combination of Conceptual & Relational Models Using UML to combine the conceptual and the relational model. We've placed the association link adjacent to the foreign key (FK) which implements it. However, this is actually rarely done when FK's are shown in UML, which can make it hard to tell which FK represents which relationship EmployeesDepts works for * 0..1 PK empno ename addr FK deptno FK mgr PK deptno dname manages * 0..1

60 © Ellis Cohen Why Different Models? Dept works for Emps empno ename addr deptno mgr Depts deptno dname Visual CONCEPTUAL Model (Crow Magnum ER Diagram) Visual RELATIONAL Model (Relational Schema) Employee manages Used for communication among system & UI architects and users, interested more in functionality than implementation Used for communication among database designers and administrators, focused on implementation

61 © Ellis Cohen Defining and Changing Attributes

62 © Ellis Cohen Identifying Rows in Tables Numeric Id Uniquely identifies row without providing information about row's real identity or contents - e.g Structured Id (e.g. SKU) Often structured to reflect structure or practice of organization - e.g. XM-304-T Can causes problems if structure or practice of organization changes Short Name Name that uniquely identifies row to DBA - e.g. ACCT Long Name / Title Name that clearly and uniquely identifies row to user - e.g. ACCOUNTING

63 © Ellis Cohen Naming Columns Use a consistent naming approach Column name includes (whole or partial) name of table [in Emps] empno and [in Depts] deptno (or) [in Emps] empid and [in Depts] deptid [in Emps] ename and [in Depts] dname Column name is independent of table name [in Emps] id and [in Depts] id to identify the primary key [in Emps] name and [in Depts] name to identify the employee & department name –Simplest naming model –Tends to require relative names [Emps.id] with queries joining multiple tables –Possibility of spurious join columns when using natural joins

64 © Ellis Cohen Naming Foreign Keys Use name of foreign key, with table name included, if necessary: e.g. [in Emps] deptno (or deptid) Where that name is ambiguous, replace or combine with a name that reflects the role in the local table e.g. [in Emps] use mgr or (better) mgrno to refer to the empno of the employee's manager

65 © Ellis Cohen Altering Column & Tables ALTER TABLE Emps ADD ( numkids int not null default 0 ) –Adds column ALTER TABLE Emps MODIFY ( sal NUMBER(13,2) DEFAULT 250 ) –Modifies column datatype and/or default ALTER TABLE Emps DROP (numkids, sal) –Drops columns ALTER TABLE Emps RENAME TO Employees –Renames entire table (can't rename columns)

66 © Ellis Cohen Assigning Sequential & Unique Values to Attributes

67 © Ellis Cohen Choosing a Primary Key User-provided Id or Name Provided by user when tuple in created System-provided Id Provided by an id generator defined in the database system. Useful as a surrogate key even if a name or a user-provided key is available.

68 © Ellis Cohen System-Provided Id Generator Id Values –Sequential (1,2,3,…) –Globally unique id (GUID) Every id generated is globally unique Useful if merging separate databases Id Generation –Explicit (used by Oracle) Handled through a named database object, which is explicitly incremented, gotten and stored in some column of a new row Via PL/SQL, can be made to look automatic –Automatic (used by SQL Server) Associated with a table column (A specified column in each newly created row gets a new id, sort of like Oracle ROWIDs)

69 © Ellis Cohen Sequences in Oracle Defining and using sequences CREATE SEQUENCE empseq START WITH 8000 INCREMENT BY 10 SELECT empseq.nextval FROM dual Imagine a single operation NewEmp( ename, deptno, job, sal, comm ) Inserts a tuple into emp with empseq.nextval  empno filled in from empseq.nextval  hiredate filled in with today's date  ename, deptno, job, sal, comm filled in from the given values An operation like this can be defined as a stored PL/SQL procedure UGLY This is UGLY! You'd really like to hide it

70 © Ellis Cohen Generating Globally Unique Ids If you are not using a system that can provide GUID's, you can make one by concatenating together –Unique mac or ip address or name of machine –Name of database –Name of schema (i.e. user) –System-provided per-schema id (e.g. from an Oracle sequence) or timestamp For example cs.bu.edu-testdb-scott In Oracle, the function SYS_GUID() will return a globally unique id as a 16 byte RAW value

71 © Ellis Cohen Metadata and System Data

72 © Ellis Cohen Metadata & System Data Metadata is information about the data that users store in the database. For example, –the names of a user's tables –the names and types of table columns. More generally, system data, is data stored by the database to support its proper functioning. Databases generally store metadata and other system data in the database itself. –In relational databases, this information is stored in tables.

73 © Ellis Cohen Basic Column Metadata Note that the column metadata for the Emps table can itself be represented as a table

74 © Ellis Cohen Oracle Examples SELECT * FROM Dictionary where table_name like 'USER_TAB%' order by table_name; – Dictionary holds info about all tables in the DB – this selects info about user table metadata SELECT table_name FROM User_Tables; – this lists the names of the user's tables SELECT table_name, column_name FROM User_Tab_Columns ORDER BY table_name, column_name; – this lists the names of the columns of each of the user's tables