1. CS4222 Principles of Database System
11/22/2018
CS4222 Principles of Database System
8. ER/EER –to-Relational Mapping
Huiping Guo
Department of Computer Science
California State University, Los Angeles

2. Outline SQL: Database Definition SQL: Constraints
11/22/2018
Outline
SQL: Database Definition
SQL: Constraints
ER-to-Relational Mapping
Step 1: Mapping of Regular Entity Types
Step 2: Mapping of Binary M:N Relationship Types
Step 3: Mapping of Binary 1:N Relationship Types
Step 4: Mapping of Binary 1:1 Relationship Types
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3. Database Definition Language (DDL)
Subset of SQL commands to create and modify database objects
relations, attributes, constraints,…
Can create an entire database using SQL rather than a graphical user interface
With DDL you gain more control over exactly what database objects are built and what they are named
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4. Databases
PostgreSQL
CREATE DATABASE dbname;
USE dbname;
You don’t have the privilege to create a database!
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5. Creating a Table Syntax CREATE TABLE tablename (
Fieldname1 DataType NULL | NOT NULL,
Fieldname2 DataType NULL | NOT NULL )
Null means that the field may contain null (empty) values. Not Null means that the field cannot contain null values.
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6. Datatype Numeric types Character types Date/time types More dataypes
Integer, smallint, bigint, decimal, real,…
Character types
Varchar(n): variable-length with limit
Char(n): fixed-length
Text: variable unlimited length
Date/time types
Date, time,...
More dataypes
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7. Creating a Table: Example
CREATE TABLE DEPARTMENT ( DNAME VARCHAR(10) NOT NULL, DNUMBER INTEGER NOT NULL, MGRSSN CHAR(9), MGRSTARTDATE CHAR(9) );
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8. Dropping a Table Syntax DROP Table tablename
Used to remove a relation (base table) and its definition
The relation can no longer be used in queries, updates, or any other commands since its description no longer exists
Example: Remove the DEPARTMENT table from the database
DROP Table DEPARTMENT
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9. Alter table Used to add an attribute to an existing relation
Used to altering an existing attribute in a relation
Remove an attribute from a relation
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10. Adding attributes to an Existing Table
Syntax
ALTER TABLE tablename
ADD fieldname DataType NULL | NOT NULL
Example: ALTER TABLE EMPLOYEE ADD JOB VARCHAR(12);
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11. Altering existing attributes in a Table
Syntax
ALTER TABLE tablename
MODIFY fieldname DataType NULL | NOT NULL
Change the length on the job field to 15 in the Employee table
ALTER TABLE Employee
MODIFY job VARCHAR(15) NULL
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12. Dropping an attribute Syntax
ALTER TABLE tablename
DROP COLUMN attributeName
Example: Remove the job attribute from the Employee table
ALTER TABLE Employee
DROP COLUMN job
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13. Contraints Create primary key constraints Create unique constraints
Create foreign key constraints
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14. Primary Key Constraint With Create Table
Two ways with Create Table
First form cannot create multi-field PK
First way
Create Table tablename (
fieldname1 datatype null | not null Primary Key,
fieldname2 datatype null | not null )
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15. Primary Key Constraint With Create Table (Cont.)
Second way
Create Table tablename (
Fieldname1 datatype null | not null,
Fieldname2 datatype null | not null,
Primary Key (fieldname1,fieldname2) )
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16. Example
Create a table called Department. Set the primary key to Dnumber.
CREATE TABLE DEPARTMENT ( DNAME VARCHAR(10) NOT NULL, DNUMBER INTEGER primary key, MGRSSN CHAR(9), MGRSTARTDATE CHAR(9) );
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17. Unique Constraint With Create Table
Sometimes want to make sure a non-primary key column have unique values
Two ways with Create Table
First form cannot create multi-field constraint
First form does not allow for naming constraint
Syntax
Create Table tablename (
Fieldname1 datatype null | not null Primary key,
Fieldname2 datatype null | not null Unique )
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18. Unique Constraint With Create Table (Cont.)
Second way: syntax
Create Table tablename (
Fieldname1 datatype null | not null,
Fieldname2 datatype null | not null,
Unique (fieldname1, fieldname2) )
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19. 11/22/2018
Example
Create a table called tblX with an integer ID field and an integer anotherfield. Both fields are not null. Make anotherfield unique.
Create Table tblX (
ID int Not Null,
anotherfield int Not Null Unique )
anotherfield int Not Null,
Unique(anotherfield)
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20. Primary key and Foreign Key
Parent
Table
“One”
Primary Key
Foreign Key
Child
Table
“Many”
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21. Foreign key constraints
Parent table and child table
Parent table: the table whose primary key is involved in the relationship
Child table: the table whose foreign key is involved in the table
The purpose of foreign key constraints
Make sure that all foreign keys in the child table match the primary keys in the parent table
Which table to create the foreign key constraint?
The child table
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22. Foreign Key Constraints With Create Table
Syntax 1:
Create Table tablename (
child_field1 datatype null | not null,
child_field2 datatype null | not null,
Foreign Key (child_field1, child_field2) References parent_tablename (parent_field1, parent_field2) )
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23. Foreign Key Constraints With Create Table
Syntax 2:
Create Table tablename (
Fieldname1 datatype References parent_tablename,
Fieldname2 datatype null | not null, )
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24. Example
Create a table called Dependent with 3 attributes: essn, name and sex. Create a foreign key constraint between essn and the ssn in the Employee table.
Create Table Dependent (
essn Integer Not Null,
name varchar(20) Not Null,
sex char,
Foreign Key (essn) References Employee (ssn) )
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25. Foreign Key Implications
Foreign Key constraint requires that every FK field in child table matches existing PK in Parent table
Constraint will not allow "orphan" records if updating or deleting PK value in parent table
use Cascade Update or delete
CASCADE specifies that when a referenced row is deleted/modified, row(s) referencing it should be automatically deleted as well.
use Restrict Update or delete
RESTRICT prevents deletion/modification of a referenced row
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26. Example CREATE TABLE products ( product_no integer PRIMARY KEY,
name text,
price numeric );
CREATE TABLE orders (
order_id integer PRIMARY KEY,
shipping_address text,
... );
From:
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27. Example (cont.) CREATE TABLE order_items (
product_no integer REFERENCES products
ON DELETE RESTRICT
ON MODIFY restrict,
order_id integer REFERENCES orders
ON DELETE CASCADE
ON MODIFY CASCADE,
quantity integer,
PRIMARY KEY (product_no, order_id) );
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28. ER-to-Relational Mapping
Problem
ER Diagram
Tables
Modeling
(Conceptual design)
Mapping
(Logical design)
Mapping
Get tables from the ER/EER diagram
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29. GOALS during Mapping
Preserve all information (that includes all attributes)
Maintain the constraints to the extent possible
Relational Model cannot preserve all contstraints- e.g., max cardinality ratio such as 1:10 in ER; exhaustive classification into subtypes, e.g., STUDENTS are specialized into Domestic and Foreign
Minimize null values
The mapping procedure described has been implemented in many commercial tools.
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30. ER-to-Relational Mapping
Step 1: Mapping of Regular Entity Types
Step 2: Mapping of Binary M:N Relation Types
Step 3: Mapping of Binary 1:N Relation Types
Step 4: Mapping of Binary 1:1 Relation Types
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31. Step 1: Mapping of Regular Entity Types
11/22/2018
Step 1: Mapping of Regular Entity Types
For each regular entity type E in the ER schema, create a relation R that includes all the simple attributes of E
If an attribute is composite, includes only the simple attributes of the composite attribute
Choose one of the key attributes of E as the primary key for R
If the chosen key of E is composite, the set of simple attributes that form it will together form the primary key of R.
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32. Step 1: Mapping of Regular Entity Types
Employees
ssn
name
sex
f_name
l_name
ssn
f_name
l_name
sex
Employees
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33. Step 1: Mapping of Regular Entity Types
CREATE TABLE Employees(
ssn CHAR(11),
f_name CHAR(20),
l_name CHAR(20),
sex CHAR(5),
PRIMARY KEY (ssn) )
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34. Step 2: Mapping of Binary M:N Relationship Types
11/22/2018
Step 2: Mapping of Binary M:N Relationship Types
For each regular binary M:N relationship type R, create a new relation S to represent R.
Attributes of the relation S much include
Primary Keys of each participating entity set, which form the primary key of the relationship set S
The primary keys are also the foreign keys of S
All descriptive attributes of the relationship set
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35. Step 2: Mapping of Binary M:N Relationship Types
sex
pname
hours
pno
since
name
Works_on
Projects
Employees
ssn
Employees
ssn
name
sex
ssn
pno
since
Works_On
pno
Projects
pname
hours
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36. Step 2: Mapping of Binary M:N Relationship Types
CREATE TABLE Works_On(
ssn CHAR(11),
pno INTEGER,
since DATE,
PRIMARY KEY (ssn, pno),
FOREIGN KEY (ssn) REFERENCES Employees (ssn),
FOREIGN KEY (pno) REFERENCES Projects (pno) )
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37. Step 3: Mapping of Binary 1:N Relationship Types
11/22/2018
Step 3: Mapping of Binary 1:N Relationship Types
Two approaches
Approach 1
For each regular binary 1:N relationship type R, create a new relation S to represent R.
Attributes of the relation S must include
Primary Keys of each participating entity set
The primary keys are also the foreign keys of S
All descriptive attributes of the relationship set
The primary key of S is the primary key of the participating entity type on the N side
Map the two participating entity types to two relations
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38. Step 3: Mapping of Binary 1:N Relationship Types
sex
dname
budget
dno
since
name
Manage
Departments
Employees
ssn 1 N
Employees
ssn
name
sex
ssn
dno
since
Manage
dno
Departments
dname
budget
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39. Step 3: Mapping of Binary 1:N Relationship Types
CREATE TABLE Manage(
ssn CHAR(11),
dno INTEGER,
since DATE,
PRIMARY KEY (dno),
FOREIGN KEY (ssn) REFERENCES Employees (ssn),
FOREIGN KEY (dno) REFERENCES Departments (dno) )
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40. Step 3: Mapping of Binary 1:N Relationship Types
Approach 2
Since each department has at most one manager, we could instead combine Manages and Departments.
Employees
ssn
name
sex
Departments
dno
dname
budget
Mg_Ssn
Since
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41. Step 3: Mapping of Binary 1:N Relationship Types
CREATE TABLE Departments(
dno INTEGER,
dname CHAR(20),
budget REAL,
mg_ssn CHAR(11),
since DATE,
PRIMARY KEY (dno),
FOREIGN KEY (mg_ssn) REFERENCES
Employees (ssn) )
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42. Step 4: Mapping of Binary 1:1 Relationship Types
11/22/2018
Step 4: Mapping of Binary 1:1 Relationship Types
For each binary 1:1 relationship type R in the ER schema, identify the relations S and T that correspond to the entity types participating in R.
There are three possible approaches:
Approach 1: Create a new relation for the relationship type
Approach 2: Merged relationship type with one of the entity type
Approach 3:
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43. Example One department has at most one manager
sex
dname
budget
dno
since
name
Manage
Departments
Employees
ssn 1 1
One department has at most one manager
One manager manages at most one department
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44. Approach 1: Map Manage to a table
CREATE TABLE Manage(
ssn CHAR(11),
dno INTEGER UNIQUE,
since DATE,
PRIMARY KEY (ssn),
FOREIGN KEY (ssn) REFERENCES Employees (ssn),
FOREIGN KEY (dno) REFERENCES Departments(dno)) OR
CREATE TABLE Manage(
ssn CHAR(11) UNIQUE,
dno INTEGER,
since DATE,
PRIMARY KEY (dno),
FOREIGN KEY (ssn) REFERENCES Employees (ssn),
FOREIGN KEY (dno) REFERENCES Departments) (dno) )
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45. Approach 2: Combine Manage and Employees
CREATE TABLE Emp_Mgr(
ssn CHAR(11),
name CHAR(20),
sex CHAE,
dno INTEGER UNIQUE,
since DATE,
PRIMARY KEY (ssn),
FOREIGN KEY (dno) REFERENCES Departments (dno) )
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46. Approach 3: Combine Manage and Departments
CREATE TABLE Dept_Mgr(
dno INTEGER,
dname CHAR(20),
budget REAL,
ssn CHAR(11) UNIQUE,
since DATE,
PRIMARY KEY (dno),
FOREIGN KEY (ssn) REFERENCES Employees (ssn) )
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47. Summary of Mapping constructs and constraints
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