ER Diagrams Simplified

15,152 views

Published on

If find how to create
E-R Diagrams ? Here is a Doc making things for you simple and easy .

Entity Relationship Diagrams (ERDs) illustrate the logical structure of databases.

I find many students find uneasiness in creating ER-Diagrams conceptually , but they understand how to create tables and columns for a Software project . Here is an approach creating to first create Tables and from tables ..ER Diagrams

Published in: Self Improvement, Technology
0 Comments
4 Likes
Statistics
Notes
  • Be the first to comment

No Downloads
Views
Total views
15,152
On SlideShare
0
From Embeds
0
Number of Embeds
53
Actions
Shares
0
Downloads
639
Comments
0
Likes
4
Embeds 0
No embeds

No notes for slide

ER Diagrams Simplified

  1. 1. How to create E-R Diagrams from Database Tables ? Ecologic Corporation Training Department Chandigarh www.ecologic.co.in Ecologic Corporation , Chandigarh , www.ecologic.co.in 1 26/04/2009
  2. 2. Learning Objectives Conversion of an ER model to relational schemas Entity sets, relationship sets, attributes, redundancy removal, specialization, and aggregation SQL constructs to create tables and constraints Ecologic Corporation , Chandigarh 2 160022, www.ecologic.co.in 26/04/2009
  3. 3. Doing from E-R to Relational To create a relational database, we need a set of relational schemas. The first step of database design is to produce an E-R diagram that conforms to the user requirement. The subsequent step is to convert the E-R diagram into a collection of relational schemas, including constraints. Ecologic Corporation , Chandigarh 3 160022, www.ecologic.co.in 26/04/2009
  4. 4. Step 1 : Conversion For each entity set, create a corresponding relational schema. For each relationship set, create a corresponding relational schema. Convert attributes in the E-R diagram to columns (attributes) in the relational schemas. Ecologic Corporation , Chandigarh 4 160022, www.ecologic.co.in 26/04/2009
  5. 5. Entity Sets to Schemas A strong entity set is converted to a relation with no additional column. A weak entity set becomes a table that includes the primary key of the identifying strong entity set. E.g., payment = ( loan_number, payment_number, payment_date, payment_amount ) Ecologic Corporation , Chandigarh 5 160022, www.ecologic.co.in 26/04/2009
  6. 6. Relationship Sets to Schemas A many-to-many relationship set is represented as a relation. Its attributes include the primary keys of the participating entity sets and the attributes of the relationship set. Example: borrower = (customer_id, loan_number ) Ecologic Corporation , Chandigarh 6 160022, www.ecologic.co.in 26/04/2009
  7. 7. Removing Redundancy: Onemany-to-one or one-to-many relationshipto that is total on the to Many or Many set One A many-side can be represented by adding the attributes of the relationship set plus the primary key of the “one” side to the relation that correponds to the “many” side Example: Instead of creating a separate relation for the relationship set account_branch, we can just add an attribute branch_name to the relation that represents the entity set account. Ecologic Corporation , Chandigarh 7 160022, www.ecologic.co.in 26/04/2009
  8. 8. The next step: Removing Redundancy : One to One For a one-to-one relationship set, it can be represented by the relation that corresponds to the entity set on either side after adding the attributes from the relationship set and the primary key of the entity from the other side. Ecologic Corporation , Chandigarh 8 160022, www.ecologic.co.in 26/04/2009
  9. 9. Removing Redundancy : Identifying Relationship The schema corresponding to the identifying relationship set linking a weak entity set to its identifying strong entity set is redundant. Example: The payment schema already contains the attributes that would appear in the loan_payment schema (i.e., loan_number and payment_number). Ecologic Corporation , Chandigarh 9 160022, www.ecologic.co.in 26/04/2009
  10. 10. Composite Attributes Composite attributes are flattened out by creating a separate attribute for each component attribute Example: given entity set customer with composite attribute name with component attributes first_name and last_name, the schema corresponding to the entity set has two attributes name.first_name and name.last_name Ecologic Corporation , Chandigarh 10 160022, www.ecologic.co.in 26/04/2009
  11. 11. Multivalued Attributes A multivalued attribute M of an entity E is represented by a separate schema EM Schema EM has attributes corresponding to the primary key of E and an attribute corresponding to the multivalued attribute M. E.g.,: dependent_names of employee is represented as employee_dependent_names = ( employee_id, dname) Each value of the multivalued attribute maps to a tuple of a relation on schema EM. E.g., an employee entity with primary key 123 and dependents Jack and Jane maps to two tuples: (123, Jack) and (123, Jane). Ecologic Corporation , Chandigarh 11 160022, www.ecologic.co.in 26/04/2009
  12. 12. Representing Specialization (1) Method 1: Form a schema for the higher-level entity set Form a schema for each lower-level entity set and include the primary key of the higher- level entity set and local attributes schema attributes person name, street, city customer name, credit_rating employee Ecologic Corporation , Chandigarh name, salary 12 160022, www.ecologic.co.in 26/04/2009
  13. 13. Representing Specialization (2) Method 2: Form a schema for each entity set with all local and inherited attributes schema attributes person name, street, city customer name, street, city, credit_rating employee name, street, city, salary Ecologic Corporation , Chandigarh 13 160022, www.ecologic.co.in 26/04/2009
  14. 14. Comparison of the Two Methods of Representing Specialization Method 1: getting information about an employee requires accessing two relations, the one corresponding to the low-level schema and the one corresponding to the high-level schema Method 2: If specialization is total, the schema for the generalized entity set (person) is not required to store information Drawback: street and city may be stored redundantly for people who are both customers and employees Ecologic Corporation , Chandigarh 14 160022, www.ecologic.co.in 26/04/2009
  15. 15. Representing Aggregation Create a schema containing the primary key of the aggregated relationship the primary key of the associated entity set any attributes of the relationship Ecologic Corporation , Chandigarh 15 160022, www.ecologic.co.in 26/04/2009
  16. 16. Example of Representing Aggregation manages (employee_id, branch_name, title, manager_name) Ecologic Corporation , Chandigarh 16 160022, www.ecologic.co.in 26/04/2009
  17. 17. History of SQL Developed as Sequel in System R project at the IBM San Jose Research Laboratory Renamed Structured Query Language (SQL) Became ANSI and ISO standards: SQL-86, SQL-89, SQL-92, SQL:1999, SQL:2003 Most of SQL-92 features are offered by commercial systems, plus varying feature sets from later standards and special proprietary features. Ecologic Corporation , Chandigarh 17 160022, www.ecologic.co.in 26/04/2009
  18. 18. Data Definition Language Defines The schema for each relation The domain of each attribute Integrity constraints Indexes for each relation Security and authorization information for each relation The physical storage structure of each relation on disk Corporation , Chandigarh Ecologic 18 160022, www.ecologic.co.in 26/04/2009
  19. 19. Domain Types in SQL char(n), varchar(n) Fixed length or variable length character strings int, smallint Integers numeric(p,d) Fixed point number real, double precision, float(n) Floating point and double-precision floating point numbers Ecologic Corporation , Chandigarh 19 160022, www.ecologic.co.in 26/04/2009
  20. 20. Create Table SQL Construct create table r (A1 D1, A2 D2, ..., An Dn, (integrity-constraint1), ..., (integrity-constraintk)) r is the name of the relation each Ai is an attribute name in the schema of relation r Di is the data type of values in the domain of attribute Ai Ecologic Corporation , Chandigarh 20 160022, www.ecologic.co.in 26/04/2009
  21. 21. Example: Creating a Table create table branch (branch_name char(15) not null, branch_city char(30), assets integer) Ecologic Corporation , Chandigarh 21 160022, www.ecologic.co.in 26/04/2009
  22. 22. Integrity Constraints in SQL not null primary key (A1, ..., An ) create table branch (branch_name char(15), branch_city char(30), assets integer, primary key (branch_name)) primary key declaration on an attribute automatically ensures not null in SQL-92 onwards Ecologic Corporation , Chandigarh 22 160022, www.ecologic.co.in 26/04/2009
  23. 23. Drop and Alter Table Constructs The drop table command deletes all information about the dropped relation from the database. The alter table command can be used to add attributes to an existing relation: alter table r add A D where A is the name of the attribute to be added to relation r and D is the domain of A. Ecologic Corporation , Chandigarh 23 160022, www.ecologic.co.in 26/04/2009
  24. 24. Examples of ER Diagrams Ecologic Corporation , Chandigarh 24 160022, www.ecologic.co.in 26/04/2009
  25. 25. Shopping Cart ER Diagram Ecologic Corporation , Chandigarh 25 160022, www.ecologic.co.in 26/04/2009
  26. 26. North Wind DB ER Diagram Ecologic Corporation , Chandigarh 26 160022, www.ecologic.co.in 26/04/2009
  27. 27. ER Diagrams Rules Simplified Ecologic Corporation , Chandigarh 27 160022, www.ecologic.co.in 26/04/2009
  28. 28. Guess What this diagram is Saying? Ecologic Corporation , Chandigarh 28 160022, www.ecologic.co.in 26/04/2009
  29. 29. Summary In general, each entity set or relationship set is translated into a relation. There are special cases, including many- to-one, one-to-many, and one-to-one relationships; identifying relationships; multi-valued and composite attributes; specialization and aggregation. Tables and constraints are created, altered, and dropped through SQL DDL. Ecologic Corporation , Chandigarh 29 160022, www.ecologic.co.in 26/04/2009

×