Converting ER Diagrams to Tables
After designing an ER Diagram,
 ER diagram is converted into the tables in relational model.
 This is because relational models can be easily implemented by RDBMS like MySQL , Oracle etc.
Following rules are used for converting an ER diagram into the tables
Rule01: For Strong Entity Set With Only Simple Attributes
A strong entity set with only simple attributes will require only one table in relational model.
 Attributes of the table will be the attributes of the entity set.
 The primary key of the table will be the key attribute of the entity set.
Example
Schema : Student ( Roll_no , Name , Sex )
Also Read Entity Sets in DBMS
Rule02: For Strong Entity Set With Composite Attributes
 A strong entity set with any number of composite attributes will require only one table in relational model.
 While conversion, simple attributes of the composite attributes are taken into account and not the composite attribute itself.
Example
Roll_no  First_name  Last_name  House_no  Street  City 
     
Schema : Student ( Roll_no , First_name , Last_name , House_no , Street , City )
Also Read Types of Attributes in DBMS
Rule03: For Strong Entity Set With Multi Valued Attributes
A strong entity set with any number of multi valued attributes will require two tables in relational model.
 One table will contain all the simple attributes with the primary key.
 Other table will contain the primary key and all the multi valued attributes.
Example
Rule04: Translating Relationship Set into a Table
A relationship set will require one table in the relational model.
Attributes of the table are
 Primary key attributes of the participating entity sets
 Its own descriptive attributes if any.
Set of nondescriptive attributes will be the primary key.
Example
Schema : Works in ( Emp_no , Dept_id , since )
NOTE
If we consider the overall ER diagram, three tables will be required in relational model
 One table for the entity set “Employee”
 One table for the entity set “Department”
 One table for the relationship set “Works in”
Rule05: For Binary Relationships With Cardinality Ratios
The following four cases are possible
Case01: Binary relationship with cardinality ratio m:n
Case02: Binary relationship with cardinality ratio 1:n
Case03: Binary relationship with cardinality ratio m:1
Case04: Binary relationship with cardinality ratio 1:1
Also read Cardinality Ratios in DBMS
Case01: For Binary Relationship With Cardinality Ratio m:n
Here, three tables will be required
 A ( a1 , a2 )
 R ( a1 , b1 )
 B ( b1 , b2 )
Case02: For Binary Relationship With Cardinality Ratio 1:n
Here, two tables will be required
 A ( a1 , a2 )
 BR ( a1 , b1 , b2 )
NOTE Here, combined table will be drawn for the entity set B and relationship set R.
Case03: For Binary Relationship With Cardinality Ratio m:1
Here, two tables will be required
 AR ( a1 , a2 , b1 )
 B ( b1 , b2 )
NOTE Here, combined table will be drawn for the entity set A and relationship set R.
Case04: For Binary Relationship With Cardinality Ratio 1:1
Here, two tables will be required. Either combine ‘R’ with ‘A’ or ‘B’
Way01:
 AR ( a1 , a2 , b1 )
 B ( b1 , b2 )
Way02:
 A ( a1 , a2 )
 BR ( a1 , b1 , b2 )
Thumb Rules to Remember While determining the minimum number of tables required for binary relationships with given cardinality ratios, following thumb rules must be kept in mind  For binary relationship with cardinality ration m : n , separate and individual tables will be drawn for each entity set and relationship.
 For binary relationship with cardinality ratio either m : 1 or 1 : n , always remember “many side will consume the relationship” i.e. a combined table will be drawn for many side entity set and relationship set.
 For binary relationship with cardinality ratio 1 : 1 , two tables will be required. You can combine the relationship set with any one of the entity sets.

Rule06: For Binary Relationship With Both Cardinality Constraints and Participation Constraints
 Cardinality constraints will be implemented as discussed in Rule05.
 Because of the total participation constraint, foreign key acquires NOT NULL constraint i.e. now foreign key can not be null.
Case01: For Binary Relationship With Cardinality Constraint and Total Participation Constraint From One Side
Because cardinality ratio = 1 : n , so we will combine the entity set B and relationship set R.
Then, two tables will be required
 A ( a1 , a2 )
 BR ( a1 , b1 , b2 )
Because of total participation, foreign key a1 has acquired NOT NULL constraint, so it can’t be null now.
Case02: For Binary Relationship With Cardinality Constraint and Total Participation Constraint From Both Sides
If there is a key constraint from both the sides of an entity set with total participation, then that binary relationship is represented using only single table.
Here, Only one table is required.
 ARB ( a1 , a2 , b1 , b2 )
Rule07: For Binary Relationship With Weak Entity Set
Weak entity set always appears in association with identifying relationship with total participation constraint.
Here, two tables will be required
 A ( a1 , a2 )
 BR ( a1 , b1 , b2 )
Next Article Practice Problems On Converting ER Diagrams to Tables
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