4 ER to Tables

Conversion of E-R Diagram to Tables

• For each entity set and relationship set there is a unique table, which is assigned the name of the corresponding entity set or relationship set. Each table has a number of columns (generally corresponding to attributes), which have unique names. Primary keys allow entity sets and relationship sets to be expressed uniformly as tables, which represent the contents of the database.

Representing Entity sets as Tables

Composite and Multi-Value Attributes

• In order to convert an entity having composite attributes, the composite attributes are flattened out by creating a separate attribute for each component attribute.

Representing Generalization as Tables

• There are two approaches to convert an E-R diagram representing generalization.

• Method 1• E-R diagram containing generalization as shown in figure

4.20 can be converted in following tables:• Account: Account_Number, Balance• Saving_Account: Account_Number, Interest_rate• Checking _Account: Account_Number,

Overdraft_Amount• The problem of this approach is that in order to obtain

Balance we have to join two tables.

• Method 2• In second method we create only two

tables as shown below:• Saving_Account: Account_Number,

Balance, Interest_rate• Checking _Account: Account_Number,

Balance, Overdraft_Amount• The problem of this is the redundancy of

Balance field.