What are relationship in rdbms?

Answer 1

Relational databases store information in tables. A database for an online shop might have tables for Products, Customers and Orders.

An Order can consist of multiple Products

An Order is placed by one Customer

These are examples of how data are related. In database language you would say an Order has a many-to-manyrelationship with Product, because an order can contain multiple Products, but a Product can be part of many Orders.

Customer would have a one-to-many relationship with Order, because a (one) customer can place multiple (many) orders over time, and each (many) order belongs to exactly one customer.

Many-to-many and one-to-one are the most common database relationships.

Now, a database table consists of rows or records. Each record in a Customer table represents one customer. If you wanted to create relationships between the Customers in the customer table and orders in the Order table you would use keys.

In a database table each record has a unique key, called the primary key. This is often a number. The primary key is used to link rows in one table to rows in another table. The primary key of a Customer row might be copied to an Order row, linking that Order to the Customer.

This is how relationships are created between data in a relational database.