One-to-Many Relationship

In this relationship, each row/record of the Let’sfirst table can be related to multiple rows in the second table.

Example:

Let’s consider 2 tables “Parent” and “Child“. Now the “Parent” table has columns “ParentID“, “Name“… and so on, and “Child” has columns or attributes “ChildID“, “Name“, “Age“, “ParentID” and so on.

In the above example we used the logic of parent and child and you can see that a parent can have many children, but a child cannot have many/multiple parents.

Create tables ‘parent‘ and ‘child‘ as mentioned in the above table and let’s see one-to-many relation between these 2 tables.

We can see that each record from the table parent is associated with one or more than one record(s) in the child table representing one-to-many relationships. Here, a one-to-many relationship occurs when the vice-versa of the above condition should not be true. If the vice-versa or reverse condition for a one-to-many relationship becomes true then it is a many-to-many relationship.

SQL Query:

Let’s see the query to retrieve data representing one-to-many relationshipsparent-to-child:

SELECT parent.parentID, parent.Name, parent.age, parent.Address, child.ChildID, child.Name, child.age
FROM parent
JOIN child ON parent.parentID = child.ParentID

The above query retrieves all possible records from parent-to-child relationship entities.

Explanation: In the output of the above query, you can see that the parent’s name is occurring more than one time with different child names representing a parent associated with one or more than one child. For example, we can see that HarishKumar Verma has two children, Jiya Verma, and Vihaan Verma, Vishvaas Modi has 3 children i.e. Vrisha Modi, Mihir Modi, and Farah Modi, and last Vidhit Chopra has only a single child i.e. Nickie Chopra representing one record/row associated with one or more than one child.

Each record from the child table is associated with only one record in the parent table and it also fits the logic that a parent may have one or more than one child but a child cannot have multiple parents. Similarly, you can also design the relation as per the business logic which fits into which relationship better.

In database design, understanding and implementing relationships between entities is crucial. These relationships, such as one-to-one, one-to-many, and many-to-many, establish connections between tables using key constraints.

Let’s explore each type of relationship with examples and SQL implementation.