Which constraints is used to establish parent-child relation between two or more tables ?

Understanding Parent-Child Relationships in Databases

In relational database management systems (RDBMS), parent-child relationships between tables are established using foreign key constraints. Here’s a detailed explanation:

1. Definition of Foreign Key: A foreign key is a column or a set of columns in one table (the child table) that establishes a link between data in two tables. It represents a reference to the primary key in another table (the parent table).

2. Role of Constraints: The foreign key constraint ensures referential integrity between the two tables. This means that the values in the foreign key column must match values in the primary key column of the parent table or be null. This helps to maintain consistency and integrity of the data across tables.

3. Parent Table: The table that has the primary key. It is considered the "parent" in the relationship. Each row in this table can be related to multiple rows in the child table.

4. Child Table: The table that contains the foreign key. This is the "child" table that references the parent table. Each row in this table may relate to exactly one row in the parent table, but a parent row can relate to multiple child rows.

5. Example: For instance, if we have a "Customers" table (parent) with a primary key of CustomerID and an "Orders" table (child) with a CustomerID foreign key, each order in the "Orders" table can be linked back to exactly one customer in the "Customers" table.

The use of foreign key constraints is essential for maintaining data integrity and defining the relationships between different entities in a relational database.