? Composite Key in SQL

A Composite Key is a primary key that consists of two or more columns combined to uniquely identify each row in a table.

? Why Use a Composite Key?

• When a single column cannot uniquely identify a record.

• To enforce uniqueness based on the combination of multiple columns.

? How to Define a Composite Key

1. During Table Creation

CREATE TABLE orders (    order_id INT,    product_id INT,    quantity INT,    PRIMARY KEY (order_id, product_id));

• Here, the combination of order_id and product_id uniquely identifies each row.

2. On an Existing Table

ALTER TABLE ordersADD CONSTRAINT pk_orders PRIMARY KEY (order_id, product_id);

? Example Use Case

• In an order details table, an order can have multiple products.

• Neither order_id nor product_id alone is unique.

• The combination of order_id and product_id uniquely identifies each record.

? Key Points

• Composite keys enforce uniqueness on multiple columns.

• They are commonly used in many-to-many relationship tables (junction tables).

• All columns in the composite key together must be unique.

Want me to show how composite keys work with foreign keys or examples in specific databases?