What does the ON clause do?

The ON clause is used in SQL to specify conditions for certain operations, particularly in the context of joins and foreign key constraints. Here are the primary uses of the ON clause:

In Joins: The ON clause is used to define the condition for joining two or more tables. It specifies how the tables are related by indicating which columns should be matched.

Example:
```
SELECT *
FROM orders
JOIN customers ON orders.customer_id = customers.customer_id;
```
In this example, the ON clause specifies that the customer_id column in the orders table should match the customer_id column in the customers table to combine the rows from both tables.
In Foreign Key Constraints: The ON clause can also be used with foreign key constraints to define cascading actions that should occur when a referenced record is deleted or updated.

Example:
```
CREATE TABLE orders (
    order_id SERIAL PRIMARY KEY,
    customer_id INTEGER REFERENCES customers(customer_id) ON DELETE CASCADE
);
```
Here, the ON DELETE CASCADE part of the foreign key constraint specifies that if a record in the customers table is deleted, all related records in the orders table should also be deleted.

In summary, the ON clause is essential for defining relationships between tables in SQL queries and for specifying behavior in foreign key constraints.