What other constraints are reasonable to use in a database besides PRIMARY KEY, UNIQUE, NOT NULL, and CHECK?

Besides PRIMARY KEY, UNIQUE, NOT NULL, and CHECK, databases often use the following constraints:

• FOREIGN KEY — ensures referential integrity between tables, guaranteeing that the value in the column exists in the related table.
• DEFAULT — sets a default value for a column if no value is specified during data insertion.
• INDEX (although not exactly a constraint) — speeds up searches on the column but does not affect data integrity.
• EXCLUSION CONSTRAINT (in some DBMSs, such as PostgreSQL) — allows setting exclusion conditions for rows.

Example of using FOREIGN KEY:

CREATE TABLE orders (
    order_id INT PRIMARY KEY,
    customer_id INT,
    FOREIGN KEY (customer_id) REFERENCES customers(customer_id)
);

This guarantees that you cannot insert a customer_id into orders that does not exist in the customers table.