What types of keys have you used in SQL tables and what are their features?

In SQL tables, the following key types are commonly used:

• Primary Key (PRIMARY KEY) — uniquely identifies each record in the table. Cannot contain NULL or duplicate values.
• Foreign Key (FOREIGN KEY) — references the primary key of another table, ensuring data integrity between tables.
• Unique Key (UNIQUE KEY) — guarantees the uniqueness of values in a column or set of columns, allows NULLs (depending on the DBMS).
• Composite Key (Composite Key) — a key consisting of multiple columns, used for unique identification of a record.

Features:

• Primary key ensures uniqueness and does not allow NULL.
• Foreign key maintains the relationship between tables and ensures referential integrity.
• Unique key is similar to primary but can allow NULLs and is not necessarily the main identifier.

Example of creating a table with keys:

CREATE TABLE orders (
    order_id INT PRIMARY KEY,
    customer_id INT,
    order_number VARCHAR(50) UNIQUE,
    FOREIGN KEY (customer_id) REFERENCES customers(customer_id)
);