1.3.2 SQL selecting data (multiple table)

Question 1

what does the command NESTED SELECT do?

Answer 1

a select within another select statement.

SELECT * FROM users WHERE age > (SELECT AVG(age) FROM users);

(retrieves users with an age greater than the average age)

Question 2

what does the command JOIN do?

Answer 2

combines data from 2 or more tables based on a related column/attribute.

SELECT users.name, orders.order_id FROM users
JOIN orders
ON users.user_id = orders.user_id;
(retrieves user names and their corresponding order IDs)