SELECT product_id
FROM Products
WHERE low_fats = 'Y'
AND recyclable = 'Y'

SELECT name 
FROM Customer 
WHERE referee_id != 2 OR referee_id IS null

SELECT name, population, area
FROM WORLD
WHERE area >= 3000000
OR population >= 25000000

SELECT DISTINCT author_id as id
FROM Views
WHERE viewer_id >= 1
AND author_id = viewer_id
ORDER BY author_id

SELECT tweet_id
FROM Tweets
WHERE length(content) > 15

SELECT unique_id, name
FROM Employees e
LEFT JOIN EmployeeUNI eu
ON e.id = eu.id

SELECT product_name, year, price
FROM Sales s
LEFT JOIN Product p
ON s.product_id = p.product_id

SELECT customer_id, COUNT(*) as count_no_trans
FROM Visits 
WHERE visit_id NOT IN (SELECT DISTINCT visit_id FROM Transactions)
GROUP BY customer_id

SELECT w1.id 
FROM Weather w1, Weather w2
WHERE DATEDIFF(w1.recordDate, w2.recordDate) = 1
AND w1.temperature > w2.temperature

-- OR
SELECT w1.id
FROM Weather w1, Weather w2
WHERE w1.temperature > w2.temperature
AND SUBDATE(w1.recordDate, 1) = w2.recordDate

SELECT machine_id, ROUND(AVG(end - start), 3) AS processing_time
FROM 
(SELECT machine_id, process_id, 
    MAX(CASE WHEN activity_type = 'start' THEN timestamp END) AS start,
    MAX(CASE WHEN activity_type = 'end' THEN timestamp END) AS end
 FROM Activity 
  GROUP BY machine_id, process_id) AS subq
GROUP BY machine_id

SELECT name, bonus
FROM Employee e
LEFT JOIN Bonus b
ON e.empId = b.empId
WHERE bonus < 1000
OR bonus IS NULL

SELECT a.student_id, a.student_name, b.subject_name, COUNT(c.subject_name) AS attended_exams
FROM Students a
JOIN Subjects b
LEFT JOIN Examinations c
ON a.student_id = c.student_id
AND b.subject_name = c.subject_name
GROUP BY 1, 3
ORDER BY 1, 3