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 

SELECT name 
FROM Employee 
WHERE id IN
  (SELECT managerId 
   FROM Employee 
   GROUP BY managerId 
   HAVING COUNT(*) >= 5
  )

-- OR
SELECT a.name
FROM Employee a
JOIN Employee b
WHERE a.id = b.managerId
GROUP BY b.managerId
HAVING COUNT(*) >= 5

SELECT 
  s.user_id, 
  ROUND(
    COALESCE(
      SUM(
        CASE WHEN ACTION = 'confirmed' THEN 1 END
      ) / COUNT(*), 0),2) 
  AS confirmation_rate 
FROM Signups s 
LEFT JOIN Confirmations c 
ON s.user_id = c.user_id 
GROUP BY s.user_id;  

-- odd id, "boring", rating desc
SELECT *
FROM Cinema
WHERE id % 2 <> 0 
AND description <> "boring"
ORDER BY rating DESC

-- avg(selling), round 2
SELECT p.product_id, 
  ROUND(SUM(price * units) / SUM(units), 2) AS average_price
FROM Prices p
LEFT JOIN UnitsSold s
ON p.product_id = s.product_id
AND purchase_date BETWEEN start_date AND end_date
GROUP BY p.product_id

-- avg(exp_yr), round 2, by project
SELECT project_id, ROUND(AVG(experience_years), 2) average_years
FROM Project p 
LEFT JOIN Employee e
ON p.employee_id = e.employee_id
GROUP BY project_id

-- % desc, contest_id asc, round 2
SELECT r.contest_id,
       ROUND(COUNT(DISTINCT r.user_id) * 100 / (SELECT COUNT(DISTINCT user_id) FROM Users), 2) AS percentage
FROM Register r
GROUP BY r.contest_id
ORDER BY percentage DESC, r.contest_id ASC;

--quality - avg(rating/position), poor query % - %(rating < 3), round 2
SELECT query_name, 
    ROUND(AVG(rating/position), 2) AS quality, 
    ROUND(SUM(IF(rating < 3, 1, 0)) * 100/ COUNT(rating), 2) AS poor_query_percentage
FROM Queries
GROUP BY query_name

-- OR
SELECT query_name, 
    ROUND(AVG(rating/position), 2) AS quality, 
    ROUND(SUM(
        CASE WHEN rating < 3 THEN 1 ELSE 0 END
    ) * 100/ COUNT(rating), 2) AS poor_query_percentage
FROM Queries
GROUP BY query_name

-- month, country, count(trans), total(amt), count(approved_trans), total(amt)
SELECT DATE_FORMAT(trans_date, '%Y-%m') month, country, 
        COUNT(state) trans_count, 
        SUM(IF(state = 'approved', 1, 0)) approved_count, 
        SUM(amount) trans_total_amount,
        SUM(IF(state = 'approved', amount, 0)) approved_total_amount
FROM Transactions
GROUP BY 1, 2

-- OR
SELECT DATE_FORMAT(trans_date, '%Y-%m') month, country, 
        COUNT(state) trans_count, 
        SUM(CASE WHEN state = 'approved' THEN 1 ELSE 0 END) approved_count, 
        SUM(amount) trans_total_amount,
        SUM(CASE WHEN state = 'approved' THEN amount ELSE 0 END) approved_total_amount
FROM Transactions
GROUP BY 1, 2

SELECT
    ROUND((COUNT(CASE WHEN d.order_date = d.customer_pref_delivery_date THEN 1 END) / COUNT(*)) * 100, 2)  immediate_percentage
FROM Delivery d
WHERE d.order_date = (
    SELECT
    MIN(order_date)
    FROM Delivery
    WHERE customer_id = d.customer_id
    );

-- OR
SELECT ROUND(AVG(temp.order_date=temp.customer_pref_delivery_date) * 100, 2) immediate_percentage
FROM (
    SELECT *, RANK() OVER(partition by customer_id ORDER BY order_date) od
    FROM Delivery) temp
WHERE temp.od = 1

WITH login_date AS (SELECT player_id, MIN(event_date) AS first_login
FROM Activity
GROUP BY player_id),

recent_login AS (
SELECT *, DATE_ADD(first_login, INTERVAL 1 DAY) AS next_day
FROM login_date)

SELECT ROUND((SELECT COUNT(DISTINCT(player_id))
FROM Activity
WHERE (player_id, event_date) IN 
(SELECT player_id, next_day FROM recent_login)) / (SELECT COUNT(DISTINCT player_id) FROM Activity), 2) AS fraction

SELECT teacher_id, COUNT(DISTINCT subject_id) cnt
FROM Teacher
GROUP BY teacher_id

SELECT activity_date as day, COUNT(DISTINCT user_id) AS active_users
FROM Activity
WHERE activity_date BETWEEN DATE_SUB('2019-07-27', INTERVAL 29 DAY) AND '2019-07-27'
GROUP BY activity_date

SELECT s.product_id, s.year AS first_year, s.quantity, s.price
FROM Sales s
JOIN (
  SELECT product_id, MIN(year) AS year 
  FROM sales 
  GROUP BY product_id
  ) p
ON s.product_id = p.product_id
AND s.year = p.year

-- OR
WITH first_year_sales AS (
  SELECT s.product_id, MIN(s.year) as first_year
  FROM Sales s
  INNER JOIN Product p
  ON s.product_id = p.product_id
  GROUP BY s.product_id)
SELECT f.product_id, f.first_year, s.quantity, s.price
FROM first_year_sales f
JOIN Sales s 
ON f.product_id = s.product_id
AND f.first_year = s.year

SELECT class
FROM Courses
GROUP BY class
HAVING COUNT(student) >= 5

SELECT user_id, COUNT(DISTINCT follower_id) AS followers_count
FROM Followers
GROUP BY user_id
ORDER BY user_id ASC

SELECT COALESCE(
  (SELECT num
  FROM MyNumbers
  GROUP BY num
  HAVING COUNT(num) = 1
  ORDER BY num DESC
  LIMIT 1), null) 
  AS num

SELECT customer_id
FROM Customer
GROUP BY customer_id
HAVING COUNT(DISTINCT product_key) = (
  SELECT COUNT(product_key)
  FROM Product
)

SELECT e1.employee_id, e1.name, COUNT(e2.employee_id) reports_count, ROUND(AVG(e2.age)) average_age 
FROM Employees e1, Employees e2
WHERE e1.employee_id = e2.reports_to
GROUP BY e1.employee_id
HAVING reports_count > 0
ORDER BY e1.employee_id

SELECT employee_id, department_id
FROM Employee 
WHERE primary_flag = 'Y'
UNION
SELECT employee_id, department_id
FROM Employee
GROUP BY employee_id
HAVING COUNT(employee_id)=1

-- OR
SELECT employee_id,department_id
FROM Employee
WHERE primary_flag = 'Y' OR employee_id IN
    (SELECT employee_id
     FROM employee
     GROUP BY employee_id
     HAVING COUNT(department_id) = 1
    )