SQL query with time comparison - mysql

Question

I have a table of users and a table of orders. Table data is linked using a key
user_id. The user has a date of birth. It is necessary to compose a query to display one random user from the users table, over 30 years old, who has made at least 3 orders in the last six months.
I was able to make a query to sample by age:
SELECT Name from users WHERE(DATEDIFF(SYSDATE(), birthday_at)/365)>30;
but I don’t know how to solve the problem to the end

Answer 1

I like the additional effort LukStorms has shown by including details of the date calculations but one important point was missed. It may seem like a subtle difference but it is amazing how often it goes unnoticed until the dataset gets significantly larger. In the WHERE clause for the users age -
WHERE TIMESTAMPDIFF(YEAR, usr.birthday_at, CURDATE()) > 30
the result of the function call (age calculation) is being compared to a static integer. This will result in every user record having its age calculated unnecessarily and will also mean that any applicable index on the birthday_at column cannot be used. By moving the date calculation to the other side of the comparison available indices can be used -
WHERE u.birthday_at <= DATE_SUB(CURDATE(), INTERVAL 30 YEAR)
This may be insignificant for your use case but it is still a good habit to get into as it will almost certainly catch you out one day.
Furthermore, if you are retrieving the random user as part of some kind of reward scheme, I would suggest applying a random order of some kind as the single row returned will be predictable and repeatable.
SELECT u.id, u.Name
FROM users AS u
JOIN orders AS o
ON u.id = o.user_id
AND o.order_date >= DATE_SUB(CURDATE(), INTERVAL 6 MONTH)
WHERE u.birthday_at <= DATE_SUB(CURDATE(), INTERVAL 30 YEAR)
GROUP BY u.id
HAVING COUNT(o.id) >= 3
ORDER BY RAND()
LIMIT 1

Answer 2

Join to orders
Get only those over 30 years old and with orders from last 6 months
Group by the user
Filter on the count with a having
Limit to 1 without sorting (since random)
SELECT usr.Name AS UserName
FROM users AS usr
JOIN orders AS ord
ON ord.user_id = usr.user_id
WHERE TIMESTAMPDIFF(YEAR, usr.birthday_at, CURDATE()) > 30
AND ord.order_date BETWEEN DATE_ADD(LAST_DAY(DATE_SUB(CURDATE(), INTERVAL 6+1 MONTH)), INTERVAL 1 DAY)
AND LAST_DAY(DATE_SUB(CURDATE(), INTERVAL 1 MONTH))
GROUP BY usr.Name
HAVING COUNT(ord.order_id) >= 3
LIMIT 1
Test code for the date calculations
-- previous month, last day
select LAST_DAY(DATE_SUB(CURDATE(), INTERVAL 1 MONTH))
| LAST_DAY(DATE_SUB(CURDATE(), INTERVAL 1 MONTH)) |
| :---------------------------------------------- |
| 2021-10-31 |
-- 6 months ago, first day
select DATE_ADD(LAST_DAY(DATE_SUB(CURDATE(), INTERVAL 6+1 MONTH)), INTERVAL 1 DAY)
| DATE_ADD(LAST_DAY(DATE_SUB(CURDATE(), INTERVAL 6+1 MONTH)), INTERVAL 1 DAY) |
| :-------------------------------------------------------------------------- |
| 2021-05-01 |
-- someone's current age
select TIMESTAMPDIFF(YEAR, '2005-11-28', CURDATE())
| TIMESTAMPDIFF(YEAR, '2005-11-28', CURDATE()) |
| -------------------------------------------: |
| 15 |
db<>fiddle here