Join tables and update column on multiple conditions - mysql

Question

I have two tables customers and orders, where important columns are:
/* Customers: */
id | login | membershipid
100 | email1 | 0
101 | email2 | 0
/* Orders: */
userid | total
100 | 150
101 | 120
101 | 450
I want to update membershipid depending of total purchases for each customer. Lets say it will be zero if the sum is less than 500, one if sum is 500-1000, two if sum is over 1000.
I get the sum this way:
SELECT customers.login, sum( orders.total ) AS total_purchased
FROM xcart_customers AS customers
LEFT JOIN xcart_orders AS orders ON customers.id = orders.userid
GROUP BY customers.login
but I am not sure how to update back customers table. Something like:
UPDATE xcart_customers set (membershipid) = (
SELECT sum( orders.total ) AS total_purchased
FROM xcart_customers AS customers
LEFT JOIN xcart_orders AS orders ON customers.id = orders.userid
GROUP BY customers.login
)
but where and how to use conditions (and will GROUP BY work?):
CASE
WHEN
..
ELSE
END

Answer 1

UPDATE xcart_customers
join
(
SELECT customers.login, sum( orders.total ) AS total_purchased
FROM xcart_customers AS customers
LEFT JOIN xcart_orders AS orders ON customers.id = orders.userid
GROUP BY customers.login
) tmp on tmp.login = xcart_customers.login
set xcart_customers.membershipid = case when total_purchased is null or total_purchased < 500 then 0
when total_purchased between 500 and 1000 then 1
else 2
end

Answer 2

One way to do this is with a join operation. Take your query, and turn it into an inline view, and join that back to the customers table.
We might want to handle the case when total_purchased is NULL as being less than 500 (when a customer doesn't have any related rows in xcart_orders.)
As a SELECT it would look like this:
SELECT t.login
, t.membershipid
, CASE
WHEN IFNULL(s.total_purchased,0) < 500 THEN 0
WHEN s.total_purchased <= 1000 THEN 1
WHEN s.total_purchased > 1000 THEN 2
ELSE NULL
END AS new_membershipid
FROM customers t
JOIN ( SELECT c.login
, SUM(o.total) AS total_purchased
FROM xcart_customers c
LEFT
JOIN xcart_orders o
ON o.userid = c.id
GROUP BY c.login
) s
ON s.login = t.login
To convert that into an UPDATE, replace the SELECT ... FROM with UPDATE
and add a SET clause at the end of the statement (before the WHERE clause if there was a WHERE clause).
For example:
UPDATE customers t
JOIN ( SELECT c.login
, SUM(o.total) AS total_purchased
FROM xcart_customers c
LEFT
JOIN xcart_orders o
ON o.userid = c.id
GROUP BY c.login
) s
ON s.login = t.login
SET t.membershipid =
CASE
WHEN IFNULL(s.total_purchased,0) < 500 THEN 0
WHEN s.total_purchased <= 1000 THEN 1
WHEN s.total_purchased > 1000 THEN 2
ELSE NULL
END
NOTE: This assumes that login is UNIQUE` in the customers table.
FOLLOWUP
Given that membershipid cannot be assigned a NULL value, we can tweak the CASE expression. Do the check for > 1000 first, then a check for >= 500, else 0.
For example:
CASE
WHEN s.total_purchased > 1000 THEN 2
WHEN s.total_purchased >= 500 THEN 1
ELSE 0
END