Get last balance sign change in (My)SQL - mysql

Question

I have a Transaction table that records every amount added to or subtracted from the balance of a Customer, with the new balance:
+----+------------+------------+--------+---------+
| id | customerId | timestamp | amount | balance |
+----+------------+------------+--------+---------+
| 1 | 1 | 1000000001 | 10 | 10 |
| 2 | 1 | 1000000002 | -20 | -10 |
| 3 | 1 | 1000000003 | -10 | -20 |
| 4 | 2 | 1000000004 | -5 | -5 |
| 5 | 2 | 1000000005 | -5 | -10 |
| 6 | 2 | 1000000006 | 10 | 0 |
| 7 | 3 | 1000000007 | -5 | -5 |
| 8 | 3 | 1000000008 | 10 | 5 |
| 9 | 3 | 1000000009 | 10 | 15 |
| 10 | 4 | 1000000010 | 5 | 5 |
+----+------------+------------+--------+---------+
The Customer table stores the current balance, and looks like:
+----+---------+
| id | balance |
+----+---------+
| 1 | -20 |
| 2 | 0 |
| 3 | 15 |
| 4 | 5 |
+----+---------+
I would like to add a balanceSignSince column, that would store the timestamp at which the balance sign last changed. Transitioning to and from positive, negative, or zero counts as a balance change.
After the update, based on the above data, the Customer table should contain:
+----+---------+------------------+
| id | balance | balanceSignSince |
+----+---------+------------------+
| 1 | -20 | 1000000002 |
| 2 | 0 | 1000000006 |
| 3 | 15 | 1000000008 |
| 4 | 5 | 1000000010 |
+----+---------+------------------+
How can I write a SQL query that updates every Customer with the last time the balance sign changed, based on the Transaction table?
I suspect I can't do this without a quite complex stored procedure, but am curious to see if any clever ideas come up.

Answer 1

This uses a simulated rank() function.
select customerId, min(tstamp) from
(
select tstamp,
if (#cust = customerId and sign(#bal) = sign(balance), #rn := #rn,
if (#cust = customerId and sign(#bal) <> sign(balance), #rn := #rn + 1, #rn := 0)) as rn,
#cust := customerId as customerId, #bal := balance as balance
from
(select #rn := 0) x,
(select id, #cust := customerId as customerId, tstamp, amount, #bal := balance as balance
from trans order by customerId, tstamp desc) y
) z
where rn = 0
group by customerId;
Check it: http://rextester.com/XJVKK61181
This script returns a table like this:
+------------+----+------------+---------+
| tstamp | rn | customerId | balance |
+------------+----+------------+---------+
| 1000000003 | 0 | 1 | -20 |
| 1000000002 | 0 | 1 | -10 |
| 1000000001 | 1 | 1 | 10 |
| 1000000006 | 0 | 2 | 0 |
| 1000000005 | 2 | 2 | -10 |
| 1000000004 | 2 | 2 | -5 |
| 1000000009 | 0 | 3 | 15 |
| 1000000008 | 2 | 3 | 5 |
| 1000000007 | 3 | 3 | -5 |
| 1000000010 | 0 | 4 | 5 |
+------------+----+------------+---------+
Then selecting min(timestamp) of files where rn = 0:
+------------+-------------+
| customerId | min(tstamp) |
+------------+-------------+
| 1 | 1000000002 |
+------------+-------------+
| 2 | 1000000006 |
+------------+-------------+
| 3 | 1000000009 |
+------------+-------------+
| 4 | 1000000010 |
+------------+-------------+

Answer 2

Updated answer with the restriction that this needs to work on the existing data
The following query should work for most cases, there is still an issue with customers having only a single transaction or no sign change. As this is a one time update, I would run the query below and then do a simple update for all users not having a timestamp set, for them it's going to be the timestamp of the first transaction:
# Find the smallest timestamp, e.g. the
# transaction which changed the signum.
SELECT
p.customerId as customerId,
MIN(t.timestamp) as balanceSignSince
FROM
transaction as t,
(
# find the latest timestamp having
# a different sign for each user.
# Here is the issue with users having
# only a single transaction or no sign
# changes.
SELECT
u.customerId as customerId,
MAX(t.timestamp) as balanceSignSince
FROM
transaction as t,
customer as c,
(
# find the timestamp of the very last
# transaction for every user.
SELECT
t.customerId as customerId,
MAX(t.timestamp) as lastTransaction
FROM
transaction as t
GROUP BY
t.customerId
) as u
WHERE
u.customerId = c.id
AND u.customerId = t.customerId
AND SIGN(c.balance) <> SIGN(t.balance)
GROUP BY
u.customerId
) as p
WHERE
p.customerId = t.customerId
AND p.balanceSignSince < t.timestamp
GROUP BY
p.customerId;
Fiddle: http://sqlfiddle.com/#!9/bd0760/13
Original Answer
This should work to get the timestamp of a sign change:
SELECT
c.id as id,
MAX(t.timestamp) as balanceSignSince
FROM
transaction as t,
customer as c
WHERE
t.customerId = c.id
AND SIGN(t.balance) <> SIGN(c.balance)
This needs to be executed before the customer table is updated with the new balance. If you have a trigger on transation:insert you should probably put the above into the query updating the customer table.