Update MySQL Table from Subquery/Joined Same Table - mysql

Question

I've seen many questions along this issue, but can't get this to work.
I want to UPDATE multiple columns in a table (but will start with one) based upon a calculated value from the same table.
It is a list of transactions per customer, per month.
TransID | Cust | Month | Value | PastValue | FutureValue
1 | 1 | 2018-01-01 | 45 |
2 | 1 | 2018-02-01 | 0 |
3 | 1 | 2018-03-01 | 35 |
4 | 1 | 2018-04-01 | 80 |
.
UPDATE tbl_transaction a
SET PrevMnthValue =
(SELECT COUNT(TransactionID) FROM tbl_transaction b WHERE b.Cust=a.Cust AND b.Month<a.Month)
But we get the dreaded 'Can't update a table using a where with a subquery of the same table).
I've tried to nest the subquery as this has been touted as a workaround:
UPDATE tbl_transactions a
SET
PastValue =
(
SELECT CNT FROM
(
SELECT
COUNT(TransactionID) AS CNT
FROM tbl_transactions b
WHERE
b.CustomerRef=a.CustomerRef AND b.Month<a.Month
) x
),
FutureValue =
(
SELECT CNT FROM
(
SELECT
COUNT(TransactionID) AS CNT
FROM tbl_transactions b
WHERE
b.CustomerRef=a.CustomerRef AND b.Month>a.Month
) x
)
But I get an UNKNOWN a.CustomerRef in WHERE clause. Where am I going wrong?

Answer 1

You can't update and read from one table at the same time.
MySQL documentation tell about it
You cannot update a table and select from the same table in a subquery.
At first you must select necessary data and save them to somewhere, for example to temporary table
CREATE TEMPORARY TABLE IF NOT EXISTS `temp` AS (
SELECT
COUNT(`TransactionID`) AS CNT,
`CustomerRef`,
`Month`
FROM `tbl_transactions`
GROUP BY `Custom,erRef`, `Month`
);
After it, you can use JOIN statement for update table
UPDATE `tbl_transactions` RIGTH
JOIN `temp` ON `temp`.`CustomerRef` = `tbl_transactions`.`CustomerRef`
AND `temp`.`Month` < `tbl_transactions`.`Month`
SET `tbl_transactions`.`PastValue` = `temp`.`cnt`
UPDATED: if you want to update several columns by different condition you can combine temporary table, UPDATE + RIGHT JOIN and CASE statement. For example:
UPDATE `tbl_transactions`
RIGTH JOIN `temp` ON `temp`.`CustomerRef` = `tbl_transactions`.`CustomerRef`
SET `tbl_transactions`.`PastValue` = CASE
WHEN `temp`.`Month` < `tbl_transactions`.`Month` THEN `temp`.`cnt`
ELSE `tbl_transactions`.`PastValue`
END,
`tbl_transactions`.`FutureValue` = CASE
WHEN `temp`.`Month` > `tbl_transactions`.`Month` THEN `temp`.`cnt`
ELSE `tbl_transactions`.`FutureValue`
END

Answer 2

You can try below
UPDATE tbl_transactions a
Join
( SELECT CustomerRef,COUNT(TransactionID) AS CNT FROM tbl_transactions b
group by CustomerRef)x
SET PastValue = CNT
WHERE x.CustomerRef=a.CustomerRef AND x.Month<a.Month