How to update the same row with condition in MySql? - mysql

Question

Here's data
KeyID | Queue | Pay
65 1 0
60 2 0
58 3 1
57 4 1
55 5 0
54 6 0
53 7 1
50 8 1
if the data like this , I need a single MySql query to update it to be like below data table which update only Queue column.
KeyID | Queue | Pay
65 0 0
60 0 0
58 1 1
57 2 1
55 0 0
54 0 0
53 3 1
50 4 1
I have try this
update tabl1
set Queue = case when Pay = 0 then Queue=Queue-1 else Queue
But Queue number not look like this.
Please suggest.
Thank you in advance

Answer 1

We can try joining to a subquery which computes the sequence:
UPDATE yourTable k1
INNER JOIN
(
SELECT KeyID, rn
FROM
(
SELECT t1.KeyID,
(SELECT SUM(t2.Pay = 1) FROM yourTable t2
WHERE t2.KeyID >= t1.KeyID) rn
FROM yourTable t1
) t
) k2
ON k1.KeyID = k2.KeyID
SET
Queue = CASE WHEN Pay = 0 THEN 0 ELSE k2.rn END;
To see how the above logic is working, here is what the intermediate join table looks like:
KeyID | Queue | Pay | rn
65 | 1 | 0 | 0
60 | 2 | 0 | 0
58 | 3 | 1 | 1
57 | 4 | 1 | 2
55 | 5 | 0 | 2
54 | 6 | 0 | 2
53 | 7 | 1 | 3
50 | 8 | 1 | 4
That is, the innermost correlated subquery generates the queue sequence by counting the number of times Pay is 1.
Note that if you are using MySQL 8+, then there is a much simpler query using analytic functions:
UPDATE yourTable
SET Queue = CASE WHEN Pay = 0
THEN 0
ELSE SUM(Pay = 1) OVER (ORDER BY KeyID DESC) END;