Update table if all results from multiple UNION queries is 0 - mysql

Question

I have 2 tables with stock from two different sellers.
If I can not source the product from any of these two sellers then I would like to set a value in another table to '0'.
First of all I am getting some required values from the two seller stock tables combined with a UNION
(SELECT A.`id`, B.`product_id`, A.`internal_id`, CASE WHEN A.`in_stock` = 'Stock available' THEN '1' ELSE '0' END as `in_stock` FROM `pricing_dm` A LEFT JOIN `pricing_vendor_mapping` B ON A.`internal_id`= B.`internal_id` WHERE B.`vendor_id` = '1' AND B.`product_id` > '0'
UNION
SELECT A.`id`, B.`product_id`, A.`internal_id`, CASE WHEN A.`in_stock` = 'In Stock' THEN '1' ELSE '0' END as `in_stock` FROM `pricing_m` A LEFT JOIN `pricing_vendor_mapping` B ON A.`internal_id`= B.`internal_id` WHERE B.`vendor_id` = '2' AND B.`product_id` > '0') t1
If I get a result like below where in_stock = 0 for both sellers then I would like to update another field with 0
id product_id internal_id in_stock
1 123 5555 0
1 123 4567 0
If I can source the product from one (like below) or both sellers then I don't want to do anything.
id product_id internal_id in_stock
1 123 5555 0
1 123 4567 1
Basically how can I check if all results for a product_id are 'in_stock = 0' and based on that do an update of another table.
Thank you

Answer 1

There are easier ways to do this if the tables can be joined, but You could get the max in_stock number and knowing that if it's over 0 you could do it with a CTE like this (I am not able to test this in mysql so it may need some syntax correction)
WITH CTE (in_stock) AS (
SELECT MAX(in_stock) from (
(SELECT A.`id`, B.`product_id`, A.`internal_id`, CASE WHEN A.`in_stock` = 'Stock available' THEN '1' ELSE '0' END as `in_stock` FROM `pricing_dm` A LEFT JOIN `pricing_vendor_mapping` B ON A.`internal_id`= B.`internal_id` WHERE B.`vendor_id` = '1' AND B.`product_id` > '0'
UNION
SELECT A.`id`, B.`product_id`, A.`internal_id`, CASE WHEN A.`in_stock` = 'In Stock' THEN '1' ELSE '0' END as `in_stock` FROM `pricing_m` A LEFT JOIN `pricing_vendor_mapping` B ON A.`internal_id`= B.`internal_id` WHERE B.`vendor_id` = '2' AND B.`product_id` > '0') t1 ))
UPDATE some_table SET some_column = 0
where CTE.in_stock < 1

Answer 2

I used nested queries and a variable to meet your criteria.
See example DDL in this link:https://www.db-fiddle.com/f/8jUMnbEFpGWA4fmmP67yw9/0 I think this meets your requirements.
Here is my Query, with comments:
#check initial another table values
select * from another_table;
#check the given resultset
#there are 3 pairs,
#2 of which meet the condition for not updating
#1 of the pairs meets the condition for doing the requested update
SELECT A.`id`, B.`product_id`, A.`internal_id`, CASE WHEN A.`in_stock` = 'Stock available' THEN '1' ELSE '0' END as `in_stock` FROM `pricing_dm` A LEFT JOIN `pricing_vendor_mapping` B ON A.`internal_id`= B.`internal_id` WHERE B.`vendor_id` = '1' AND B.`product_id` > '0'
UNION
SELECT A.`id`, B.`product_id`, A.`internal_id`, CASE WHEN A.`in_stock` = 'In Stock' THEN '1' ELSE '0' END as `in_stock` FROM `pricing_m` A LEFT JOIN `pricing_vendor_mapping` B ON A.`internal_id`= B.`internal_id` WHERE B.`vendor_id` = '2' AND B.`product_id` > '0';
#the requested update statement
set #chk1 := 0;
update another_table
set crab_people = 0
where product_id in
#this nested query will return all product_id
#that fits the criteria to update another_table
(select product_id_not_in_stock from
(select
case
when product_id = #chk1 and in_stock = 0 then #chk1
when product_id != #chk1 and in_stock = 0 then #chk1 := product_id
end as product_id_not_in_stock
from
(SELECT A.`id`, B.`product_id`, A.`internal_id`, CASE WHEN A.`in_stock` = 'Stock available' THEN '1' ELSE '0' END as `in_stock` FROM `pricing_dm` A LEFT JOIN `pricing_vendor_mapping` B ON A.`internal_id`= B.`internal_id` WHERE B.`vendor_id` = '1' AND B.`product_id` > '0'
UNION
SELECT A.`id`, B.`product_id`, A.`internal_id`, CASE WHEN A.`in_stock` = 'In Stock' THEN '1' ELSE '0' END as `in_stock` FROM `pricing_m` A LEFT JOIN `pricing_vendor_mapping` B ON A.`internal_id`= B.`internal_id` WHERE B.`vendor_id` = '2' AND B.`product_id` > '0') as t1
order by product_id, in_stock DESC, internal_id) as t2
where product_id_not_in_stock is not null
group by product_id_not_in_stock
having count(product_id_not_in_stock) > 1);
#confirm the changes went though
#expect crab_people from another_table to be updated
#where the product id is not in stock from both tables: pricing_dm and pricing_m
select * from another_table;