Select with innerjoin and dynamic update - mysql

Question

I created a query in my magento database to find product the have a "New From" dat but no "New To".
SELECT
catalog_product_entity.entity_id,
catalog_product_entity.sku,
a.value as New_From,
b.value as New_To
FROM `catalog_product_entity`
INNER JOIN catalog_product_entity_datetime as a on a.entity_id = catalog_product_entity.entity_id and a.attribute_id = '93'
INNER JOIN catalog_product_entity_datetime as b on b.entity_id = catalog_product_entity.entity_id and b.attribute_id = '94'
WHERE a.value IS NOT NULL AND b.value IS NULL
Now what I would like to accomplish since it concerns 500+ items to run a UPDATE statement on this query closing "New To" 1 month after having been opened.
Any ideas how to run that?
So to clarify:
Table 1: (catalog_product_entity)
| entity_id | sku |
| --------- | --- |
| 1 | ABC |
| 2 | DEF |
Table 2: (catalog_product_entity_datetime)
| id | entity_id | attribute_id | value |
| --- | --------- | ------------ | ----- |
| 1 | 1 | 93 | 2013-06-12 00:00:00 |
| 2 | 1 | 94 | 2013-07-12 00:00:00 |
| 3 | 1 | 98 | Some other attribute |
| 4 | 2 | 93 | 2014-08-20 00:00:00 |
| 5 | 2 | 94 | NULL | <- This I want updating
| 6 | 2 | 98 | Some other attribute |
I want to update the NULL values in table 2 with the date from that same table + 1 month. My search returns:
| entity -id | sku | New_From | New_To |
| ----------- | --- | ------------------- | ------------------- |
| 2 | DEF | 2014-08-20 00:00:00 | NULL |
So I would like to write the query that updates that field to the New_From + 1 month.
| entity -id | sku | New_From | New_To |
| ----------- | --- | ------------------- | ------------------- |
| 2 | DEF | 2014-08-20 00:00:00 | 2014-09-20 00:00:00 |
In my mind I will end up with something like this: (but this one does not work)
UPDATE catalog_product_entity_datetime
SET value = TIMESTAMP(a.value+300000000)
INNER JOIN catalog_product_entity_datetime as a on a.entity_id = catalog_product_entity_datetime.entity_id and a.attribute_id = '93'
WHERE attribute_id=94

Answer 1

Try this:
UPDATE catalog_product_entity_datetime t1
JOIN catalog_product_entity t2
ON t1.entity_id = t2.entity_id
AND t1.attribute_id = 94
JOIN catalog_product_entity_datetime t3
ON t2.entity_id = t3.entity_id
AND t3.attribute_id = 93
SET t1.value = CASE WHEN t1.value IS NULL THEN date_add(t3.value, interval 1 month)
ELSE t1.value END
;
Demo Here