Update row based on multiple relations - mysql

Question

I have three tables:
Kits (kit_id, kit_weight)
Kit_Components (kit_id, quantity, component_id)
Components (component_id, weight)
For each entry in the kits table there can be one or more Kit_Component entries. Each component has a weight column which can either be the weight or null if we haven't weighed it yet. What I need to do is run an SQL query to update the weight column of the Kits table based on the total weight times quantity of all its components or if any of the weights are null set its value to null but I'm not even sure its possible, is it?
Note: I'd like to avoid scripts, triggers or procedures. I have code that does this when a component is saved or a kit is updated but I'd like to be able to do this in bulk.
EDIT: To further clarify I can SUM the weights * quantity however this doesn't deal with component rows being NULL as NULL acts as 0 in a SUM (I've tested this)
E.g. Kit1 has 1xComponentA with a weight of 14 and 2xComponentB with a weight of NULL
SELECT kit_id, SUM(component.weight * kit_component.quantity) FROM kit_component INNER JOIN component ON kit_component.component_id = component.id GROUP BY kit_component.kit_id
This would return 14 for kit1, however this is wrong because ComponentB has no weight so instead should return NULL.
Hugo Kornelis:
"If the data in a group (as formed by GROUP BY) has some NULLs and some
non-NULL data, the NULLs are ignored and the result is the sum of the
remaining numbers: SUM {1, 3, NULL, 5} = SUM {1, 3, 5} = 9
If all data in the group is NULL, the NULLs are ignored as well, leaving
no rows to be summed at all: the result is the sum of the empty set; by
definition this is NULL. SUM {NULL, NULL} = SUM {} = NULL."

Answer 1

Based on your edit, your problems seem to be to make the following query return NULL when any value going into it is NULL:
SELECT kit_id, SUM(component.weight * kit_component.quantity)
FROM kit_component INNER JOIN
component
ON kit_component.component_id = component.id
GROUP BY kit_component.kit_id
You can do this with additional logic:
SELECT kit_id,
(case when count(component.weight) = count(*) and
count(component.quantity) = count(*)
then SUM(component.weight * kit_component.quantity)
end)
FROM kit_component INNER JOIN
component
ON kit_component.component_id = component.id
GROUP BY kit_component.kit_id
Remember count(<field>) counts the number of non-NULL values in the field. So, the counts are essentially saying "all values are non-null" or, equivalently, "no values are null".

Answer 2

After looking around a bit more I realised the problem was the way that SUM handles groupings that have some NULL values. After finding this post SQL query to return NULL for SUM(expression) whenever an included value IS NULL I have work out a resolution and it is as follows:
UPDATE kits
LEFT JOIN
(SELECT
kit_id,
IF(SUM(component.weight is NULL), NULL, SUM(component.weight * kit_component.quantity)) AS total_weight
FROM
kit_component
INNER JOIN component ON kit_component.component_id = component.id
GROUP BY kit_component.kit_id) AS weights ON kits.id = weights.kit_id
SET
kits.weight = weights.total_weight
This will update the kits tables weight to null if any of its components weights are null or total weight if all components have valid values.