SQL add table subset for each distinct value in another table

Question

I want to add a subset of a table (t2) to another table (t1) for each customer_id once.

t1 represents ordered materials. t2 represents a static table representing high-seller materials.

The goal is to create a table with material lists for each customer. (t2) does not include any customer_id reference but a region_id key.

Basically what I need is "add this subset material list to your existing material list for each customer based on his region"

I can't figure out how to join/union those tables without a customer_id, as I will get duplicated entries.

t1 looks like this:

Customer_id	region_id	material_id
155	NAM	AA-123-526
155	NAM	AA-425-123
157	NAM	AA-123-623
157	NAM	AA-612-244
158	SOA	AA-123-456
158	SOA	AA-655-876

t2 looks like this:

region_id	material_id
NAM	BB-724-623
NAM	BB-421-125
DACH	BB-123-622
DACH	BB-421-231
SOA	BB-123-551
SOA	BB-421-125

desired output:

Customer_id	region_id	material_id
155	NAM	AA-123-526
155	NAM	AA-425-123
155	NAM	BB-724-623
155	NAM	BB-421-125
157	NAM	AA-123-623
157	NAM	AA-612-244
157	NAM	BB-724-623
157	NAM	BB-421-125
158	SOA	AA-123-456
158	SOA	AA-655-876
158	SOA	BB-123-551
158	SOA	BB-421-125

Answer 1

Sample data

SELECT * FROM t1
SELECT * FROM t2

Solution

SELECT  t1.Customer_id, t1.region_id, t1.material_id FROM t1
UNION ALL
SELECT DISTINCT t1.Customer_id, t2.region_id, t2.material_id
FROM t2 INNER JOIN t1 ON t2.region_id = t1.region_id
ORDER BY t1.Customer_id