How to create a transactional master table via MySQL by joining multiple separate tables on common date and product key plus specific dimensions?

Question

I am dealing with the following challenge:

I have multiple (sparse) tables that all share a common date field + common "product_id"s. Additionally each table has one additional field which tracks a specific kind of transactional data for the product, such as "units sold", "units purchased", "received_support_requests", etc.

Table 1

DATE	PRODUCT ID	UNITS SOLD
2022-01-01	1	10
2022-01-02	2	40

Table 2

DATE	PRODUCT ID	UNITS PURCHASED
2022-01-01	2	456
2022-01-04	5	34

Table 3

DATE	PRODUCT ID	RECEIVED SUPPORT REQUESTS
2022-01-04	5	2
2022-01-05	2	1

My goal is to somehow join all of these tables so that I get a master table which shows what happened to each product on a specific day as shown below using MySQL:

DATE	PRODUCT ID	UNITS SOLD	UNITS PURCHASED	RECEIVED SUPPORT REQUESTS
2022-01-01	1	10	0	0
2022-01-01	2	0	456	0
2022-01-02	2	40	0	0
2022-01-04	5	0	34	2
2022-01-05	2	0	0	1

The tables are all very long > 50000 and contain a big list of products > 3000

I first though about building a kind of ("date" - "product id") scaffold and then just left join all of the tables to this scaffold table. Unfortunately the combination of each date, with each product gets too big.

How would you accomplish such a join in a more efficient way?

Answer 1

SELECT date, 
       product_id,
       COALESCE(table1.amount, 0) sold,
       COALESCE(table2.amount, 0) purchased,
       COALESCE(table3.amount, 0) supported
FROM ( SELECT date FROM table1 
       UNION
       SELECT date FROM table2 
       UNION
       SELECT date FROM table3 ) dates
CROSS 
JOIN ( SELECT product_id FROM table1 
       UNION
       SELECT product_id FROM table2 
       UNION
       SELECT product_id FROM table3 ) products
NATURAL LEFT JOIN table1
NATURAL LEFT JOIN table2
NATURAL LEFT JOIN table3
HAVING sold + purchased + supported;