Combining multiple related tables with different dates - sql

Question

Suppose I have a company that each month buys a certain amount of computers, and the ratios of desktop to laptops as well as the brand changes sometimes (This is made up scenario, my actual scenario has nothing to do with computers). Ex:
For the accounting department:
On 1/1:
Laptops 50% (Dell) Desktop 50% (Acer)
For the next few months, I use that buying structure. However, I decide I want to change brands:
On 4/1:
Laptops 50% (Apple) Desktop 50% (Asus)
Then later, I decide I need more laptops:
On 7/1:
Laptops 75% (Apple) Desktop 25% (Asus)
For a certain department, I would like to get the following data in a report:
Date
Company
Weight
1/1
Dell
.5
1/1
Acer
.5
4/1
Apple
.5
4/1
Asus
.5
7/1
Apple
.75
7/1
Asus
.25
Here is what the database table structure looks like:
Table: department
id
name
1
Accounting
2
Developers
Table: computer_type
id
name
1
Laptop
2
Desktop
Table: purchase_weights
date
computer_type_id
weight
department_id
1/1
1
.5
1
1/1
2
.5
1
7/1
1
.25
1
7/1
2
.75
1
Table: purchase_companies
date
computer_type_id
company
1/1
1
Dell
1/1
2
Acer
4/1
1
Apple
4/1
2
Asus
As you can see, the tables need to be joined in a way that the dates that are missing in one table get forward filled. There is no entry in the weights table for 4/1 when the company changed, and there is no entry in the company table when the weight changed. The constraints can also be seen, where if you change the company for laptops, all departments will start to buy that company if their weights include that computer type. Any insight would be very helpful, thanks!
Things I have tried:
Full outer join (does not include missing dates)
SELECT * FROM purchase_weights pw FULL OUTER JOIN purchase_companies pc ON pw.computer_type_id = pc.computer_type_id AND pw.date = pc.date WHERE pw.department_id = 1
Normal join (does a cross multiplication)
SELECT * FROM purchase_weights pw JOIN purchase_companies pc ON pw.computer_type_id = pc.computer_type_id WHERE pw.department_id = 1