SQL statement with conditions on year

Question

I am a newbie to SQL and I am trying to add conditions in my SQL query.

I have 4 product pricing file for each year and another table for products.

price_table_2020
price_table_2019
price_table_2019

All the tables have the data as follows

price_table_2020 :

product  price   product_description
-------------------------------------
ball      100    plastic ball
ink        80    ink for pen
pen      1000    pen
bucket    200    bucket

price_table_2019 :

product  price   product_description
------------------------------------
ball      90     plastic ball
ink       70     ink for pen
pen      900     pen
bucket   100     bucket

price_table_2018 :

product  price   product_description
-------------------------------------
ball      80     plastic ball
ink       60     ink for pen
pen      800     pen
bucket   300     bucket

Product table looks like this:

product invoice_year
---------------------
pen     2019
ball    2020
ink     2020

Depending upon the invoice year, I am supposed to get the respective product's price from the respective tables.

Expected output as below:

product invoice_year   price
----------------------------
pen     2019           900
ball    2020           100
ink     2020            80

I am able to get the price from one of the table as below

select A.price, B.product, B.product_description 
from product A 
inner join price_table_2020 B on A.product = B.product

Can someone help how to do if I have to include the logic based on product.invoice_year ? Thanks.

Answer 1

What you are trying to do is difficult, to say the least. I would highly recommend restructuring your tables to put all the price data in one table, adding a year column to that table to indicate which year the price is applicable to. Then your query becomes simple:

SELECT pr.product,
       pr.invoice_year,
       pc.price
FROM product pr
JOIN price_table pc ON pc.product = pr.product
                   AND pc.year = pr.invoice_year
ORDER BY pr.invoice_year, pr.product

Output (for your sample data):

product     invoice_year    price
pen         2019            900
ball        2020            100
ink         2020            80

Demo on db-fiddle

If you have to maintain your current table structure, you could emulate the price table above using a UNION of all the price tables:

SELECT pr.product,
       pr.invoice_year,
       pc.price
FROM product pr
JOIN (
  SELECT *, 2020 AS year
  FROM price_table_2020
  UNION ALL
  SELECT *, 2019
  FROM price_table_2019
  UNION ALL
  SELECT *, 2018
  FROM price_table_2018
) pc ON pc.product = pr.product
    AND pc.year = pr.invoice_year
ORDER BY pr.invoice_year, pr.product

The output of this query is the same. The downside of this approach is that you have to edit the query every time you add a new price table.

Demo on db-fiddle

Answer 2

This is a very bad data model. Data belongs in tables, not in the database structure. The year is mere data (as can be seen from your product table), and it should be data in a price table. Instead you have a separate price table per year. This makes writing queries a nuisance, as can be seen below. I recommend you change this data model.

select p.*, coalesce(p2018.price, p2019.price, p2020.price) as invoice_price
from products p
left join price_table_2018 p2018 on p2018.product = p.product and p.invoice_year = 2018
left join price_table_2019 p2019 on p2019.product = p.product and p.invoice_year = 2019
left join price_table_2020 p2020 on p2020.product = p.product and p.invoice_year = 2020
order by p.product;