Solution

To determine the prices of all products on a specific date (2019-08-16), considering that the default price before any changes is 10, we can follow a systematic approach. This involves identifying the most recent price change for each product up to the specified date and handling cases where no price changes have occurred by assigning the default price.

• Identify Distinct Products: Retrieve all unique product_ids from the PriceChanges table.
• Determine the Latest Price Before or On the Specified Date: For each product, find the most recent new_price that became effective on or before 2019-08-16.
• Assign Default Price Where Applicable: If a product has no price changes before or on 2019-08-16, assign the default price of 10.
• Order the Results: Sort the final output by product_id in ascending order.

SQL Query

SELECT p.product_id,
       COALESCE(
           (SELECT new_price 
            FROM PriceChanges pc 
            WHERE pc.product_id = p.product_id AND pc.effective_date <= '2019-08-16' 
            ORDER BY pc.effective_date DESC LIMIT 1), 
           10
       ) AS price
FROM (SELECT DISTINCT product_id FROM PriceChanges) p
ORDER BY p.product_id;

Step-by-Step Approach

Step 1: Identify Distinct Products

Retrieve all unique product_ids from the PriceChanges table to ensure that each product is considered in the final report.

SQL Query:

SELECT DISTINCT product_id
FROM PriceChanges;

Explanation:

• SELECT DISTINCT product_id:
  • Selects each unique product_id from the PriceChanges table to avoid processing duplicate entries.
• FROM PriceChanges:
  • Specifies the PriceChanges table as the data source.

Output After Step 1:

Assuming the example input, the subquery would produce:

+------------+
| product_id |
+------------+
| 1          |
| 2          |
| 3          |
+------------+

Step 2: Determine the Latest Price Before or On 2019-08-16

For each product, identify the most recent new_price that became effective on or before 2019-08-16. This step ensures that we capture the accurate price applicable on the specified date.

SQL Query:

SELECT new_price 
FROM PriceChanges pc 
WHERE pc.product_id = p.product_id 
  AND pc.effective_date <= '2019-08-16' 
ORDER BY pc.effective_date DESC 
LIMIT 1

Explanation:

• SELECT new_price:
  • Retrieves the new_price for the product.
• FROM PriceChanges pc:
  • Specifies the PriceChanges table with an alias pc for reference.
• WHERE pc.product_id = p.product_id AND pc.effective_date <= '2019-08-16':
  • Filters the records to include only those where:
    • The product_id matches the current product being evaluated.
    • The effective_date is on or before 2019-08-16.
• ORDER BY pc.effective_date DESC:
  • Orders the filtered records in descending order of effective_date to prioritize the most recent price change.
• LIMIT 1:
  • Restricts the result to the top record, effectively selecting the latest applicable new_price.

Purpose in Main Query:
This subquery is used within the COALESCE function to fetch the latest price for each product up to the specified date. If no such price exists, COALESCE will assign the default value of 10.

Step 3: Assign Default Price Where Applicable and Order the Results

Combine the distinct products with their corresponding latest prices or assign the default price of 10 if no price changes occurred before or on 2019-08-16. Finally, sort the results by product_id.

SQL Query:

SELECT p.product_id,
       COALESCE(
           (SELECT new_price 
            FROM PriceChanges pc 
            WHERE pc.product_id = p.product_id AND pc.effective_date <= '2019-08-16' 
            ORDER BY pc.effective_date DESC LIMIT 1), 
           10
       ) AS price
FROM (SELECT DISTINCT product_id FROM PriceChanges) p
ORDER BY p.product_id;

Explanation:

• SELECT p.product_id, COALESCE(... ) AS price:
  • Selects each product_id and determines its price using the COALESCE function.
• COALESCE(..., 10):
  • Attempts to retrieve the latest new_price from the subquery.
  • If the subquery returns NULL (i.e., no price changes before or on 2019-08-16), it assigns the default price of 10.
• ORDER BY p.product_id:
  • Sorts the final results in ascending order of product_id for organized presentation.

Final Output:

Based on the example input, the final output would be:

+------------+-------+
| product_id | price |
+------------+-------+
| 1          | 35    |
| 2          | 50    |
| 3          | 10    |
+------------+-------+