Solution in postgres. Beats 98.81 %
Intuition\n Describe your first thoughts on how to solve this problem. \n\n# Approach\n Describe your approach to solving the problem. \n\n# Complexity\n- Time 
# Intuition\n<!-- Describe your first thoughts on how to solve this problem. -->\n\n# Approach\n<!-- Describe your approach to solving the problem. -->\n\n# Complexity\n- Time complexity:\n<!-- Add your time complexity here, e.g. $$O(n)$$ -->\n\n- Space complexity:\n<!-- Add your space complexity here, e.g. $$O(n)$$ -->\n\n# Code\n```\n-- Write your PostgreSQL query statement below\nSELECT p.product_id, COALESCE(ROUND(SUM(u.units*p.price)/SUM(u.units)::decimal, 2), 0)average_price\nFROM Prices p\nLEFT JOIN UnitsSold u\nON u.product_id = p.product_id AND u.purchase_date BETWEEN p.start_date AND p.end_date\nGROUP BY p.product_id\n```
['PostgreSQL']