added mock interview problem.

problem1.sql

@@ -0,0 +1,17 @@
+"""
+1. We need to find all sales that occurred in the first year each product was sold
+2. We first compute the first year that a product was sold.
+3. Then we join it with the sales table on product id and year to fetch all the first year sales 
+"""
+
+
+with cte1 as
+(   
+    select product_id, min(year) as year
+    from sales
+    group by product_id
+)
+
+select c.product_id, c.year as first_year, quantity, price
+from cte1 c
+join sales s on c.product_id = s.product_id and c.year = s.year