added mock interview problem

problem1.sql

@@ -0,0 +1,13 @@
+"""
+1. We need to report the customer ids from the Customer table that bought all the products in the Product table.
+2. So we first check the distinct products bought by each customer
+3. We filter out the customers who dont have the count same as number of unique products
+"""
+
+with cte as 
+(select customer_id , count(distinct product_key) 'num' 
+    from Customer 
+    group by customer_id
+    having num = (select count(*) from Product)
+)
+select cte.customer_id from cte;