Find users with a large cart, whose total number of items exceeds 300.

To find users whose total number of items in the cart exceeds 300, you can use an SQL query with grouping and a HAVING condition:

SELECT userID, SUM(quantity) AS total_quantity
FROM carts
GROUP BY userID
HAVING SUM(quantity) > 300;

This query sums the number of items for each user and selects only those whose total exceeds 300.