Coalesce with Roll Up not working correctly (MySql)

Question

I am trying to coalesce on the roll up (against the year revenue), The roll up is calculating correctly, however instead of inputting 'Grand Total' at the end of the table, 'Socks' is being inputted again.

Any ideas what i am doing wrong?

select coalesce(product_name, 'total') as product_name,  sum(price) as year_revenue
from orders 
    join product on orders.ProductID = product.ProductID
group by month(order_data) with rollup;

'Blue Shirt', '69.93'

'Denim Jeans', '197.91'

'White Blazer', '94.97'

'Socks', '109.94'

'Skinny Jeans', '73.96'

'Mini Skirt', '31.98'

'White Blazer', '74.97'

'Black Blazer', '40.99'

'Shorts', '19.98'

'Mini Skirt', '85.96'

'Flare Blouse', '33.98'

'Socks', '7.98'

'Socks', '842.55'

Answer 1

The reason for this is that you are grouping by MONTH(order_data) , not product_name . When WITH ROLLUP happens, it is the grouped by column value that gets replaced by NULL . If you were to change your query to:

SELECT MONTH(order_data) AS month, product_name, SUM(price) AS year_revenue
FROM orders
JOIN product ON orders.ProductID = product.ProductID
GROUP BY month WITH ROLLUP

You would see the NULL values in the month column.

To achieve what you want, try changing your query to this:

SELECT IF(month IS NULL, 'Total', product_name) AS product_name, year_revenue
FROM (SELECT MONTH(order_data) as month, product_name, SUM(price) AS year_revenue
    FROM orders
    JOIN product ON orders.ProductID = product.ProductID
    GROUP BY month WITH ROLLUP)