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GROUP BY Extensions in SQL

Introduction
GROUP BY – Recap
ROLLUP
Summary

Instruction

Okay! All that's left for you to do is to write your own query with COALESCE.

Exercise

Show the total price paid for all deliveries in each category and a grand total for all categories. Show the category column and the sum of the total_price. The columns names should be category and total. Instead of NULL values in the category column, show two hyphens (--).

Stuck? Here's a hint!

Use COALESCE(category,'--') in the SELECT clause.