N：M MySQL关系中的DISTINCT和COUNT

Question

I have a N:M relation that connects customers and purchase Customer_id,Purchase_id . I would like to know how many purchases each specific user made, from one query.

I tried this:

SELECT DISTINCT(Customer_id) AS ID,COUNT(Purchase_id) AS P FROM CustomerPruchases;

but it only gives me one row for the first customer in the DB. I want it for all.

Answer 1

If you group then aggregate functions like count() apply to each group and not to the complete table

SELECT Customer_id as ID, 
       COUNT(Purchase_id) AS P_COUNT
FROM CustomerPruchases
group by ID