QuerydrillCustomers who ordered
An SQL exercise from Querydrill. Solve it with a SQL editor in your browser, checked on the rows it returns and again on changed data, and it is checked the moment you submit.
The task
How many different customers have placed at least one order? Return a single number.
What this exercise teaches
COUNT(*) counts rows; COUNT(DISTINCT x) counts unique values of x.
A hint if you get stuck
COUNT(DISTINCT column) counts unique values.
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