Question

I’m trying to select 1000 customers that have placed exactly 1 order. Everything is in the Orders table.

select * from Order having count(CustomerID) = 1 limit 1000

So basically all records that have only one occurence of the CustomerID in the entire table. This returns an empty result set and there are 100,000s in the table.

Answer 1

You need to GROUP in order to (meaningfully) use the HAVING clause. The following works as expected:

SELECT
    *
FROM
    `Order`
GROUP BY
    CustomerID
HAVING
    COUNT(CustomerID) = 1
LIMIT 1000

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UPDATE
Adding WHERE (see comments):

SELECT
    *
FROM
    `Order`
WHERE
    Language = 'EN'
GROUP BY
    CustomerID
HAVING
    COUNT(CustomerID) = 1
LIMIT 1000