MySQL“分组依据”限制组返回

Question

If I apply a limit when selecting with group by, the limit is applied to the number of groups returned or the number of rows?

Myself, I want to limit the groups returned.

Here's my trial:

select * from orders group by customer_email limit 0,2

From this table:

id customer_email
0 a@a.com
1 a@a.com
2 a@a.com
3 b@b.com
4 c@c.com
5 c@c.com
6 d@d.com

I want returned rows 0,1,2,3,4,5.

Answer 1

Your question is a little ambiguous: the proposed query limits to 2 groups, and you seem to want all the rows with equal customer_email that are contained in the first 3 groups.

Anyway, here is my answer for as far as I correctly understand your question :)

SELECT *
FROM orders
INNER JOIN (
    SELECT customer_email
    FROM orders
    GROUP BY customer_email
    LIMIT 0,3
) AS temp
ON orders.customer_email = temp.customer_email

The sub query select customer_email from orders group by customer_email limit 0,3 selects the first 3 groups, then you select all the rows in order that contain the same customer_email as in the first 3 groups which would give you row 0, 1, 2, 3, 4, 5.