MySQL-按DESC排序和按GROUP BY

Question

I have a customers table with its field such as userId, customerId and created.

This is the data in customers table

userId    customerId    created
user_1    customer_1    2016-03-04 00:00:00
user_1    customer_2    2016-10-04 00:00:00
user_1    customer_3    2016-07-04 00:00:00
user_1    customer_4    2016-09-04 00:00:00
user_2    customer_5    2016-06-04 00:00:00
user_2    customer_6    2016-03-04 00:00:00

I used some following queries to get latest created of each user. And this is one of queries I tried

select *
from customers
order by created desc
group by userId

The above query didn't work properly. My desired result is:

user_1    customer_2    2016-10-04 00:00:00
user_2    customer_5    2016-06-04 00:00:00

Maybe I don't understand clearly about how order by and group by commands work. Could you give me some solutions?

Answer 1

SELECT c1.userId, c1.customerId, c1.created
FROM customers c1
INNER JOIN
(
    SELECT userId, MAX(created) AS maxCreated
    FROM customers
    GROUP BY userId
) c2
    ON c1.userId = c2.userId AND c1.created = c2.maxCreated
ORDER BY c1.created

Note that you don't need to explicitly use DESC with the ORDER BY clause, because MySQL sorts in descending order by default.

Answer 2

Try this:

SELECT 
    *
FROM 
    customers c1
WHERE 
    (userId, created) IN 
        (
            SELECT userId, MAX(created)
            FROM customers c2
            WHERE c1.userId = c2.userId
        );

Answer 3

If you try do in this way, you'll get the result you want:

SELECT * FROM customers WHERE created = (select max(created) from customers a where a.userId = customers.userId )

group by clause allows you get maximum value:

select userId,max(created) from customers group by userId

so that you get max date for each userId and after putting it into some values you can use in the another query.

Something like that gives you only one row with maximum date:

select userId,customerId,max(created) from customers order by created desc