相当于为mysql保留dense_rank

Question

I have this table

create table  sd_devices (
device_code varchar(128),
vin varchar(128),
created_at  datetime,
loaded_at   datetime
)

I would like to select for each device_code the first vin order by created_at desc, loaded_at desc .

In oracle, I would have use keep dense_rank , but here with MySQL i do not know how to do that.

Answer 1

this should do it:

SELECT DISTINCT device_code,
       vin
       FROM (
SELECT sd_devices.*
FROM sd_devices
ORDER BY created_at DESC, loaded_at DESC ) AS a
GROUP BY device_code

In the subquery you select all the rows and order them in the correct order, Then in the outside query you use DISTINCT to get rid of any Duplicate rows and it will use the first one it encounters. Because we already put it in the correct order it will grab the one row we want.