Changing sql to get rid of windowed functions

Question

I'm trying to create a view on a remote mysql database and unfortunately it appears that the version installed (5.7) does not support window functions. Everything worked on my local database but now I'm a little stuck.

Here's previous code:

create or replace view my_view as 
( 
with a as 
    (select a.*, DENSE_RANK() over (partition by SHOP order by TIMESTAMP(LOAD_TIME) DESC) rn 
     from my_table as a) 
select row_number() OVER () as id, SHOP, LOAD_TIME from a WHERE a.rn = 1
);

Mysql 5.7 doesnt support CTE either, but that isn't a big problem.

Any hints how to solve this?

Answer 1

Replacing the dense_rank() is pretty easy. However, replacing the row_number() is more difficult. MySQL does not allow variables in views. Unfortunately, that leaves you with an inefficient subquery for the row number as well:

select (select count(distinct shop)
        from mytable t2
        where t2.shop <= t.shop
       ) as id,
       shop, load_time
from mytable t
where t.load_time = (select max(t2.load_time) from mytable t2 where t2.shop = t.shop);

Or, if these are the only two columns you have, use aggregation:

select (select count(distinct shop)
        from mytable t2
        where t2.shop <= t.shop
       ) as id,
       shop, max(load_time) as load_time
from mytable t
group by shop;

This is not efficient. In a simple query, you could use variables:

select (@rn := @rn + 1) as id,
       shop, load_time
from mytable t cross join
     (select @rn := 0) params
where t.load_time = (select max(t1.load_time) from mytable t1 where t1.shop = t.shop);

If performance is an issue, then you may want to create a table rather than a view and keep it up-to-date using triggers.

Answer 2

You can handle the filtering part with a correlated subquery:

create or replace view my_view as 
select shop, load_time
from mytable t
where t.load_time = (select max(t1.load_time) from mytable t1 where t1.shop = t.shop)