SQL - How to select/delete rows with the max value depending on two others - mysql

Question

I'm an intern in a property rental company. I'm in charge of developping the CRM under Symfony.
So, ones of my entities are properties (houses) and their availabilities. See the table structure below.
The problem I'm facing for now, is that the availabilities had been defined for each day (e.g. 28/01, 29/01, 30/01) instead of being defined for a range of day (e.g. 28/01 -> 30/01). So, the table is really heavy (~710 000 rows). Furthermore, before we changed the way of editing an availability, it created a new row for a same date instead of editing it. So, there are a lot of duplications in this table.
What I want, is to lighten the DB by keeping only the rows which have the max value in date_modif_availabilities for the same date_availabilities and id_properties.
For example, if I have these rows (availabilities_duplications):
I only want to keep the row with the latest modif like this (availabilities_keep_max_value) :
The thing is, I don't know enough the SQL language. I'm able to write few basics scripts but not complex subqueries. Even with code samples that I found.
Thank you in advance for your help.

Answer 1

You could select the elements for which no element with greater modified date exists.
Something like this:
SELECT avl1.*
FROM availabilities avl1
WHERE NOT EXISTS (SELECT *
FROM availabilities avl2
WHERE avl1.date_availabilities = avl2.date_availabilities
AND avl1.id_properties = avl2.id_properties
AND avl2.date_modif_availabilities > avl1.date_modif_availabilities);
This of course has the pre-condition that the combination of the three columns date_availabilities, id_properties and date_modif_availabilities is unique.
Furthermore, it seems that all columns (except the PK) may be NULL. Looks kinda odd to me.

Answer 2

You can use subquery :
select t.*
from table t
where id_availabilities = (select t1.id_availabilities
from table t1
where t1.id_properties = t.id_properties and
t1.date_availabilities = t.date_availabilities
order by t1.date_modif_availabilities desc
limit 1
);
However, if you have concern about the performance, then you want index on (id_properties, date_availabilities and id_availabilities).