MySQL Keep record with most current date - mysql

Question

I created a table w_provider_remove to keep the Provider_no and max(prov_effective_Date) of the records in w_providers_load because I have duplicate providers but some have expired. I later try to join the w_providers_load table and the w_provider_Remove table and insert the records into the w_providers_main table. The problem is I get back to many records because it turns out I have more than one record for the particular provider with the same effective date. How can I limit it so it only inserts one of them? Or maybe there is another way to go about this were I do not need 3 tables to accomplish this task
Truncate w_provider_remove;
insert into w_provider_remove
select provider_no as provider_no, max(PROV_DATE_EFFECTIVE) as prov_date_effective
from w_provider_load
group by provider_no;
Truncate w_provider_main;
INSERT INTO w_provider_main
Select l.*
from w_provider_load as l
inner JOIN w_provider_remove as r on l.provider_no = r.provider_no AND l.prov_date_effective = r.prov_date_effective;

Answer 1

If you want to limit the rows in the table to one effective date per provider, then create a unique index:
create unique index w_provider_load_provider_effectivedate on w_provider_load(provider_no, prov_effective_date);
This will generate an error if two records for the same provider have the same effective date in the load.
You can do the same thing for the main table:
create unique index w_provider_main_provider_effectivedate on w_provider_main(provider_no, prov_effective_date);