Query LEFT JOIN on condition

Question

Let's say I have a table of clients and a table of "more_information".

I query the list of clients and their additional information (not every client has entries here, so we use LEFT JOIN):

 SELECT id, name, more_information.data
    FROM clients
    LEFT JOIN more_information
    ON more_information.client_id = clients.id

Now my problem: I want to query "more_information" only for clients that fulfill a condition, fe signed up in last 30 days, for the others I want just null for more_information.data

so something like (syntax not correct)

 SELECT id, name, more_information.data
    FROM clients
    CASE WHEN clients.registered > '2015-12-27 00:00:00' THEN
        LEFT JOIN more_information
        ON more_information.client_id = clients.id
    ELSE
        null
    END

I know I could use the CASE in the SELECT part, but that won't improve the performance what is my goal.

The reason is that the more_information JOIN queries a lot of data and has many additional conditions that make the query too slow. I don't want to query it where it is not necessary. Is that even possible with MySQL or do I need to split the query and make two separate queries?

Thanks a lot for your ideas

Answer 1

Just add to your JOIN condition

SELECT id, name, more_information.data
FROM clients
LEFT JOIN more_information
ON more_information.client_id = clients.id AND clients.registered > '2015-12-27 00:00:00'

An INDEX on registered , id and client_id would help performance.