Question

What i’ve got is these two tables:

One prefix can be used by unlimited number of links. I’m receiving a list if id‘s from click_links table as a paramater.

Now my goal is to select a list of prefixes and to mark those which are used by links in that list.

What I’ve tried: my best attempt was this:

select p.id, p.contents, c.id,
    case
    when (c.id in (544,545...))
        then 1
        else 0
    end as isused
from prefixes p 
join click_links c on c.prefix_id = p.id

but that’s not so good, ’cause there are duplicates of prefix if it’s used by many links.

How can I get a list of prefixes without duplicates and with a mark (flag if you wish) showing whether it’s ised by at least one of links in the list?

edit
ended with this query:

SELECT distinct(p.id), p.contents,
       IF(c.id IS NOT NULL, 1, 0) AS isused
FROM prefixes p
     LEFT JOIN (SELECT DISTINCT (prefix_id), id
                FROM click_links WHERE id IN(544, 545...)
               )c
      ON c.prefix_id = p.id;

Answer 1

I think this is what you are looking for, try it:

SELECT p.id, p.contents, c.id,
       IF(c.id IS NOT NULL, 1, 0) AS isused
FROM prefixes p
     LEFT JOIN (SELECT DISTINCT id
                FROM click_links WHERE id IN(544,545...)
               )c
      ON c.prefix_id = p.id;