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POSQL - min() gets the lowest value, max() the highest, what if I want the 2nd (or 5th or nth) lowest value?
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copied!<p>The problem I'm trying to solve is that I have a table like this:</p> <p>a and b refer to point on a different table. distance is the distance between the points.</p> <pre><code>| id | a_id | b_id | distance | delete | | 1 | 1 | 1 | 1 | 0 | | 2 | 1 | 2 | 0.2345 | 0 | | 3 | 1 | 3 | 100 | 0 | | 4 | 2 | 1 | 1343.2 | 0 | | 5 | 2 | 2 | 0.45 | 0 | | 6 | 2 | 3 | 110 | 0 | .... </code></pre> <p>The important column I'm looking is a_id. If I wanted to keep the closet b for each a, I could do something like this:</p> <pre><code>update mytable set delete = 1 from (select a_id, min(distance) as dist from table group by a_id) as x where a_gid = a_gid and distance > dist; delete from mytable where delete = 1; </code></pre> <p>Which would give me a result table like this:</p> <pre><code>| id | a_id | b_id | distance | delete | | 1 | 1 | 1 | 1 | 0 | | 5 | 2 | 2 | 0.45 | 0 | .... </code></pre> <p>i.e. I need one row for each value of a_id, and that row should have the lowest value of distance for each a_id.</p> <p>However I want to keep the 10 closest points for each a_gid. I could do this with a plpgsql function but I'm curious if there is a more SQL-y way.</p> <p>min() and max() return the smallest and largest, if there was an aggregate function like nth(), which'd return the nth largest/smallest value then I could do this in similar manner to the above.</p> <p>I'm using PostgeSQL.</p>

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