MySQL从左连接选择行，在一列中具有特定数量的不同值，但没有总行数限制

Question

I have 2 tables (records, features) that are connected by one foreign key-relationship.

Now I want to draw samples from those two tables where in my primary table I select by an id which is the primary key.

So if I wanted to get all entries it would look like this:

SELECT features.record_id, [...more columns...]
FROM records 
LEFT JOIN features ON records.id = features.record_id
ORDER BY records.id

What I try to achieve:

I would like to enhance the query so I can specify an exact number n of distinct values of records.record_id . I don't want n rows but all rows from the join until I have n diffferent record_id s without limiting the total number of rows in the most efficient way. I hope this is understandable, if not: please tell me and I will try to elaborate my question.

To add additinal context:

I use this inside a java-program (plain jdbc) and my goal is to collect the information to build n fully populated record-objects in the most efficient way. I already tried drawing all data and iterate over the ordered result until I constructed a sufficient number of record-objects but it didn't turn out very efficient (already takes about half a minute for 100k entries).

Answer 1

You can limit the number of rows from records with LIMIT :

SELECT features.* FROM (
  SELECT records.record_id 
  FROM records LIMIT 100
) t
LEFT JOIN features ON t.record_id = features.record_id
ORDER BY t.record_id

the column record_id is the primary key so you don't need distinct .