mysql - How can I force a subquery return only one row?

Question

Here is my query:

SELECT e.id, e.table_code,

 ( SELECT MIN(date_time) date_time,
         SUM(score) score,
         type,
         post_id,
         table_code,
         comment_id,
         context
   FROM events 
   WHERE author_id = 32
   GROUP BY type, post_id, table_code, comment_id, context
   ORDER BY MIN(date_time) DESC
   LIMIT 15 ) g

FROM events e
WHERE e.author_id = 32 AND e.date_time >= MIN(g.date_time)
ORDER BY seen ASC, date_time DESC 

It throws:

#1241 - Operand should contain 1 column(s)

Any idea how can I fix it?

---

I'm trying to:

1. Select 15 groups of latest events (the smallest id of groups)
2. And then Select all events (which are belong to a specific user) that have a bigger id that that smallest id

Noted that, probably "group" be vague. Actually it's about notifications, so the notifications which has the same post id (and are related to vote) will be summed and named as one group.

Answer 1

You can use LEFT JOIN instead of Sub-Query.

Try this:

SELECT e.id, e.table_code,g.*
FROM events e
LEFT JOIN (SELECT MIN(date_time) date_time,
         SUM(score) score,
         type,
         post_id,
         table_code,
         comment_id,
         context
   FROM events 
   WHERE author_id = 32
   GROUP BY type, post_id, table_code, comment_id, context
   ORDER BY MIN(date_time) DESC
   LIMIT 15 ) g ON e.table_code = g.table_code AND e.date_time >= g.date_time
WHERE e.author_id = 32
ORDER BY seen ASC, e.date_time DESC