Optimise MySQL SELECT with LEFT JOIN subquery I have a query which combines a LEFT JOIN and subquery. The dataset is quite big and the time to execute the statement is over 70 seconds.

SELECT
    s.siblings,
    l.id
FROM
    `list` l
        INNER JOIN
    child c ON c.id = l.child_id
        INNER JOIN
    parent p ON p.id = c.parent_id
    LEFT JOIN (
      SELECT COUNT(c.id) AS siblings, c.id, c.parent_id
      FROM child c
      GROUP BY c.id
    ) AS s ON s.parent_id = c.parent_id AND s.id != c.id
WHERE
    l.country = 1
GROUP BY l.id, s.siblings
ORDER BY l.dateadded


This query should return all lists for a country. Each list is specific to a unique baby. For each list I would like to return a count of the number of children that have the same parent.

If I remove the LEFT JOIN subquery the fetch time is 0.1 seconds. Is there a way to make the query more efficient?
The main reason for the slow query is the join on a subquery. This will not use indexes. Then, you not only join with a derived table (subquery), but as well group total result based on subquery column - GROUP BY l.id, s.Siblings

In this case it could help to:


create temporary table from subquery, it also could include subquery
for return correct parent_id
create index on this table
use temporary table in join
drop temporary table


This could have variants, but it is often faster and less server-loading than a complicated set of subqueries with joins.