mysql获取具有某些值的外部记录的记录

Question

I have two tables

Parent:
    +--+---------+
    |id|   text  |
    +--+---------+
    | 1|  Blah   |
    | 2|  Blah2  |
    | 3|  Blah3  |
    +--+---------+

 Children
    +--+---------+---------+
    |id|parent_id| table_id|
    +--+---------+---------+
    | 1|   1     | 1       |
    | 2|   1     | 3       |
    | 3|   2     | 2       |
    +--+---------+---------+

I want to find parent having children with table_id both 1 and 3. Currently I am using following query

SELECT * 
FROM  Parent
WHERE id
IN (
    SELECT parent_id
    FROM Children
    WHERE table_id = 1
)
AND id
IN (
    SELECT parent_id
    FROM Children
    WHERE table_id = 3
)

As I have thousands of records, this query runs very slow.

Is there altenative faster query to execute it?

Answer 1

You can let mysql EXPLAIN its execution plan to you: check this chapter in the manual. From there, you can optimize your query for speed. There are different approaches to that, it's a broad field - but in general, it's a good start to minimize the number and the width of the included joins and inner queries.

Answer 2

There are many ways of doing it, and one way with proper indexing would make it pretty fast as

select p.* from parent p 
join children c on c.parent_id = p.id 
where c.table_id = 1 
and exists ( 
  select 1 from children c1 
  where c1.parent_id = c.parent_id 
  and c1.table_id = 3
);

The indexes that could be added as

alter table parent add index id_idx(id);
alter table children add index parent_idx(parent_id);
alter table children add index table_id_idx(table_id);

Answer 3

SELECT c.parent_id, p.text,
SUM(CASE WHEN c.table_id=1 THEN 1 ELSE 0 END) as t_1,
SUM(CASE WHEN c.table_id=3 THEN 1 ELSE 0 END) as t_3
FROM  Children c
LEFT JOIN Parent p
ON c.parent_id = p.id
WHERE c.table_id IN ( 1, 3)
GROUP BY c.parent_id
HAVING t_1>0 AND t_3>0