suppose I have the following table:
id parent
1 1
2 1
3 1
4 1
5 2
6 5
7 5
And I want to execute the following query:
SELECT id_table.parent,id_table.count_id
FROM
(SELECT count(id) AS count_id,parent FROM item group by parent) AS id_table
WHERE id_table.count_id =
(SELECT max(count_id) AS max_count_id FROM
( SELECT count(id) AS count_id,parent
FROM item group by parent ) AS id_table2 )
to get the following result:
parent count_id
1 4
I try to transform the query to optimize performance:
SELECT id_table.parent,id_table.count_id FROM
(SELECT count(id) AS count_id,parent FROM item GROUP BY parent)
AS id_table
LEFT JOIN
(SELECT count(id) AS count_id,parent FROM item GROUP BY parent )
AS id_table2
ON id_table.parent=id_table2.parent AND id_table.count_id<id_table2.count_id
WHERE id_table2.parent IS NULL
BUT got the following result instead, which I don't want:
parent count_id
1 4
2 1
5 2
This is my reference
Could anyone tell me where I did wrong? regards
Your second query does not work as you wish because it simply does not match any records in the join. Look:
You are declaring a join between a certain query and itself, according to the following condition:
ON id_table.parent=id_table2.parent AND id_table.count_id<id_table2.count_id
This means: select all the records from the first query (because it is a LEFT join), and every record from the second query which has the same parent
and a count
lower than the first query. ¡!
As you can see, this is absurd: Since the auto-joined query returns these records:
count_id parent
4 1
2 2
2 5
... the parent
column is a de-facto PK. So. the pair (4,1) has no matching pairs (x,1) with x<4. (Idem the rest of the pairs.)
That was your mistake: The example from MySQL is not appliable here, because in the shop
table, surely there are more than one record with the same value of article
, which is not your case of ( parent
has unique values in the auto-joined query).
The only query formula based on joins that comes to my mind is like this:
SELECT count_id, parent
FROM (SELECT count(id) AS count_id,parent FROM item GROUP BY parent) query1
INNER JOIN (SELECT MAX(query2.count_id) as max_id FROM (SELECT count(id) AS count_id,parent FROM item GROUP BY parent) query2) queryMax ON query1.count_id=queryMax.max_id;
... but I doubt that such a complexity it will improve the performance of your first approach.
Based on your output which you wanted to get i got a simpler solution. Maybe i didn't understand your question, but i'll give a shot.
Why to use your query (kinda long and messy) :
SELECT id_table.parent,id_table.count_id
FROM
(SELECT count(id) AS count_id,parent FROM item group by parent) AS id_table
WHERE id_table.count_id =
(SELECT max(count_id) AS max_count_id FROM
( SELECT count(id) AS count_id,parent
FROM item group by parent ) AS id_table2 )
I acomplished the same result with much more simpler query.
SELECT parent, count(id) as count_id FROM items GROUP BY 1 ORDER BY 2 DESC LIMIT 1
To get the following result.
parent count_id
1 4
Can you check this
SELECT parent,count(*) AS parent_count FROM parent group by parent
ORDER BY parent_count DESC LIMIT 1
Hope this helps
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