Complex join with nested group-by/having clause? - sql

Question

I ultimately need a list of "import" records that include "album"
records which only have one "song" each.
This is what I'm using now:
select i.id, i.created_at
from imports i
where i.id in (
select a.import_id
from albums a inner join songs s on a.id = s.album_id
group by a.id having 1 = count(s.id)
);
The nested select (with the join) is blazing fast, but the external
"in" clause is excruciatingly slow.
I tried to make the entire query a single (no nesting) join but ran
into problems with the group/having clauses. The best I could do was
a list of "import" records with dupes, which is not acceptable.
Is there a more elegant way to compose this query?

Answer 1

How's this?
SELECT i.id,
i.created_at
FROM imports i
INNER JOIN (SELECT a.import_id
FROM albums a
INNER JOIN songs s
ON a.id = s.album_id
GROUP BY a.id
HAVING Count(* ) = 1) AS TEMP
ON i.id = TEMP.import_id;
In most database systems, the JOIN works a lost faster than doing a WHERE ... IN.

Answer 2

SELECT i.id, i.created_at, COUNT(s.album_id)
FROM imports AS i
INNER JOIN albums AS a
ON i.id = a.import_id
INNER JOIN songs AS s
ON a.id = s.album_id
GROUP BY i.id, i.created_at
HAVING COUNT(s.album_id) = 1
(You might not need to include the COUNT in the SELECT list itself. SQL Server doesn't require it, but it's possible that a different RDBMS might.)

Answer 3

Untested:
select
i.id, i.created_at
from
imports i
where
exists (select *
from
albums a
join
songs s on a.id = s.album_id
where
a.import_id = i.id
group by
a.id
having
count(*) = 1)
OR
select
i.id, i.created_at
from
imports i
where
exists (select *
from
albums a
join
songs s on a.id = s.album_id
group by
a.import_id, a.id
having
count(*) = 1 AND a.import_id = i.id)

Answer 4

All three sugested techniques should be faster than your WHERE IN:
Exists with a related subquery (gbn)
Subquery that is inner joined (achinda99)
Inner Joining all three tables (luke)
(All should work, too ..., so +1 for all of them. Please let us know if one of them does not work!)
Which one actually turns out to be the fastest, depends on your data and the execution plan. But an interesting example of different ways for expressing the same thing in SQL.

Answer 5

I tried to make the entire query a
single (no nesting) join but ran into
problems with the group/having
clauses.
You can join subquery using CTE (Common Table Expression) if you are using SQL Server version 2005/2008
As far as I know, CTE is simply an expression that works like a virtual view that works only one a single select statement - So you will be able to do the following.
I usually find using CTE to improve query performance as well.
with AlbumSongs as (
select a.import_id
from albums a inner join songs s on a.id = s.album_id
group by a.id
having 1 = count(s.id)
)
select i.id, i.created_at
from imports i
inner join AlbumSongs A on A.import_id = i.import_id