SQL Query to select bottom 2 from each category - sql

Question

In Mysql, I want to select the bottom 2 items from each category
Category Value
1 1.3
1 4.8
1 3.7
1 1.6
2 9.5
2 9.9
2 9.2
2 10.3
3 4
3 8
3 16
Giving me:
Category Value
1 1.3
1 1.6
2 9.5
2 9.2
3 4
3 8
Before I migrated from sqlite3 I had to first select a lowest from each category, then excluding anything that joined to that, I had to again select the lowest from each category. Then anything equal to that new lowest or less in a category won. This would also pick more than 2 in case of a tie, which was annoying... It also had a really long runtime.
My ultimate goal is to count the number of times an individual is in one of the lowest 2 of a category (there is also a name field) and this is the one part I don't know how to do.
Thanks

Answer 1

SELECT c1.category, c1.value
FROM catvals c1
LEFT OUTER JOIN catvals c2
ON (c1.category = c2.category AND c1.value > c2.value)
GROUP BY c1.category, c1.value
HAVING COUNT(*) < 2;
Tested on MySQL 5.1.41 with your test data. Output:
+----------+-------+
| category | value |
+----------+-------+
| 1 | 1.30 |
| 1 | 1.60 |
| 2 | 9.20 |
| 2 | 9.50 |
| 3 | 4.00 |
| 3 | 8.00 |
+----------+-------+
(The extra decimal places are because I declared the value column as NUMERIC(9,2).)
Like other solutions, this produces more than 2 rows per category if there are ties. There are ways to construct the join condition to resolve that, but we'd need to use a primary key or unique key in your table, and we'd also have to know how you intend ties to be resolved.

Answer 2

You could try this:
SELECT * FROM (
SELECT c.*,
(SELECT COUNT(*)
FROM user_category c2
WHERE c2.category = c.category
AND c2.value < c.value) cnt
FROM user_category c ) uc
WHERE cnt < 2
It should give you the desired results, but check if performance is ok.

Answer 3

Here's a solution that handles duplicates properly. Table name is 'zzz' and columns are int and float
select
smallest.category category, min(smallest.value) value
from
zzz smallest
group by smallest.category
union
select
second_smallest.category category, min(second_smallest.value) value
from
zzz second_smallest
where
concat(second_smallest.category,'x',second_smallest.value)
not in ( -- recreate the results from the first half of the union
select concat(c.category,'x',min(c.value))
from zzz c
group by c.category
)
group by second_smallest.category
order by category
Caveats:
If there is only one value for a given category, then only that single entry is returned.
If there was a unique recordID for each row you wouldn't need all the concats to simulate a unique key.
Your mileage may vary,
--Mark

Answer 4

A union should work. I'm not sure of the performance compared to Peter's solution.
SELECT smallest.category, MIN(smallest.value)
FROM categories smallest
GROUP BY smallest.category
UNION
SELECT second_smallest.category, MIN(second_smallest.value)
FROM categories second_smallest
WHERE second_smallest.value > (SELECT MIN(smallest.value) FROM categories smallest WHERE second.category = second_smallest.category)
GROUP BY second_smallest.category

Answer 5

Here is a very generalized solution, that would work for selecting first n rows for each Category. This will work even if there are duplicates in value.
/* creating temporary variables */
mysql> set #cnt = 0;
mysql> set #trk = 0;
/* query */
mysql> select Category, Value
from (select *,
#cnt:=if(#trk = Category, #cnt+1, 0) cnt,
#trk:=Category
from user_categories
order by Category, Value ) c1
where c1.cnt < 2;
Here is the result.
+----------+-------+
| Category | Value |
+----------+-------+
| 1 | 1.3 |
| 1 | 1.6 |
| 2 | 9.2 |
| 2 | 9.5 |
| 3 | 4 |
| 3 | 8 |
+----------+-------+
This is tested on MySQL 5.0.88
Note that initial value of #trk variable should be not the least value of Category field.