Teradata SQL: Max (greatest), 2nd and 3rd greatest column names - sql

Question

I have a table with 6 columns in Teradata as follows:
ID Feature1 Feature2 Feature3 Feature4 Feature5
1 12 15 1 22 350
2 121 0.9 999 756 879
...
I need to get the column names for the greatest, 2nd greatest and 3rd greatest values per row, so, I need output that looks like this:
ID Greatest 2nd_Greatest 3rd_Greatest
1 Feature5 Feature4 Feature2
2 Feature3 Feature5 Feature4
Can someone help please.
Thank you!

Answer 1

You can do this with a massive case statement, which gets even more complicated if any of the values are NULL. That would be the fastest way, though.
The easiest method might be to unpivot the data and re-summarize it:
select id,
max(case when seqnum = 1 then feature end) as greatest_feature,
max(case when seqnum = 2 then feature end) as greatest_feature2,
max(case when seqnum = 3 then feature end) as greatest_feature3,
max(case when seqnum = 1 then which end) as which_1,
max(case when seqnum = 2 then which end) as which_2,
max(case when seqnum = 3 then which end) as which_3
from (select id, feature, row_number() over (partition by id order by feature desc) as serqnum
from ((select id, feature1 as feature, 'feature1' as which from table) union all
(select id, feature2 as feature, 'feature2' as which from table) union all
(select id, feature3 as feature, 'feature3' as which from table) union all
(select id, feature4 as feature, 'feature4' as which from table) union all
(select id, feature5 as feature, 'feature5' as which from table) union all
(select id, feature6 as feature, 'feature6' as which from table)
) t
) t
group by id;

Answer 2

Refining Gordon's query:
Instead of several passes over the source table for those UNIONs you can create a list of features and then cross join it:
SELECT t.id, f.feature,
CASE f.feature
WHEN 'feature1' THEN t.feature1
WHEN 'feature2' THEN t.feature2
WHEN 'feature3' THEN t.feature3
WHEN 'feature4' THEN t.feature4
WHEN 'feature5' THEN t.feature5
END AS val
FROM tab AS t CROSS JOIN
(
SELECT * FROM (SELECT 'feature1' AS feature) AS dt
UNION ALL
SELECT * FROM (SELECT 'feature2' AS feature) AS dt
UNION ALL
SELECT * FROM (SELECT 'feature3' AS feature) AS dt
UNION ALL
SELECT * FROM (SELECT 'feature4' AS feature) AS dt
UNION ALL
SELECT * FROM (SELECT 'feature5' AS feature) AS dt
) AS f
You can create the list on the fly like above using UNIONs or as a real table.
Starting with TD14.10 there's also a TD_UNPIVOT table operator (but still no PIVOT):
SELECT *
FROM TD_UNPIVOT
(
ON (SELECT id, feature1, feature2, feature3, feature4, feature5 FROM tab)
USING
VALUE_COLUMNS('val')
UNPIVOT_COLUMN('feature')
COLUMN_LIST('feature1', 'feature2', 'feature3', 'feature4', 'feature5')
) AS dt
Also starting with TD14.10 there's LAST_VALUE which can be used for finding nth-greatest value together with the ROW_NUMBER, thus avoiding the final aggregation:
SELECT id,
feature AS "Greatest",
LAST_VALUE(feature)
OVER (PARTITION BY id ORDER BY val DESC
ROWS BETWEEN 1 FOLLOWING AND 1 FOLLOWING) AS "2nd_Greatest",
LAST_VALUE(feature)
OVER (PARTITION BY id ORDER BY val DESC
ROWS BETWEEN 2 FOLLOWING AND 2 FOLLOWING) AS "3rd_Greatest"
FROM TD_UNPIVOT
(
ON (SELECT id, feature1, feature2, feature3, feature4, feature5 FROM tab)
USING
VALUE_COLUMNS('val')
UNPIVOT_COLUMN('feature')
COLUMN_LIST('feature1', 'feature2', 'feature3', 'feature4', 'feature5')
) AS dt
QUALIFY ROW_NUMBER() OVER (PARTITION BY id ORDER BY val DESC) = 1;