Case statement with Ranking function - sql

Question

Hello looking for help with ranking.
I'm working with Teradata using SQL and I'm trying to rank a list by a specific group and then by age.
For example: I want to rank by group then only rank those under the selected group that are under 21 years old.
However, when I use the query below it seems to not take into account the members in a group and assigns only if they meet the criteria in the case statement.
select
policy,
age,
case when age <'21' then '1'else '0' end as Under21,
case when age <'21' then dense_rank () over (order by group, age desc) else '0' end as Rank_Under_21
from Table

Answer 1

You can use the partition by clause:
dense_rank () over (partition by policy, case when age < 21 then 1 end
order by group, age desc)
NB: If age is a numerical field (it should be), then don't compare it with a string: leave out the quotes. If age is of a string type, then be aware that the comparison with another string will be alphabetical, and thus '9' > '21'.

Answer 2

Your code still ranks all ages, the CASE simply replaces the high age ranks with zero.
Another solution (besides to #trincot's answer) which moves the CASE into the RANK:
CASE
WHEN age < 21
THEN Rank ()
Over (PARTITION BY policy
ORDER BY CASE WHEN age < 21 THEN age END DESC)
ELSE 0
END
This also ranks all ages, but the high ones are sorted last und thus got a high rank, which is replaced by the outer CASE with zero.

Answer 3

The below code is for MS SQL Server. Thanks to #Victor Hugo Terceros, for sample code.
DECLARE #tbl TABLE
(
age INT,
grp VARCHAR(20)
)
INSERT INTO #tbl
SELECT 1,
'A'
UNION
SELECT 12,
'A'
UNION
SELECT 20,
'A'
UNION
SELECT 19,
'B'
UNION
SELECT 30,
'B'
UNION
SELECT 11,
'B'
UNION
SELECT 4,
'C'
UNION
SELECT 14,
'C'
UNION
SELECT 5,
'B'
UNION
SELECT 16,
'D'
SELECT grp AS Policy,
age,
under21 AS Under21,
CASE
WHEN under21 = 0 THEN Dense_rank()
OVER(
partition BY grp
ORDER BY under21age DESC)
ELSE 0
END AS Rank_Under_21
FROM (SELECT CASE
WHEN age < 21 THEN 0
ELSE 1
END AS Under21,
CASE
WHEN age < 21 THEN age
ELSE 0
END AS under21age,
age,
grp
FROM #tbl) AS t

Answer 4

What I suggest is a Partition by your Group column and then Rank by Age
---------------Test Table with data---------
declare #tbl table(age int, policy varchar(20))
insert into #tbl
select 1, 'A' union
select 12, 'A' union
select 20, 'A' union
select 19, 'B' union
select 30, 'B' union
select 11, 'B' union
select 4, 'C' union
select 14, 'C' union
select 5, 'B' union
select 16, 'D'
---------------Main Query--------------------
select policy,age,
'1' as Under21,
rank() over (partition by policy order by age desc) as Rank_Under_21
from #tbl
where age <21
union
select policy,age,
'0' as Under21,
0 as Rank_Under_21
from #tbl
where age >=21
order by policy asc,age desc