Oracle Select Query, distributing frequency of labels - sql

Question

If i know how much data i am receiving, i can distribute the appearance of a label in this way:
select
value_column
, CASE WHEN TO_CHAR(VALUE_DATE, 'mm') = '01' and MOD(extract(year from VALUE_DATE), 3) = 0
THEN TO_CHAR(VALUE_DATE, 'MON-yyyy')
else ' '
END VALUE_DATE_STRING
from SomeTable
this will show the data label on January every 3rd year.
Now, if i don't know how many years are coming back, i'd like to figure this out in the same select and display a total of 5 labels.
i reckon i'd need something like this (pseudo code):
CASE WHEN MOD(allRows / 5, ROW_NUM) = 0
i guess the only challenging part is getting the allRows in the same select.. since i'm calling this sql from a telerik report, there's limited support for declaring vars and running multiple statements..

Answer 1

This might do what you want:
select value_column,
(CASE WHEN mod(rownum, trunc(cnt / 5)) = 0 o
THEN TO_CHAR(VALUE_DATE, 'MON-yyyy')
else ' '
END) as VALUE_DATE_STRING
from (select t.*, count(*) over () as cnt
from SomeTable t
) t;
It might be trunc((cnt - 1) / 5).
DIT:
If this does what you want, you don't need a CTE or subquery. I just thought that it made more sense this way (and the subquery does not effect performance). You can do:
select value_column,
(CASE WHEN mod(row_number() over (order by value_date),
trunc(count(*) over () / 5)) = 0
THEN TO_CHAR(VALUE_DATE, 'MON-yyyy')
else ' '
END) as VALUE_DATE_STRING
from SomeTable t;
By the way, you should include order by if you are using rownum. Oracle does not guarantee the ordering of rows in a result set without an order by.

Answer 2

here's my solution.. it could still be optimized, and i believe the rounding will not always work in my favor..
WITH base as (
select
value_column
, value_date
, cnt.c
, case mod(c,2) when 0 then 6 else 5 end as divider -- get 5 or 6 dates.. try to minimize truncation consequences.. this may need work.
, row_number() over (order by value_date) as row_number
from MyView
left join (select count(*) c from MyView where id = 250170) cnt on 1=1
where id = 250170
order by value_date
)
select
value_column
, value_date
, CASE WHEN MOD(row_number, trunc(c/divider)) = 0
THEN TO_CHAR(VALUE_DATE, 'MON-yyyy')
else ' '
END VALUE_DATE_STRING
from base
order by value_date
UPDATE
simplified based on Gordon's answer
select
value_column
, value_date
, (
CASE WHEN
mod(
row_number() over (order by value_date),
trunc(count(*) over () / 5)) = 0
THEN TO_CHAR(VALUE_DATE, 'MON-yyyy')
else ' '
END) as LABEL_STRING
from MyView