How to increment the counting for each non-consecutive value? - sql

Question

Below is a simple representation of my table:
ID | GA
----------
1 | 1.5
2 | 1.5
3 | 1.2
4 | 1.5
5 | 1.3
I would like to count the number of occurrence of the GA column's values BUT the count should not increment when the value is the same as the next row.
What I would like to expect is like this:
ID | GA | COUNT
-------------------
1 | 1.5 | 1
2 | 1.5 | 1
3 | 1.2 | 1
4 | 1.5 | 2
5 | 1.3 | 1
Notice that GA = 1.5 count is 2. This is because there is a row between ID 2 & 4 that breaks the succession of 1.5.
NOTE: The ordering by ID also matters.
Here's what I've done so far:
SELECT ID,GA,COUNT (*) OVER (
PARTITION BY GA
ORDER BY ID
) COUNT
FROM (
SELECT 1 AS ID,'1.5' AS GA
FROM DUAL
UNION
SELECT 2,'1.5' FROM DUAL
UNION
SELECT 3,'1.2' FROM DUAL
UNION
SELECT 4,'1.5' FROM DUAL
UNION
SELECT 5,'1.3' FROM DUAL
) FOO
ORDER BY ID;
But the result is far from expectation:
ID | GA | COUNT
-------------------
1 | 1.5 | 1
2 | 1.5 | 2
3 | 1.2 | 1
4 | 1.5 | 3
5 | 1.3 | 1
Notice that even if they are consecutive values, the count is still incrementing.

Answer 1

It seems, that you are asking for a kind of a running total, not just a global count.
Assuming, that the input data is in a table named input_data, this should do the trick:
WITH
with_previous AS (
SELECT id, ga, LAG(ga) OVER (ORDER BY id) AS previous_ga
FROM input_data
),
just_new AS (
SELECT id,
ga,
CASE
WHEN previous_ga IS NULL
OR previous_ga <> ga
THEN ga
END AS new_ga
FROM with_previous
)
SELECT id,
ga,
COUNT(new_ga) OVER (PARTITION BY ga ORDER BY id) AS ga_count
FROM just_new
ORDER BY 1
See sqlfiddle: http://sqlfiddle.com/#!4/187e13/1
Result:
ID | GA | GA_COUNT
----+-----+----------
1 | 1.5 | 1
2 | 1.5 | 1
3 | 1.2 | 1
4 | 1.5 | 2
5 | 1.3 | 1
6 | 1.5 | 3
7 | 1.5 | 3
8 | 1.3 | 2
I took sample data from #D-Shih's sqlfiddle

Answer 2

As I understand the problem, this is a variation of a gaps-and-islands problem. You want to enumerate the groups for each ga value independently.
If this interpretation is correct, then I would go for dense_rank() and the difference of row numbers:
select t.*, dense_rank() over (partition by ga order by seqnum_1 - seqnum_2)
from (select t.*,
row_number() over (order by id) as seqnum_1,
row_number() over (partition by ga order by id) as seqnum_2
from t
) t
order by id;
Here is a rextester.

Answer 3

Use a subquery with LAG and SUM anlytic functions:
SELECT id, ga,
sum( cnt ) over (partition by ga order by id) as cnt
FROM (
select t.*,
case lag(ga) over (order by id)
when ga then 0 else 1
end cnt
from Tab t
)
order by id
| ID | GA | CNT |
|----|-----|-----|
| 1 | 1.5 | 1 |
| 2 | 1.5 | 1 |
| 3 | 1.2 | 1 |
| 4 | 1.5 | 2 |
| 5 | 1.3 | 1 |
Demo: http://sqlfiddle.com/#!4/5ddd1/5