Sum based on group by and one condition

Question

I have a tableof following structure
Group SubGroup sequence length 
A         x               1            10
A         X        
    2            20
A         X               2            30
A         X               3            50 
B         Y               4  30
B         Y               5            40
I needa new column with a sum of lengths grouping Group,sub group and sequence but the condition is sum should be calculated based on the sequence of each row.Sum is sum of lengths where sequence is greater than the current row sequence number and grouping
group nad sub group
following is the output i am looking for
Group SubGroup sequence length   Sum
A         x               1            10          100(Sum of all the lengths where sesquence > current rows sequence
                                                         and group by group,sub
group and sequnce) 
A         X               2            20          50
A         X   3 30 NULL
B         Y               4  40 50
B         Y      5 50 NULL

Answer 1

Please follow basic Netiquette and post the DDL we need to answer this. Follow industry and ANSI/ISO standards in your data. You should follow ISO-11179 rules for naming data elements. You should follow ISO-8601 rules for displaying temporal data. We need
to know the data types, keys and constraints on the table. Avoid dialect in favor of ANSI/ISO Standard SQL. 
And you need to read and download the PDF for: 
https://www.simple-talk.com/books/sql-books/119-sql-code-smells/
>> I have a table of following structure <<
Your posting is crap! Did you know that GROUP and SEQUENCE are reserved words in SQL? Did you really want to use a CREATE SEQUENCE? Did you know a table has a name and key?? That having a separate “<something>_sub_group” column is so bad a design flaw
it has a name (attribute splitting)? That “<something>_length” cannot exist without being the length of something in particular? 
CREATE TABLE Rude_Postings
(rudeness_grp CHAR(2) NOT NULL
 CHECK (rudeness_grp LIKE '[A-Z][A-Z]'),
 foobar_seq INTEGER NOT NULL
 CHECK (foobar_seq > 0),
 PRIMARY KEY (rudeness_grp, foobar_seq),
 foobar_length INTEGER NOT NULL
 CHECK (foobar_length > 0)
The foobar_seq simply looks wrong. It was not unique, so it could not be a sequence by definition. I will guess that it is unique within a rudeness_grp. 
INSERT INTO Rude_Postings
VALUES
('AX', 1, 10),
('AX', 2, 20),
('AX', 3  30),
('AX', 4,  50),
('BY', 1 , 30),
('BY', 2 , 40);
>> I need a new column with a sum of lengths grouping Group,sub group and sequence but the condition is sum should be calculated based on the sequence of each row. Sum [sic: SUM is a reserved word!] is sum of lengths where sequence is greater than the
current row [sic: tables are sets not files, so there is no ordering and no concept of a current row] sequence number and grouping group and sub group << 
Here is my guess:
SELECT rudeness_grp, foobar_seq, foobar_length 
   SUM( foobar_length) 
       OVER (PARTITION BY  rudeness_grp 
               ORDER BY  foobar_seq DESC
               ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW)
     AS foobar_length_running_tot
  FROM Rude_Postings;
--CELKO-- Books in Celko Series for Morgan-Kaufmann Publishing: Analytics and OLAP in SQL / Data and Databases: Concepts in Practice Data / Measurements and Standards in SQL SQL for Smarties / SQL Programming Style / SQL Puzzles and Answers / Thinking
in Sets / Trees and Hierarchies in SQL