MYSQL: How to get a group total repeating in column while keeping the grouping -

generated results this:

------------------------ |group a|group b|amount| ------------------------ |     |   1   |   5  | |     |   2   |   4  | |   b   |   1   |   4  | |   b   |   2   |   2  | |   b   |   3   |   6  | |   c   |   1   |   1  | |   c   |   2   |   4  | ------------------------ 

looking sum in new column total of group repeats along group b:

--------------------------------------- |group a|group b|amount|sum of group a| --------------------------------------- |     |   1   |   5  |       9      | |     |   2   |   4  |       9      |  |   b   |   1   |   4  |      12      | |   b   |   2   |   2  |      12      | |   b   |   3   |   6  |      12      | |   c   |   1   |   1  |       5      | |   c   |   2   |   4  |       5      | --------------------------------------- 

i'm using several derived , joined tables, hoping easy enough calculate within current select statement or derived table. ideas? can't add additional rows using rollup modifier.

try this:

select t1.*, sumofgroup_a mytable t1 inner join (    select group_a, sum(amount) sumofgroup_a    mytable    group group_a ) t2 on t1.group_a = t2.group_a 

demo here