Oracle 11g Release 2

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

alter session set nls_date_format = 'yyyy-mm-dd';
CREATE TABLE mytable(
   id   INTEGER  NOT NULL 
  ,step VARCHAR(1) NOT NULL
  ,mdate date  NOT NULL
);
begin
INSERT INTO mytable(id,step,mdate) VALUES (1,'a','2019-12-01');
INSERT INTO mytable(id,step,mdate) VALUES (1,'a','2019-12-02');
INSERT INTO mytable(id,step,mdate) VALUES (1,'b','2019-12-02');
INSERT INTO mytable(id,step,mdate) VALUES (1,'x','2019-12-03');
INSERT INTO mytable(id,step,mdate) VALUES (1,'a','2019-12-04');
INSERT INTO mytable(id,step,mdate) VALUES (1,'b','2019-12-05');
INSERT INTO mytable(id,step,mdate) VALUES (2,'a','2019-12-05');
INSERT INTO mytable(id,step,mdate) VALUES (2,'b','2019-12-06');
end;
/
1 rows affected

select
    id,
    iteration,
    min(case when step = 'a' then mdate end) first_step_a,
    min(case when step = 'b' then mdate end) first_step_b
from (
    select 
        t.*,
        1 + sum(case when step = 'x' then 1 else 0 end) over(partition by id order by mdate) iteration
    from mytable t
) t
group by id, iteration
order by id, iteration


ID
ITERATION
FIRST_STEP_A
FIRST_STEP_B


1
1
2019-12-01
2019-12-02

1
2
2019-12-04
2019-12-05

2
1
2019-12-05
2019-12-06

ID	ITERATION	FIRST_STEP_A	FIRST_STEP_B
1	1	2019-12-01	2019-12-02
1	2	2019-12-04	2019-12-05
2	1	2019-12-05	2019-12-06