I need to query a table, containing a step id + value.
The result will list the intervals, along with their associated value.
Intervals here are defined as "succession of contiguous ids of steps, sharing same data value".

I'm having a hard time describing it in words, so please see this:

From this table

```
  Step ! Data
  ------------
   1   !  A
   2   !  A
   3   !  A
   5   !  A
   6   !  B
  10   !  A

```

I need the following report

```
  From ! To   ! Data
  -------------------
     1 !   3  !   A
     5 !   5  !   A
     6 ! null !   B
    10 ! null !   A

```

I thought lead() would help me out here, but did not succeed.