SQL rollup latest non-null column values

Question

I have a SQLAnywhere table like this:

id        int         not null,
inserted  datetime    not null,
modified  datetime    not null,
data1     varchar(20)     null
data2     varchar(20)     null

The developer didn't put a unique index on the id, and now there are a bunch of rows with the same id but different values for data1 and data2 across the rows, including some nulls. I need to fix that, rolling up all the rows with the same id into single rows in a new table using the earliest inserted time, the latest modified time, and the latest (by modified time) non-null values of data1 and data2 (unless all the rows for that id have null, in which case the rolled-up row value would be null). I can't just use the last row; I need the latest non-null value for each individual column.

So three rows like this:

1, '2016-01-01', '2016-01-02', 'first', null
1, '2016-01-05', '2016-01-07', null, 'second'
1, '2016-01-10', '2016-01-12', 'third', null

...would roll up to:

1, '2016-01-01', 2016-01-12', 'third', 'second'

I've looked at non-equi-self-joins and windowing functions, but I can't quite get them to roll up that latest non-null value for individual columns. Is there a cool SQL way to do this in one query or am I, yikes, writing a Java program for it?

Thanks

Answer 1

I would use LAST_VALUE .

select
  id
  ,min(inserted) over (partition by id)
  ,max(modified) over (partition by id)
  ,last_value(data1 ignore nulls) over (partition by id order by modified)
  ,last_value(data2 ignore nulls) over (partition by id order by modified)
from
  table_a