Question

I have Work and Person tables (these are just examples to understand problem).

Structure

Work table

id INTEGER
person_id INTEGER
dt_from DATETIME
dt_to DATETIME

Person table

person_id INTEGER
name VARCHAR(50)

Data

Work table

id | person_id | dt_from    | dt_to
-------------------------------------------------
1  | 1         | 2011-01-01 | 2011-02-02
2  | 1         | 2011-02-02 | 2011-04-04
3  | 1         | 2011-06-06 | 2011-09-09
4  | 2         | 2011-01-01 | 2011-02-02
5  | 2         | 2011-02-02 | 2011-03-03
....etc.

Person table

Just person names with person id

Expected output

Person 1 : 2011-01-01 - 2011-04-04
Person 1 : 2011-06-06 - 2011-09-09
Person 2 : 2011-01-01 - 2011-03-03

Interval must be in sequence. It cannot be broken somewhere in middle. Thats why Person 1 have two intervals.

I’m using postgres if it changes something. Have you any thougths?
I wanted do it in one query, but if there is no such solution i will do some interval merge in php.

Answer 1

There may be a way to do this in one SQL select, but it escapes me. I managed to do it with one stored function though. Here’s what I did for my testing:

create table work
(id integer, start_date date, end_date date);

insert into work values (1, '2011-01-01','2011-02-02');
insert into work values (1, '2011-02-02','2011-04-04');
insert into work values (1, '2011-06-06','2011-09-09');
insert into work values (2, '2011-01-01','2011-02-02');
insert into work values (2, '2011-02-02','2011-03-03');

create or replace function get_data() returns setof work as
$body$
declare
    res work%rowtype;
    sd  date := null;
begin
    for res in
        select
            w1.id,
            case when exists (select 1 from work w2 where w1.id=w2.id and w2.end_date=w1.start_date) then null else w1.start_date end,
            case when exists (select 1 from work w2 where w1.id=w2.id and w2.start_date=w1.end_date) then null else w1.end_date end
        from
            work w1
        order by
            id, start_date, end_date
    loop
        if res.start_date is not null and res.end_date is not null then
            return next res;
        elsif res.start_date is not null then
            sd := res.start_date;
        elsif res.end_date is not null then
            res.start_date := sd;
            return next res;
        end if;
    end loop;

    return;
end;$body$
  language 'plpgsql';

Then

select * from get_data() order by id, start_date;

returned this result:

 id | start_date |  end_date
----+------------+------------
  1 | 2011-01-01 | 2011-04-04
  1 | 2011-06-06 | 2011-09-09
  2 | 2011-01-01 | 2011-03-03
(3 rows)

which is, I think, what you’re after.