Joining a record from a single column date range - sql

Question

I have two tables:
Person
+---------+-----------+
| Name | Added |
+---------+-----------+
| Roger | 2/1/2001 |
| Natalie | 5/5/2001 |
| George | 6/6/2001 |
| Paul | 12/5/1999 |
+---------+-----------+
Stage
+-------------+----------+
| Description | Start |
+-------------+----------+
| 1 | 1/1/1980 |
| 2 | 4/1/2001 |
| 3 | 6/1/2001 |
+-------------+----------+
I want to join Person with stage such that I get the following result.
Result
+---------+-----------+--------+
| Name | Added | Stage |
+---------+-----------+--------+
| Roger | 2/1/2001 | 1 |
| Natalie | 5/5/2001 | 2 |
| George | 6/6/2001 | 3 |
| Paul | 12/5/1999 | 1 |
+---------+-----------+--------+
So, the stage 1 matches (added >= 1/1/1980 AND added < 4/1/2001), stage 2 matches (added >= 4/1/2001 AND added < 6/1/2001), stage 3 (added >= 6/1/2001) etc... This works, but I think it's kind of ugly (and only happens to work because the description is sequential as well).
SELECT person.name,
person.added,
(SELECT MAX(description) FROM stage d2 WHERE person.added >= d2.start) description
FROM person
Is there a way to do this in a regular join, and if description were a string rather than a sequential number? Thanks.

Answer 1

Instead of a subquery, you could use row_number():
select name, added, description
from (
select p.name, p.added, s.description
, row_number() over (
partition by p.name
order by s.start desc
) as rn
from person p
inner join stage s
on s.start <= p.added
) t
where rn = 1
test setup: http://rextester.com/SIAUAZ29747
with Person (Name,Added_date) as (
select 'Roger' , to_date('2001-02-01','yyyy-mm-dd') from dual union all
select 'Natalie' , to_date('2001-05-05','yyyy-mm-dd') from dual union all
select 'George' , to_date('2001-06-06','yyyy-mm-dd') from dual union all
select 'Paul' , to_date('1999-12-05','yyyy-mm-dd') from dual
),
Stage ( Description , Start_date ) as (
select 1, to_date('1980-01-01','yyyy-mm-dd') from dual union all
select 2, to_date('2001-04-01','yyyy-mm-dd') from dual union all
select 3, to_date('2001-06-01','yyyy-mm-dd') from dual
)
select name, to_char(added_date,'yyyy-mm-dd') added, description
from (
select p.name, p.added_date, s.description
, row_number() over (
partition by p.name
order by s.start_date desc
) as rn
from person p
inner join stage s
on s.start_date <= p.added_date
) t
where rn = 1
order by added_date
returns:
+---------+------------+-------------+
| NAME | ADDED | DESCRIPTION |
+---------+------------+-------------+
| Paul | 1999-12-05 | 1 |
| Roger | 2001-02-01 | 1 |
| Natalie | 2001-05-05 | 2 |
| George | 2001-06-06 | 3 |
+---------+------------+-------------+

Answer 2

Problems of this type can often be solved with no joins at all. Instead, combine the two tables (as illustrated below) with UNION ALL and use the LAST_VALUE() function:
select name, added, description
from (
select name, added,
last_value(description ignore nulls)
over (order by added, description) as description
from ( select name, null as description, added
from person
union all
select null, description, start_date
from stage
)
)
where name is not null
order by added, name -- if needed
;
NAME ADDED DESCRIPTION
------- ---------- -----------
Paul 12/05/1999 1
Roger 02/01/2001 1
Natalie 05/05/2001 2
George 06/06/2001 3
Big THANK YOU to #MT0 for providing the setup (CREATE TABLE statements).

Answer 3

Here is a version that joins the rows in Person to Stage with a 1:1 correspondence (unlike the accepted solution which will join Person to multiple rows in Stage and then have to filter out the unwanted rows):
Oracle Setup:
CREATE TABLE Person (Name,Added) AS
SELECT 'Roger' , DATE '2001-02-01' FROM DUAL UNION ALL
SELECT 'Natalie' , DATE '2001-05-05' FROM DUAL UNION ALL
SELECT 'George' , DATE '2001-06-06' FROM DUAL UNION ALL
SELECT 'Paul' , DATE '1999-12-05' FROM DUAL;
CREATE TABLE Stage ( Description , Start_date ) AS
SELECT 1, DATE '1980-01-01' FROM DUAL UNION ALL
SELECT 2, DATE '2001-04-01' FROM DUAL UNION ALL
SELECT 3, DATE '2001-06-01' FROM DUAL;
Query:
SELECT name, added, description
FROM person p
INNER JOIN
(
SELECT description,
start_date,
LEAD( start_date ) OVER ( ORDER BY start_date ) AS end_date
FROM stage
) s
ON ( s.start_date <= p.added AND ( s.end_date IS NULL OR p.added < s.end_date ) );
Output:
NAME ADDED DESCRIPTION
------- ------------------- -----------
Paul 1999-12-05 00:00:00 1
Roger 2001-02-01 00:00:00 1
Natalie 2001-05-05 00:00:00 2
George 2001-06-06 00:00:00 3