Field correlation with date constraint - sql

Question

We're stuck in a huge challenge here. Let's assume the tables of one db were not properly planned in first hand. That's what it is and I need a solution for that.
There's a table A with 2 fields. Let's think that I have an assistant that supports my job every day, but i just registered when he/she started to assist me. It means the 'Stop Date' (not existent in the table) of each Assistant is the day before the Start Date of the next one.
Assistant | Start Date
James | 07/01/17
Frank | 01/03/18
Erika | 01/06/18
There's a second table B with that registers how many hours my assistant worked:
Date | Worked Hours
12/31/17 | 7.5
01/01/18 | 7.5
01/02/18 | 9
01/03/18 | 8
01/04/18 | 9
01/05/18 | 7.5
01/06/18 | 9
01/07/18 | 10
Given the information above, I need to write a SQL to return a table like below, considering the Start Dates of each person:
Assistant | Date | Worked Hours
Basically I need to correlate somehow the Date and Start Date to return the Assistant, but it involve's date comparisons that I have no idea how to do.
Any ideas how to solve this?

Answer 1

You can use a correlated subquery:
select b.*,
(select a.assistant
from a
where a.date <= b.date
order by a.date desc
fetch first 1 row only
) as assistant
from b;
Note all databases support the ANSI standard fetch first 1 row only, so you may need to use limit or top or whatever is appropriate for your database.

Answer 2

You can try this.
DECLARE #TableA TABLE (Assistant VARCHAR(10), [Start Date] DATE)
INSERT INTO #TableA VALUES
('James','07/01/17'),
('Frank','01/03/18'),
('Erika','01/06/18')
DECLARE #TableB TABLE ([Date] DATE, [Worked Hours] DECIMAL(18,2))
INSERT INTO #TableB VALUES
('12/31/17', 7.5),
('01/01/18', 7.5),
('01/02/18', 9 ),
('01/03/18', 8 ),
('01/04/18', 9 ),
('01/05/18', 7.5),
('01/06/18', 9 ),
('01/07/18', 10 )
;WITH CTE AS (
SELECT *, RN = ROW_NUMBER() OVER( PARTITION BY [Date] ORDER BY [Start Date] DESC)
FROM
#TableA A
INNER JOIN #TableB B ON A.[Start Date] <= B.Date
)
select Assistant, Date, [Worked Hours] FROM CTE WHERE RN = 1
Result:
Assistant Date Worked Hours
---------- ---------- ---------------------------------------
James 2017-12-31 7.50
James 2018-01-01 7.50
James 2018-01-02 9.00
Frank 2018-01-03 8.00
Frank 2018-01-04 9.00
Frank 2018-01-05 7.50
Erika 2018-01-06 9.00
Erika 2018-01-07 10.00

Answer 3

use 'lead()' to find the next record
use infinity to keep the final interval unclosed
CREATE TABLE a
( assistant text primary key
, startdate date
);
SET datestyle = 'mdy';
insert into a(assistant,startdate) VALUES
('James', '07/01/17' )
,('Frank', '01/03/18' )
,('Erika', '01/06/18' )
;
CREATE TABLE b
( ddate DATE NOT NULL primary key
, workedhours DECIMAL(4,1)
);
insert into b(ddate,workedhours) VALUES
('12/31/17', 7.5)
,('01/01/18', 7.5)
,('01/02/18', 9)
,('01/03/18', 8)
,('01/04/18', 9)
,('01/05/18', 7.5)
,('01/06/18', 9)
,('01/07/18', 10)
;
WITH aa AS (
SELECT a.assistant
, a.startdate
, lead(a.startdate, 1, 'infinity'::date) OVER (ORDER BY a.startdate)
AS enddate
FROM a
)
-- SELECT * FROM a ;
SELECT aa.startdate, aa.enddate, aa.assistant
, SUM(b.workedhours) AS workedhours
FROM aa
LEFT JOIN b ON b.ddate >= aa.startdate
AND b.ddate < aa.enddate
GROUP BY 1,2,3
;
Output:
CREATE TABLE
SET
INSERT 0 3
CREATE TABLE
INSERT 0 8
startdate | enddate | assistant | workedhours
------------+------------+-----------+-------------
2017-07-01 | 2018-01-03 | James | 24.0
2018-01-03 | 2018-01-06 | Frank | 24.5
2018-01-06 | infinity | Erika | 19.0
(3 rows)