Postgres GROUP BY looking at dates ranges - sql

Question

I have a table with the history of the "Code" value changes. Every month this table gets a new record with the new value of the "Code" for the specified month.
+----------+------------+------------+------+
| Employee | FromDate | ToDate | Code |
+----------+------------+------------+------+
| Employee | 01/07/2016 | 31/07/2016 | 4 |
| Employee | 01/06/2016 | 30/06/2016 | 2 |
| Employee | 01/05/2016 | 31/05/2016 | 2 |
| Employee | 01/04/2016 | 30/04/2016 | 3 |
| Employee | 01/03/2016 | 31/03/2016 | 3 |
| Employee | 01/02/2016 | 29/02/2016 | 4 |
| Employee | 01/01/2016 | 31/01/2016 | 4 |
+----------+------------+------------+------+
I need to group by this data to get a new record every time "Code" changes and take the min value for the "From date" and the max value for the "To date". Data must be ordered descending by "FromDate". With my query I got this result:
+----------+------------+------------+------+
| Employee | FromDate | ToDate | Code |
+----------+------------+------------+------+
| Employee | 01/05/2016 | 30/06/2016 | 2 |
| Employee | 01/03/2016 | 30/04/2016 | 3 |
| Employee | 01/01/2016 | 31/07/2016 | 4 |
+----------+------------+------------+------+
It works fine but if the same "Code" has more the one date range (see the 4 code in the first table) I got a single row per code. I would like get this result with the 4 code in 2 records because its period is not continuos but it's broke by others codes (3 and 2):
+----------+------------+------------+------+
| Employee | FromDate | ToDate | Code |
+----------+------------+------------+------+
| Employee | 01/07/2016 | 31/07/2016 | 4 |
| Employee | 01/05/2016 | 30/06/2016 | 2 |
| Employee | 01/03/2016 | 30/04/2016 | 3 |
| Employee | 01/01/2016 | 29/02/2016 | 4 |
+----------+------------+------------+------+
I use this query:
SELECT
d."Employee",
MIN (d."FromDate") AS "FromDate",
MAX (d."ToDate") AS "ToDate",
d."Code"
FROM
(
SELECT
"Employees"."FromDate",
"Employees"."ToDate",
"Employees"."Code",
"Employees"."Employee"
FROM
schema_estelspa."Employees"
ORDER BY
"Employees"."FromDate" DESC
) d
GROUP BY
d."Code",
d."Employee"
ORDER BY
(MIN(d."FromDate")) DESC
Is there any trick to get the result I desired?
Date format is: dd/MM/yyyy

Answer 1

Here you need to make date range and make from_date as one part of group by column. you also need to self join to achieve this result. I prepared following SQL in teradata. Please make necessary changes for your database(coalesc is used as if null expression, you can use nvl or case statement as well)
Query:
SELECT E.EMPLOYEE, E.CODE,COALESCE(ET1.FROMdATE,E.FROMDATE)FROM_DATE ,MAX(E.TODATE)TO_D
FROM EMP_TEST E
LEFT OUTER JOIN EMP_TEST ET1
ON E.EMPLOYEE=ET1.EMPLOYEE
AND E.CODE=ET1.CODE
AND E.FromDate=ET1.ToDate+1
GROUP BY 1,2,3
ORDER BY FROM_DATE
Output:
Employee Code FROM_DATE TO_D
1 Employee 4 1/1/2016 2/29/2016
2 Employee 2 5/1/2016 6/30/2016
3 Employee 4 7/1/2016 7/31/2016
4 Employee 3 3/1/2016 4/30/2016

Answer 2

Standard recursive solution for connecting-the-dots
in practice, half-open intervals (lower_limit <= X < upper_limit) are easier to work with
Recursion starts with any segment that does not have a lower neigbor
adjacent segments are glued to the right side, building longer chains
the final query suppresses partial results
Note: the code below does not deal with overlapping intervals.
-- Table
CREATE TABLE ecode
( employee varchar NOT NULL
, code INTEGER NOT NULL
, fromdate DATE NOT NULL
, uptodate DATE NOT NULL
);
SET datestyle = 'DMY' ;
-- Data
INSERT INTO ecode(employee, fromdate, uptodate, code) VALUES
('Employee','01/07/2016','31/07/2016', 4)
, ('Employee','01/06/2016','30/06/2016', 2)
, ('Employee','01/05/2016','31/05/2016', 2)
, ('Employee','01/04/2016','30/04/2016', 3)
, ('Employee','01/03/2016','31/03/2016', 3)
, ('Employee','01/02/2016','29/02/2016', 4)
, ('Employee','01/01/2016','31/01/2016', 4)
;
-- Convert to half-open interval
UPDATE ecode SET uptodate = uptodate + '1 day'::interval;
-- SELECT * FROM ecode;
WITH RECURSIVE zzz AS (
SELECT employee, code, fromdate, uptodate
FROM ecode e0
WHERE NOT EXISTS ( -- first one in series
SELECT * FROM ecode nx
WHERE nx.employee = e0.employee
AND nx.code = e0.code
AND nx.uptodate = e0.fromdate
)
UNION ALL -- append consecutive intervals
SELECT e1.employee, e1.code, zzz.fromdate, e1.uptodate
FROM ecode e1
JOIN zzz ON zzz.employee = e1.employee
AND zzz.code = e1.code
AND zzz.uptodate = e1.fromdate
)
SELECT * FROM zzz
-- suppress the partial results
WHERE NOT EXISTS (SELECT * FROM ecode nx
WHERE nx.employee = zzz.employee
AND nx.code = zzz.code
AND nx.fromdate = zzz.uptodate
)
ORDER BY employee, code, fromdate
;
Result:
DROP SCHEMA
CREATE SCHEMA
SET
CREATE TABLE
SET
INSERT 0 7
UPDATE 7
employee | code | fromdate | uptodate
----------+------+------------+------------
Employee | 2 | 2016-05-01 | 2016-07-01
Employee | 3 | 2016-03-01 | 2016-05-01
Employee | 4 | 2016-01-01 | 2016-03-01
Employee | 4 | 2016-07-01 | 2016-08-01
(4 rows)