Postgres 13

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select version();


version


PostgreSQL 13.4 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 8.4.1 20200928 (Red Hat 8.4.1-1), 64-bit


CREATE TABLE log (
    Request      VARCHAR(30)
  , Location     VARCHAR(10)
  , RequestDate  date
  , CompleteDate date
);

INSERT INTO log VALUES 
  ('R-001', 'a',  '2020-1-1',    '2020-1-5' )
, ('R-002', 'a',  '2020-3-17',   '2020-3-20')
, ('R-004', 'b',  '2020-2-27',   '2020-3-1' )
, ('R-003', 'b',  '2020-4-22',   '2020-4-23')
, ('R-005', 'a',  '2020-7-3',    '2020-7-7' )
;
5 rows affected

SELECT log.*
     , LAG(CompleteDate) OVER (PARTITION BY Location ORDER BY RequestDate) AS prev_date
     , RequestDate - LAG(CompleteDate) OVER (PARTITION BY Location ORDER BY RequestDate) AS diff
  FROM log
 ORDER BY Location, RequestDate 
;


request
location
requestdate
completedate
prev_date
diff


R-001
a
2020-01-01
2020-01-05
null
null

R-002
a
2020-03-17
2020-03-20
2020-01-05
72

R-005
a
2020-07-03
2020-07-07
2020-03-20
105

R-004
b
2020-02-27
2020-03-01
null
null

R-003
b
2020-04-22
2020-04-23
2020-03-01
52


DROP TABLE log;
CREATE TABLE log (
    Request      VARCHAR(30)
  , Location     VARCHAR(10)
  , RequestDate  VARCHAR(15)
  , CompleteDate VARCHAR(15)
);

INSERT INTO log VALUES 
  ('R-001', 'a',  '1/1/2020',    '1/5/2020' )
, ('R-002', 'a',  '3/17/2020',   '3/20/2020')
, ('R-004', 'b',  '2/27/2020',   '3/1/2020' )
, ('R-003', 'b',  '4/22/2020',   '4/23/2020')
, ('R-005', 'a',  '7/3/2020',    '7/7/2020' )
;
5 rows affected

WITH logs AS (
        SELECT Request, Location
             , TO_DATE(RequestDate, 'MM/DD/YYYY')  AS RequestDate
             , TO_DATE(CompleteDate, 'MM/DD/YYYY') AS CompleteDate
          FROM log
     )
SELECT log.*
     , LAG(CompleteDate) OVER (PARTITION BY Location ORDER BY RequestDate) AS prev_date
     , RequestDate - LAG(CompleteDate) OVER (PARTITION BY Location ORDER BY RequestDate) AS diff
  FROM logs AS log
 ORDER BY Location, RequestDate 
;


request
location
requestdate
completedate
prev_date
diff


R-001
a
2020-01-01
2020-01-05
null
null

R-002
a
2020-03-17
2020-03-20
2020-01-05
72

R-005
a
2020-07-03
2020-07-07
2020-03-20
105

R-004
b
2020-02-27
2020-03-01
null
null

R-003
b
2020-04-22
2020-04-23
2020-03-01
52

request	location	requestdate	completedate	prev_date	diff
R-001	a	2020-01-01	2020-01-05	null	null
R-002	a	2020-03-17	2020-03-20	2020-01-05	72
R-005	a	2020-07-03	2020-07-07	2020-03-20	105
R-004	b	2020-02-27	2020-03-01	null	null
R-003	b	2020-04-22	2020-04-23	2020-03-01	52