Postgres Consecutive Days, gaps and islands, Tabibitosan

Question

SQL FIDDLE

I have the following database table:

date	name
2014-08-10	bob
2014-08-10	sue
2014-08-11	bob
2014-08-11	mike
2014-08-12	bob
2014-08-12	mike
2014-08-05	bob
2014-08-06	bob

SELECT t.Name,COUNT(*) as frequency
FROM (
    SELECT Name,Date,
            row_number() OVER (
            ORDER BY Date
            ) - row_number() OVER (
            PARTITION BY Name ORDER BY Date
            ) + 1 seq
    FROM orders
    ) t
GROUP BY Name,seq;

Tried running the Tabibitosan method of finding gaps and islands produces the below table which is incorrect. The name "mike" should actually have a count of 2 since the 11th and 12th days are consecutive. How do I fix this?

name	frequency
mike	1
bob	3
bob	2
mike	1
sue	1

Correct expected output below:

name	frequency
bob	3
bob	2
mike	2
sue	1

Roundish

Can you also add the exact expected result? – Gladdie

@JimJones just updated my question with what the expected output should be – Roundish

BTW, It's Tabibitosan, and not Tababitosan 😉 – Ilailaire

This will not work because date is not unique. So being ordered by date rows 2014-08-11 bob and 2014-08-11 mike will get different row_number() indroducing a shift for next entries of bob and mike, but the same when partitioned by name, which ruins the methods idea. – Qualifier

Answer 1

You are using the wrong logic. Basically, you want dates that are sequential, so you want to subtract the sequence from the date:

SELECT t.Name, COUNT(*) as frequency
FROM (SELECT o.*,
             row_number() OVER (PARTITION BY Name ORDER BY Date) as seqnum
      FROM orders o
     ) t
GROUP BY Name, date - seqnum * interval '1 day';

Here is a db<>fiddle.

Sunroom

Answer 2

Gaps and Islands problem for Date datatype Demo1 in Postgresql:

Run this:

drop table if exists foobar; 
CREATE TABLE foobar( tick text, date_val date ); 
insert into foobar values('XYZ', '2021-01-03');  --island 1 has width 2
insert into foobar values('XYZ', '2021-01-04');  --island 1
insert into foobar values('XYZ', '2021-05-09');  --island 2 has width 3
insert into foobar values('XYZ', '2021-05-10');  --island 2 
insert into foobar values('XYZ', '2021-05-11');  --island 2
insert into foobar values('XYZ', '2021-07-07');  --island 3 has width 4
insert into foobar values('XYZ', '2021-07-08');  --island 3
insert into foobar values('XYZ', '2021-07-09');  --island 3
insert into foobar values('XYZ', '2021-07-10');  --island 3 
insert into foobar values('XYZ', '2022-10-10');  --island 4 has width 1

select * from foobar; 

select tick, island_width, min_val, max_val, 
       min_val - lag(max_val) over (order by max_val) 
       as gap_width from  
( 
  select tick, count(*) as island_width, 
         min(date_val) min_val, max(date_val) max_val 
  from ( 
    select t.*, 
    row_number() over ( partition by tick order by date_val ) as seqnum 
    from foobar t where tick = 'XYZ' 
    ) t 
  group by tick, date_val - seqnum * interval '1 day' 
) t2 order by max_val desc

Which Prints gaps and islands of the date col thustly:

┌──────┬────────────┐ 
│ tick │  date_val  │ 
├──────┼────────────┤  
│ XYZ  │ 2021-01-03 │ 
│ XYZ  │ 2021-01-04 │ 
│ XYZ  │ 2021-05-09 │ 
│ XYZ  │ 2021-05-10 │ 
│ XYZ  │ 2021-05-11 │ 
│ XYZ  │ 2021-07-07 │  
│ XYZ  │ 2021-07-08 │  
│ XYZ  │ 2021-07-09 │  
│ XYZ  │ 2021-07-10 │  
│ XYZ  │ 2022-10-10 │  
└──────┴────────────┘ 
┌──────┬──────────────┬────────────┬────────────┬───────────┐ 
│ tick │ island_width │  min_val   │  max_val   │ gap_width │ 
├──────┼──────────────┼────────────┼────────────┼───────────┤ 
│ XYZ  │            1 │ 2022-10-10 │ 2022-10-10 │       457 │ 
│ XYZ  │            4 │ 2021-07-07 │ 2021-07-10 │        57 │ 
│ XYZ  │            3 │ 2021-05-09 │ 2021-05-11 │       125 │ 
│ XYZ  │            2 │ 2021-01-03 │ 2021-01-04 │         ¤ │ 
└──────┴──────────────┴────────────┴────────────┴───────────┘ 

The column island_width gives the width of continuous data. The gap_width gives you the width of missing data.

Gaps and Islands problem for Integer datatype Demo2 in Postgresql:

Run This:

drop table if exists foobar; 
CREATE TABLE foobar( tick text, the_value int); 
insert into foobar values('XYZ', -5);  --island 1 has width 2 
insert into foobar values('XYZ', -4);  --end island 1 
insert into foobar values('XYZ', 0);   --island 2 has width 3 
insert into foobar values('XYZ', 1);   --island 2 
insert into foobar values('XYZ', 2);   --end island 2 
insert into foobar values('XYZ', 34);  --island 3 has width 4 
insert into foobar values('XYZ', 35);  --island 3 
insert into foobar values('XYZ', 36);  --island 3 
insert into foobar values('XYZ', 37);  --island 3 
insert into foobar values('XYZ', 85);  --island 4 has width 1 
  
select * from foobar; 
  
select tick, island_width, min_val, max_val, 
       min_val - lag(max_val) over (order by max_val) 
       as gap_width from 
( 
  select tick, count(*) as island_width, 
         min(the_value) min_val, max(the_value) max_val 
  from ( 
    select t.*, 
    row_number() over ( partition by tick order by the_value) as seqnum 
    from foobar t where tick = 'XYZ' 
    ) t 
  group by tick, the_value - seqnum * 1 
) t2 order by max_val desc

Which Prints gaps and islands of the integer col thustly:

┌──────┬───────────┐ 
│ tick │ the_value │ 
├──────┼───────────┤ 
│ XYZ  │        -5 │ 
│ XYZ  │        -4 │ 
│ XYZ  │         0 │ 
│ XYZ  │         1 │ 
│ XYZ  │         2 │ 
│ XYZ  │        34 │ 
│ XYZ  │        35 │ 
│ XYZ  │        36 │ 
│ XYZ  │        37 │ 
│ XYZ  │        85 │ 
└──────┴───────────┘ 
┌──────┬──────────────┬─────────┬─────────┬───────────┐ 
│ tick │ island_width │ min_val │ max_val │ gap_width │ 
├──────┼──────────────┼─────────┼─────────┼───────────┤ 
│ XYZ  │            1 │      85 │      85 │        48 │ 
│ XYZ  │            4 │      34 │      37 │        32 │ 
│ XYZ  │            3 │       0 │       2 │         4 │ 
│ XYZ  │            2 │      -5 │      -4 │         ¤ │ 
└──────┴──────────────┴─────────┴─────────┴───────────┘

The column island_width gives the width of continuous data. The gap_width gives you the width of missing data. This allows you to say: "show me the gaps between islands" or "show me the island sizes and start/stop location"

Beak