Generate_series in Postgres from start and end date in a table

问题

I have been trying to generate a series of dates (YYYY-MM-DD HH) from the first until the last date in a timestamp field. I've got the generate_series() I need, however running into an issue when trying to grab the start and end dates from a table. I have the following to give a rough idea:

with date1 as
(
SELECT start_timestamp as first_date
FROM header_table
ORDER BY start_timestamp DESC
LIMIT 1
),
date2 as
(
SELECT start_timestamp as first_date
FROM header_table
ORDER BY start_timestamp ASC    
LIMIT 1
)
    select generate_series(date1.first_date, date2.first_date
                         , '1 hour'::interval)::timestamp as date_hour

from
(   select * from date1
    union
    select * from date2) as foo

Postgres 9.3

回答1:

You certainly don't need a CTE for this. That would be more expensive than necessary.
And you don't need to cast to timestamp because the result already is of data type timestamp when you feed date types to generate_series().

In Postgres 9.3 or later, this is most elegantly solved with a LATERAL join:

SELECT to_char(ts, 'YYYY-MM-DD HH24') AS formatted_ts
FROM  (
   SELECT min(start_timestamp) as first_date
        , max(start_timestamp) as last_date
   FROM   header_table
   ) h
  , generate_series(h.first_date, h.last_date, interval '1 hour') g(ts);

Optionally with to_char() to get the result as text in the format you mentioned.
In earlier (or any) versions:

SELECT generate_series(min(start_timestamp)
                     , max(start_timestamp)
                     , interval '1 hour') AS ts
FROM   header_table;

But calling set-returning functions in the SELECT list is a non-standard feature and frowned upon by some. Use the first query if you can.

Note a subtle difference in NULL handling:

The equivalent of

max(start_timestamp)

is obtained with

ORDER BY start_timestamp DESC NULLS LAST
LIMIT 1

Without NULLS LAST NULL values come first in descending order (if there can be NULL values in start_timestamp). You would get NULL for last_date and your query would come up empty.

Details:

Why do NULL values come first when ordering DESC in a PostgreSQL query?

回答2:

How about using aggregation functions instead?

with dates as (
      SELECT min(start_timestamp) as first_date, max(start_timestamp) as last_date
      FROM header_table
     )
select generate_series(first_date, last_date, '1 hour'::interval)::timestamp as date_hour
from dates;

Or even:

select generate_series(min(start_timestamp),
                       max(start_timestamp),
                       '1 hour'::interval
                      )::timestamp as date_hour
from header_table;

回答3:

try this:

with dateRange as
  (
  SELECT min(start_timestamp) as first_date, max(start_timestamp) as last_date
  FROM header_table
  )
select 
    generate_series(first_date, last_date, '1 hour'::interval)::timestamp as date_hour
from dateRange

NB: You want the 2 dates in a row, not on separate rows.

see this sqlfiddle demo

来源：https://stackoverflow.com/questions/29023336/generate-series-in-postgres-from-start-and-end-date-in-a-table

标签

sql

postgresql

aggregate-functions

generate-series