PostgreSQL, min, max og antal datoer inden for rækkevidde

Givet denne tabel (som du burde have angivet):

CREATE TEMP TABLE tbl (
   id        int PRIMARY KEY
  ,mydatetxt text
 );

INSERT INTO tbl VALUES
  (1, '01.02.2011')
 ,(2, '05.01.2011')
 ,(3, '06.03.2012')
 ,(4, '07.08.2011')
 ,(5, '04.03.2013')
 ,(6, '06.08.2011')
 ,(7, '')             -- empty string
 ,(8, '02.02.2013')
 ,(9, '04.06.2010')
 ,(10, '10.10.2012')
 ,(11, '04.04.2012')
 ,(12, NULL)          -- NULL
 ,(13, '04.03.2013'); -- min date a 2nd time

Forespørgslen skal producere det, du beskriver:

WITH base AS (
   SELECT to_date(mydatetxt, 'DD.MM.YYYY') AS the_date
   FROM   tbl
   WHERE  mydatetxt <> ''  -- excludes NULL and ''
   )
SELECT min(the_date) AS dmin
      ,max(the_date) AS dmax
      ,count(*) AS ct_incl
      ,(SELECT count(*)
        FROM   base b1
        WHERE  b1.the_date < max(b.the_date)
        AND    b1.the_date > min(b.the_date)
       ) AS ct_excl
FROM   base b

-> SQLfiddle-demo

CTE'er kræver Postgres 8.4 eller nyere.
Overvej at opgradere til den seneste punktudgivelse af 9.1, som i øjeblikket er 9.1.9 .