Select the nearest value before and after

I have a table like this

let say the table is开发者_Python百科 show_date

different | date
----------------------  
    -2    | 2011-04-18
----------------------
    -1    | 2011-04-19
----------------------
     4    | 2011-04-24
----------------------
     5    | 2011-04-25 
----------------------

I want to select 2 values, the first one is the first negative value and the second one is the first positive value. in this sample will result -1 (2011-04-19) and 4 (2011-04-24)

I've try order by abs(different) but it give the 2 nearest which is -1 and -2

(SELECT * FROM show_date WHERE different < 0 ORDER BY different DESC LIMIT 1)
UNION ALL
(SELECT * FROM show_date WHERE different >= 0 ORDER BY different LIMIT 1)

SELECT
  MIN(CASE WHEN different > 0 THEN different ELSE NULL END) AS MinPositive,
  MAX(CASE WHEN different < 0 THEN different ELSE NULL END) AS MaxNegative
FROM show_date

SELECT 
   min (a.nr), 
   max (b.nr) 
FROM 
  demo a, 
  demo b 
WHERE a.nr > 0 
  AND b.nr < 0;

 min | max
-----+----
   1 | -1
(1 Row)

An alternative approach - above solution gets problematic for large tables:

SELECT min(date)
  FROM tbl 
  WHERE date > 0
UNION 
SELECT max(date) 
  FROM tbl 
  WHERE date < 0;