Reverse in SQL by pair-CodePudding

I'm very new to SQL and I'd like to do something like this :

Going from

12 34 56 78

78 56 34 12

I tried the reverse function, but the result is 87 65 43 21.

Is there a way to do this ? Thank you for reading !

CodePudding user response：

You can use string split to split your string into rows, then string_agg to get a string back with the order you want:

select string_agg(value, ' ') within group(order by cast(value as int) desc)
  from string_split('12 34 56 78', ' ')

Fiddle

However if you don't want you numbers in increasing order, you shouldn't use string_split with row_number as suggested in the other answer because the order is NOT guaranteed. Use another function instead of string_split, such as DelimitedSplit8K from here.

Your select becomes:

select string_agg(Item, ' ') within group(order by ItemNumber desc)
  from DelimitedSplit8K('12 34 56 78', ' ');

The code of DelimitedSplit8K:

CREATE FUNCTION [dbo].[DelimitedSplit8K]
--===== Define I/O parameters
        (@pString VARCHAR(8000), @pDelimiter CHAR(1))
RETURNS TABLE WITH SCHEMABINDING AS
 RETURN
--===== "Inline" CTE Driven "Tally Table” produces values from 0 up to 10,000...
     -- enough to cover VARCHAR(8000)
  WITH E1(N) AS (
                 SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL 
                 SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL 
                 SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1 UNION ALL SELECT 1
                ),                          --10E 1 or 10 rows
       E2(N) AS (SELECT 1 FROM E1 a, E1 b), --10E 2 or 100 rows
       E4(N) AS (SELECT 1 FROM E2 a, E2 b), --10E 4 or 10,000 rows max
 cteTally(N) AS (--==== This provides the "zero base" and limits the number of rows right up front
                     -- for both a performance gain and prevention of accidental "overruns"
                 SELECT 0 UNION ALL
                 SELECT TOP (DATALENGTH(ISNULL(@pString,1))) ROW_NUMBER() OVER (ORDER BY (SELECT NULL)) FROM E4
                ),
cteStart(N1) AS (--==== This returns N 1 (starting position of each "element" just once for each delimiter)
                 SELECT t.N 1
                   FROM cteTally t
                  WHERE (SUBSTRING(@pString,t.N,1) = @pDelimiter OR t.N = 0) 
                )
--===== Do the actual split. The ISNULL/NULLIF combo handles the length for the final element when no delimiter is found.
 SELECT ItemNumber = ROW_NUMBER() OVER(ORDER BY s.N1),
        Item       = SUBSTRING(@pString,s.N1,ISNULL(NULLIF(CHARINDEX(@pDelimiter,@pString,s.N1),0)-s.N1,8000))
   FROM cteStart s
;

CodePudding user response：

I'm sure the string_split will be useful here. Here's what it can do For your particular case (numbers are always increasing).

select *
  from STRING_SPLIT('12 34 56 78', ' ')
 order by value desc;

dbfiddle

UPD. Here's what you can do when numbers are not always increasing

with splitted as(
select value, row_number() over(order by (select null)) rn
  from string_split('12 34 78 56', ' '))

select string_agg(value, ' ') within group(order by rn desc)
  from splitted

dbfiddle