Reverse in SQL by pair - sql

Question

I'm very new to SQL and I'd like to do something like this :
Going from
12 34 56 78
to
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 !

Answer 1

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
;

Answer 2

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