Swap two adjacent rows of a column in sql - sql

Question

I'm trying to solve this following problem:
Write a sql query to swap two adjacent rows in a column of a table.
Input table
Name Id
A 1
B 2
C 3
D 4
E 5
Output table
Name Id
A 2
B 1
C 4
D 3
E 5
Description:- 1 is associated with A and 2 with B, swap them, thus now 1 is associated with B and 2 with A, Similarly do for C and D, Since E doesn't has any pair, leave it as it is.
Note:- This may be solved using CASE Statements, but I am trying for a generalized solution, Say currently it is only 5 rows, it may be 10,20 etc..
Eg:
SELECT
*,CASE WHEN Name = A then 2 ELSEIF Name = B then 1 etc...
FROM YourTable

Answer 1

You can use window functions to solve this.
on MySQL (>= 8.0):
SELECT ID, IFNULL(CASE WHEN t.rn % 2 = 0 THEN LAG(Name) OVER (ORDER BY ID) ELSE LEAD(Name) OVER (ORDER BY ID) END, Name) AS Name
FROM (
SELECT ID, Name, ROW_NUMBER() OVER (ORDER BY ID) AS rn
FROM table_name
) t
demo on dbfiddle.uk
on SQL-Server:
SELECT ID, ISNULL(CASE WHEN t.rn % 2 = 0 THEN LAG(Name) OVER (ORDER BY ID) ELSE LEAD(Name) OVER (ORDER BY ID) END, Name) AS Name
FROM (
SELECT ID, Name, ROW_NUMBER() OVER (ORDER BY ID) AS rn
FROM table_name
) t
demo on dbfiddle.uk

Answer 2

If you have sql-server, you can try this.
DECLARE #YourTable TABLE (Name VARCHAR(10), Id INT)
INSERT INTO #YourTable VALUES
('A', 1),
('B', 2),
('C', 3),
('D', 4),
('E', 5)
;WITH CTE AS (
SELECT *, ROW_NUMBER()OVER(ORDER BY Name) AS RN FROM #YourTable
)
SELECT T1.Name, ISNULL(T2.Id, T1.Id) Id FROM CTE T1
LEFT JOIN CTE T2 ON T1.RN + CASE WHEN T1.RN%2 = 0 THEN - 1 ELSE 1 END = T2.RN
Result:
Name Id
---------- -----------
A 2
B 1
C 4
D 3
E 5

Answer 3

You didn't specify your DBMS, but the following is standard ANSI SQL.
You can use a values() clause to provide the mapping of the IDs and then join against that:
with id_map (source_id, target_id) as (
values
(1, 2),
(2, 1)
)
select t.name, coalesce(m.target_id, t.id) as mapped_id
from the_table t
left join id_map m on m.source_id = t.id
order by name;
Alternatively if you only want to specify the mapping once for one direction, you can use this:
with id_map (source_id, target_id) as (
values
(1, 2)
)
select t.name,
case id
when m.source_id then m.target_id
when m.target_id then m.source_id
else id
end as mapped_id
from the_table t
left join id_map m on t.id in (m.source_id, m.target_id)
order by name;
Online example: https://rextester.com/FBFH52231