Get pair-wise single row

I have some data in a MS SQL Server database. Sample data is:

Sl	ColumnA	ColumnB
1	111	112
2	112	111
3	113	114
4	114	113
5	115	116
6	116	115

Now I need result like.

ColumnA	ColumnA
111	112
113	114
115	116

Notice here:

In Sl 1 number Column A = Sl 2 Column B
and Sl 1 number Column B = Sl 2 Column A

Same as:

In Sl 3 number Column A = Sl 4 Column B
and Sl 3 number Column B = Sl 4 Column A

Answer

One approach is to use CASE expressions to find the least/greatest value in each record:

SELECT DISTINCT
    CASE WHEN ColumnA < ColumnB  THEN ColumnA ELSE ColumnB END AS ColumnA,
    CASE WHEN ColumnA >= ColumnB THEN ColumnA ELSE ColumnB END AS ColumnB
FROM yourTable
ORDER BY 1;

Demo

Note that in certain other databases (such as MySQL and Postgres), there are formal LEAST and GREATEST functions, which can simplify and eliminate the need for the bulky CASE expressions used above.