How to use data from multiple similar columns as rows in SQL

I have a table as below:

ItemName	1mth_presale_cost	2mth_presale_cost	1mth_postsale_cost	2mth_postsale_cost
1000	10.1	12.1	12.5	15.1
1001	20.2	15.2	25.2	17.3

I want the result to be like below table:

ItemName	1mth_cost	2mth_cost
1000	10.1	12.1
1000	12.5	15.1
1001	20.2	15.2
1001	25.2	17.3

I don’t want to use UNION for this.

Answer

First each row converted into two sets then apply conditional CASE statement for getting desired result.

-- SQL Server
SELECT t.ItemName
     , CASE WHEN p.id = 1 THEN [1mth_presale_cost] ELSE [1mth_postsale_cost] END  "1mth_cost"
     , CASE WHEN p.id = 1 THEN [2mth_presale_cost] ELSE [2mth_postsale_cost] END  "2mth_cost"
FROM test t
CROSS JOIN (SELECT 1 id UNION SELECT 2 id) p
ORDER BY t.ItemName, p.id

Please check from url https://dbfiddle.uk/?rdbms=sqlserver_2017&fiddle=3e1fdaf829c39554941c068b0c8a3e04