How to merge data of two column into Single one in SQL ?

In SQL if you have one table which More than two Fields and if you want to merge data of Two or More then two fields then follow the below steps.

Select  column name from  table name
Union
select column name 1 from table name

Select Lastname from employee
union
select Username from employee

Data after execution of above query

above table show concatenation of column in to one, I take lastname and username column and merge into a single column which is show above.