I want to merge two columns into one, how to do it?

Question

PostgreSQL

And how to combine the values ​​​​in columns A and B, in which there are entries in cell A is 1, and in cell B it is 2, how to make them be in the same column, but in different cells?
So that it gives out like this:
Name_of_new_column
1
2

Answer 1

SELECT
ColumnA AS NewColumn
FROM table
UNION
SELECT
ColumnB
FROM table