replace into or insert into, I'm doing something wrong ?

Hi all

I'm trying to copy data from a table column into another table column

I've tried the following, however when I try to copy a different column, its adding data into the first column :

example :

REPLACE INTO TableA (ColumnA) SELECT (ColumnA) FROM TableB

Code (markup):

Then :

REPLACE INTO TableA (ColumnB) SELECT (ColumnB) FROM TableB

Code (markup):

Adds data to TableA ColumnB but it does'nt replace data its adding rows(data) only one row as well, where am I going wrong ?

 

Is TableA ColumnB a Primary Key or Unique? If not REPLACE INTO will insert instead of replace.