Update One Oracle Column with theData from Another Table

The Good Oracle Update

UPDATE TABLE_A SET COLUMN_A = (SELECT TABLE_B.COLUMN_B FROM TABLE_B WHERE TABLE_A.COLUMN_A = TABLE_B.COLUMN_B)
WHERE EXISTS (SELECT TABLE_B.COLUMN_B FROM TABLE_B WHERE TABLE_A.COLUMN_A = TABLE_B.COLUMN_B)

The Bad Oracle Update
(unless you want to wipe out the data in TABLE_A.COLUMN_A when there is no match)

UPDATE TABLE_A SET COLUMN_A = (SELECT TABLE_B.COLUMN_B FROM TABLE_B WHERE TABLE_A.COLUMN_A = TABLE_B.COLUMN_B)