set a column with value from different table

Say that you have Table A:

• username

• email

And Table B:

• item_id

• username

You want to add email to table B where the value taken from table A. After adding a column on Table B, this query can be used:

UPDATE tableB
SET    email = tableA.Email
FROM   tableA
WHERE  tableB.username = tableA.username
AND    tableB.email IS DISTINCT FROM tableA.email;  -- optional, see below
​

The final WHERE clause prevents updates that wouldn't change anything - which is practically always a good idea (almost full cost but no gain, exotic exceptions apply). If both old and new value are guaranteed to be NOT NULL, simplify to:

AND   tableB.email <> tableA.email

Taken from: https://stackoverflow.com/questions/13473499/update-a-column-of-a-table-with-a-column-of-another-table-in-postgresql​