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