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:

SET    email = tableA.Email
FROM   tableA
WHERE  tableB.username = tableA.username
AND IS DISTINCT FROM;  -- 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 <>

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