# 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:

```sql
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>


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