Update using select in DB2

Question

I have 2 tables

Table A 
address_id email
12341      [email protected]
12342      [email protected]
12343      [email protected]
12344      [email protected]
12345      [email protected]
12346      [email protected]
12347      [email protected]
12348      [email protected]
12349      [email protected]

Table B 
address_id email
12342      [email protected]
12344      [email protected]
12349      [email protected]

I want to update emails in table A with emails in Table B where the address_id matches.

I have tried the below query :

update TableA A 
set email=(select email from TableB B where A.address_id=B.address_id) 

But its updating all the emails to null where the address_id is not matching in TableA.

Appreciate your help..

Answer 1

An UPDATE without WHERE clause will update all the rows of the table. For those rows that don't have a matching address_id in table B, the subquery returns an empty result set, so the value is updated to NULL.

---

There's probably a more elegant way to do it but this should only update the matching rows:

update TableA A 
set email = (select email from TableB B where A.address_id = B.address_id)
where exists
      (select 1 from TableB B where A.address_id = B.address_id) ;

Another option is to use MERGE:

MERGE INTO TableA AS A
USING
  ( SELECT address_id, email
    FROM TableB
  ) AS B
  ON A.address_id = B.address_id
WHEN MATCHED THEN
  UPDATE SET email = B.email ;