How do I do this update?

Question

I am an Oracle idiot coming from MSSQL world. I have a specific
update I need to accomplish. Say I have two tables:
Table A with 2 columns 'Id' and 'Name':
101 Matt
102 Kevin
103 Matt
104 Peter
Table B with 2 columns 'Id' and 'Name':
300 Matt
400 John
Basically, for any name in table A that matches a name in table
B, I need to update Id column of table A with the corresponding
Id value found in table B. That is, the result should be:
Table A:
300 Matt
102 Kevin
300 Matt
104 Peter
I tried
"UPDATE A SET A.Id = (SELECT B.Id FROM B WHERE A.Name =
B.Name);"
The result is not what I want (as follows). Apprently the engine
decides to pad NULL for me. Urr..
Table A:
300 Matt
Kevin
300 Matt
Peter
Then I tried "MERGE" statement. But MERGE forces me to
specify "... WHEN NOT MATCHED THEN INSERT ..." as in follows:
"MERGE INTO A USING B ON (A.Name = B.Name)
WHEN MATCHED THEN UPDATE SET A.Id = B.Id
WHEN NOT MATCHED THEN INSERT ... /* Can I skip this? */"
Well, I just don't want to insert any new rows into TABLE A if
the ON condition is false.
Any expert out there please help me?
As a courtesy, in MSSQL, I can say
"UPDATE A SET A.Id = B.Id FROM A, B WHERE A.Name = B.Name
Nice and sweet and clean. Does Oracle have equivalent?
Thanks,
Matt

Answer 1

The problem is that your update isn't constrained so every row
gets updated. Try...
UPDATE A
SET A.Id = (SELECT B.Id FROM B WHERE A.Name = B.Name)
where exists (select * from b where a.name = b.name); Richard