Question on Updating the rows of a table using a SELECT Statement

Question

Hi,
I am trying to set all of the R_IDs in one table to the ID from another table using a join.
Here is the SQL I came up with. I don't this is correct though.
UPDATE TABLEA A
SET R_ID=(Select Id from TABLEB B
WHERE A.DEE=B.DEE AND
B.RNUM=1);
Does this make sense?

Answer 1

You can try the join view.
UPDATE ( SELECT table_a.id   AS a_id,
                table_b.id   AS b_id,
                table_a.col1 AS a_col1,
                table_b.col1 AS b_col1,
                table_a.col2 AS a_col2,
                table_b.col2 AS b_col2,
                table_a.col3 AS a_col3,
                table_b.col3 AS b_col3
           FROM table_a,
                table_b
          WHERE table_a.id = table_b.id)
   SET a_col1 = b_col1,
       a_col2 = b_col2,
       a_col3 = b_col3;
However it may result in
ORA-01779: cannot modify a column which maps to a non key-preserved tableThis is because the key-preserving property of a table does not depend on the actual data in the table. It is, rather, a property of its schema. For example, if in table_b there was at most one record for each id value, then table_a.id would be unique in the result of a join of table_a and table_b, but table_a would still not be a key-preserved table. By adding the unique/primary key on the id column of table_b we can assure that if table_b.id were part of the result set of the join view then it would be unique. This makes table_a key preserved.
Thus, in order to update a join view we must assure that the columns involved in the join view from the source table are unique. We do this by creating a unique or primary key on those columns.
However, if you do not mind taking responsibility for making sure that there is one-to-one cardinality between the source and destination rows, then you might try the correlated subquery
UPDATE table_a
   SET (col1, col2, col3 ) =
          ( SELECT col1, col2, col3
              FROM table_b
             WHERE table_b.id = table_a.id )
       WHERE EXISTS
           ( SELECT col1, col2, col3
               FROM table_b
               WHERE table_b.id = table_a.id )