# Update with Joins
# Examples: what works and what doesn't
create table tgt ( id, val ) as select 1, 'a' from dual union all select 2, 'b' from dual ; Table TGT created. create table src ( id, val ) as select 1, 'x' from dual union all select 2, 'y' from dual ; Table SRC created. update ( select t.val as t_val, s.val as s_val from tgt t inner join src s on t.id = s.id ) set t_val = s_val ; SQL Error: ORA-01779: cannot modify a column which maps to a non key-preserved table 01779. 00000 - "cannot modify a column which maps to a non key-preserved table" *Cause: An attempt was made to insert or update columns of a join view which map to a non-key-preserved table. *Action: Modify the underlying base tables directly.
Imagine what would happen if we had the value
1 in the column
src.id more than once, with different values for
src.val. Obviously, the update would make no sense (in ANY database - that's a logical issue). Now, we know that there are no duplicates in
src.id, but the Oracle engine doesn't know that - so it's complaining. Perhaps this is why so many practitioners believe Oracle "doesn't have UPDATE with joins"?
What Oracle expects is that
src.id should be unique, and that it, Oracle, would know that beforehand. Easily fixed! Note that the same works with composite keys (on more than one column), if the matching for the update needs to use more than one column. In practice,
src.id may be PK and
tgt.id may be FK pointing to this PK, but that is not relevant for updates with join; what is relevant is the unique constraint.
alter table src add constraint src_uc unique (id); Table SRC altered. update ( select t.val as t_val, s.val as s_val from tgt t inner join src s on t.id = s.id ) set t_val = s_val ; 2 rows updated. select * from tgt; ID VAL -- --- 1 x 2 y
The same result could be achieved with a MERGE statement (which deserves its own Documentation article), and I personally prefer MERGE in these cases, but the reason is not that "Oracle doesn't do updates with joins." As this example shows, Oracle does do updates with joins.