Help in writing update

Question

Please help me in writing this update statement
UPDATE fro set ckey =
(SELECT ckey FROM dcn WHERE EXISTS
(select 1 from dro, fro
where dro.rok = fro.rok and
dcn.gcn = dro.gcn
This returns ERROR at line 2:
ORA-01427: single-row subquery returns more than one row
The table structures are as follows:
In table dcn, the column ckey is primary key. GCN is also unique in this table.
In table dro, the column rok is primary key
fro     dro     dcn
rok     rok     ckey
ckey     gcn     gcn
Thanks
Acchu

Answer 1

I think it should be something like:
UPDATE fro a
SET    ckey = ( select ckey
                from   dcn b,
                       dro c
                where  c.rok = a.rok and
                       b.gcn = c.gcn )
WHERE EXISTS ( select  1
                from   dcn b,
                       dro c
                where  c.rok = a.rok and
                       b.gcn = c.gcn )You can try to drop WHERE EXISTS.... clause for better performance if there are no triggers FOR UPDATE on table fro. Since current value in column fro.ckey is null, it will be updated to null for all unmatched records, which will have no impact on data.