Checking NULL in multi column Update

Question

I have a Update query that updates 2 columns based on the value from other table. Now there may not be a corresponding record in 2nd table and so my first table can be updated with NULL. Now how do i perform a NVL check for this
Query
Update Table1
Set (col2,col3) =
( select col2, col3
from table2
where table2.col1 = table1.col1)
At present i have to repeat the entire condition in SET clause into where clause to avoid this. Is there any better way ?

Answer 1

Eric:
The restriction is that each row in table1 must match with only one row in table2 (or none in which case it will be eliminated from the join and, therefore, not updated). Many rows in table1 can match with a single row in table2.
SQL> CREATE TABLE t (id NUMBER, descr VARCHAR2(10));
Table created.
SQL> INSERT INTO t VALUES (1, 'One');
1 row created.
SQL> INSERT INTO t VALUES (1, 'Un');
1 row created.
SQL> INSERT INTO t VALUES (2, 'Two');
1 row created.
SQL> COMMIT;
Commit complete.
SQL> CREATE TABLE t1 (id NUMBER, descr VARCHAR2(10));
Table created.
SQL> ALTER TABLE t1 ADD CONSTRAINT t1_pk PRIMARY KEY (id);
Table altered.
SQL> INSERT INTO t1 VALUES(1, 'Uno');
1 row created.
SQL> INSERT INTO t1 VALUES (3, 'Tres');
1 row created.
SQL> COMMIT;
Commit complete.
SQL> UPDATE (SELECT t.id, t.descr, t1.descr t1descr
  2          FROM t, t1
  3          WHERE t.id = t1.id)
  4  SET descr = t1descr;
2 rows updated.
SQL> SELECT * FROM t;
        ID DESCR
         1 Uno
         1 Uno
         2 TwoThe uniqueness of table2 must be declared, it is not determined at runtime,
SQL> ROLLBACK;
Rollback complete.
SQL> ALTER TABLE t1 DROP CONSTRAINT t1_pk;
Table altered.
SQL> UPDATE (SELECT t.id, t.descr, t1.descr t1descr
  2          FROM t, t1
  3          WHERE t.id = t1.id)
  4  SET descr = t1descr;
SET descr = t1descr
ERROR at line 4:
ORA-01779: cannot modify a column which maps to a non key-preserved tableJohn