Update with an Outer Join

Question

I want to update a column in a table by selecting a value from one of three tables (in a particular order) based on whether the key value in the table being updated is in one (or more) of those tables.
In other words:
There are four tables
The main table, main_table, has columns "key" and "title"; all rows have a key value but their title values are null
The three column lookup tables, table_1, table_2, and table_3; each also has columns "key" and "title", but in this case, each row has its key and title values not null
Each main_table's title is set as follows:
If the key value is in table_1, use the title from tabke_1
If the key value is not in table_1 but it is in table_2, use the title from table_2
If the key value is not in table_1 or table_2 but it is in table_3, use the title from table_3
If the key value is not in any of the three other tables, set it to null
I am also trying not to use the "(+)" terminology.
Is there any way of writing a single UPDATE statement that does this?
I can't find any way of doing it off the top of my head without using three separate UPDATE statements (one using table_1, one using table_2, and one using table_3, with the latter two limited to "main_table.title is null".
-- Don

Answer 1

This?
SQL> select * from main_table;
       KEY TITLE
         1
         2
         3
         4
         5
         6
6 rows selected
SQL>
SQL> update main_table mt set title =
2 (select title from
3 (
4 select v1.key, v1.title from
5 (
6 select key, title, 1 sortcol from table_1 union
7 select key, title, 2 sortcol from table_2 union
8 select key, title, 3 sortcol from table_3
9 ) v1
10 inner join
11 (
12 select key, min(sortcol) sortcol from
13 (
14 select key, 1 sortcol from table_1 union
15 select key, 2 sortcol from table_2 union
16 select key, 3 sortcol from table_3
17 )
18 group by key
19 ) v2
20 on v1.key=v2.key and v1.sortcol = v2.sortcol
21 )
22 where key = mt.key);
6 rows updated
SQL> select * from main_table;
       KEY TITLE
         1 t11
         2 t12
         3 t23
         4 t24
         5
         6 t36
6 rows selectedAnd the content of table_1, table_2, table_3 is:
SQL> select * from table_1;
       KEY TITLE
         1 t11
         2 t12
SQL> select * from table_2;
       KEY TITLE
         3 t23
         4 t24
         1 t21
SQL> select * from table_3;
       KEY TITLE
         6 t36
         4 t34