What is the purpose of the subquery at the end of a multitable insert?

Question

I am learning about multitable inserts and am confused about the required subquery at the end of the insert statement. Take the following for example:
INSERT ALL
INTO t1 VALUES('value1')
INTO t1 VALUES('value2')
SELECT 1 FROM DUAL;
What is the subquery "SELECT 1 FROM DUAL" for? The subquery must be there; and it can be any select statement, not just the simple one I have here. Does the subquery simply act as a termination for the inserts?

Answer 1

>
INSERT ALL
INTO t1 VALUES('value1')
INTO t1 VALUES('value2')
SELECT 1 FROM DUAL;
What is the subquery "SELECT 1 FROM DUAL" for?It is there for returning a single row from DUAL. Oracle evaluates the rows returned by the subquery and executes each of those "INTO" clauses once for each row returned by the subquery.
Thus, each of the "INTO ... VALUES" clause is evaluated once for that 1 row returned by DUAL.
Alternatively, you could write just one "INTO...VALUES" clause and return multiple rows in the subquery, thusly:
SQL>
SQL> drop table t1;
Table dropped.
SQL> create table t1 (x varchar2(10));
Table created.
SQL>
insert all
into t1 values (x)
select 'value1' as x from dual union all
select 'value2'      from dual union all
select 'value3'      from dual;
3 rows created.
SQL>
SQL> select * from t1;
X
value1
value2
value3
SQL>
SQL>
... Does the subquery simply act as a termination for the inserts?Well, no. The semi-colon character (";") or a forward slash character ("/") on the first column of the last line acts as the SQL terminator.
The subquery is there because Oracle needs some data to evaluate, process as per the conditions and insert into (one or more) tables.
Check the link to the documentation where it is explained in detail:
http://download.oracle.com/docs/cd/B19306_01/server.102/b14200/statements_9014.htm#SQLRF01604
isotope