PL/SQL :: Materialized View - Update Query Without Drop And Recreate

i have a little doubt in Materialized view. i created a materialized view and log with the following query,

create table test_mv (a number ,b number )
alter table test_mv add constraint t_pk primary key ( a );
insert into test_mv values (1,2);

[Code]...

Now i want to update the query in the MV as 'Select a from test_mv' . for that i tried with

*'ALTER MATERIALIZED VIEW product_sales_mv AS SELECT a from test_mv;'*

But it throwing error,

Error starting at line 5 in command:

alter  MATERIALIZED VIEW product_sales_mv   AS SELECT  b  from test_mv

Error report:

SQL Error: ORA-00933: SQL command not properly ended +00933. 00000 - "SQL command not properly ended"+

*Cause:+   
*Action:+

i guess i am doing wrong. i want to update it without drop and recreate