How to insert 20 records only in one table

Question

hi to all,
i want insert 20 records only in a table,but suppose i want to enter 21th record it will not inserted .
suppose delete one record then insert, it will insert.
but always count(*) is not greater than 20.
is there any solution for that,
pls help me

Answer 1

Yes there is a solution for that using a materialied view:
SQL> select * from v$version;
BANNER
Oracle Database 10g Express Edition Release 10.2.0.1.0 - Product
PL/SQL Release 10.2.0.1.0 - Production
CORE    10.2.0.1.0      Production
TNS for 32-bit Windows: Version 10.2.0.1.0 - Production
NLSRTL Version 10.2.0.1.0 - Production
SQL> drop table t purge;
Table dropped.
SQL> drop materialized view t_mv;
Materialized view dropped.
SQL> create table t (
  2  x integer,
  3  y varchar2(30)
  4  );
Table created.
SQL> CREATE MATERIALIZED VIEW LOG on t
  2  WITH ROWID (x, y)
  3  including new values;
Materialized view log created.
SQL> CREATE MATERIALIZED VIEW t_mv
  2  REFRESH FAST ON COMMIT AS
  3  SELECT count(*) cnt
  4  FROM t;
Materialized view created.
SQL> alter table t_mv ADD CONSTRAINT chk check(cnt<=20);
Table altered.
SQL> insert into t select object_id, object_name from all_objects where rownum <
21;
20 rows created.
SQL> commit;
Commit complete.
SQL> insert into t values(21,'KO');
1 row created.
SQL> commit;
commit
ERROR at line 1:
ORA-12008: error in materialized view refresh path
ORA-02290: check constraint (TEST.CHK) violated
SQL>
SQL> select count(*) from t;
  COUNT(*)
        20
SQL>Not sure that trigger based solution works due to multi versioning read consistency.