Oracle SQL

Hello Albert,

generally this the explanation for that error :

ORA-12015: cannot create a fast refresh materialized view from a complex query
Cause: Neither ROWIDs and nor primary key constraints are supported for complex queries.
Action: Reissue the command with the REFRESH FORCE or REFRESH COMPLETE option or create a simple
materialized view.

also to Diagnosing ORA-12015 fast refresh materialized view / complex queries that’s depend on the database version there is some restrictions ex:

  Oracle10g – 10.2
——————–

Fast refreshable materialized views must be based on master tables, master materialized views, or synonyms of master tables
or master materialized views. Complete refresh must be used for a materialized view based on a view.

for further details about diagnosing refer to : Note.179466.1

hope this helpful.

You can use a subquery for this like

[code]
select *
from
( select *
from emp
order by sal desc )
where ROWNUM <= 5;
[/code]

Starting from Oracle 12c R1 (12.1). there is a syntax available to limit rows or start at offsets (full syntax here)

example :

[code]
SELECT *
FROM employees
ORDER BY salary
OFFSET 20 ROWS FETCH NEXT 10 ROWS ONLY;
[/code]

hope this helpful 🙂

Hello Beter,

Views in Oracle may be updateable under specific conditions. It can be tricky, and usually is not advisable.

From the Oracle 10g SQL Reference:

Notes on Updatable Views

An updatable view is one you can use to insert, update, or delete base table rows. You can create a view to be inherently updatable, or you can create an INSTEAD OF trigger on any view to make it updatable.

To learn whether and in what ways the columns of an inherently updatable view can be modified, query the USER_UPDATABLE_COLUMNS data dictionary view. The information displayed by this view is meaningful only for inherently updatable views. For a view to be inherently updatable, the following conditions must be met:

• Each column in the view must map to a column of a single table. For example, if a view column maps to the output of a TABLE clause (an unnested collection), then the view is not inherently updatable.
• The view must not contain any of the following constructs:

1. A set operator
2. a DISTINCT operator
3. An aggregate or analytic function
4. A GROUP BY, ORDER BY, MODEL, CONNECT BY, or START WITH clause
5. A collection expression in a SELECT list
6. A subquery in a SELECT list
7. A subquery designated WITH READ ONLY
8. Joins, with some exceptions, as documented in Oracle Database Administrator’s Guide

In addition, if an inherently updatable view contains pseudocolumns or expressions, then you cannot update base table rows with an UPDATE statement that refers to any of these pseudocolumns or expressions.

If you want a join view to be updatable, then all of the following conditions must be true:

• The DML statement must affect only one table underlying the join.
• For an INSERT statement, the view must not be created WITH CHECK OPTION, and all columns into which values are inserted must come from a key-preserved table. A key-preserved table is one for which every primary key or unique key value in the base table is also unique in the join view.
• For an UPDATE statement, all columns updated must be extracted from a key-preserved table. If the view was created WITH CHECK OPTION, then join columns and columns taken from tables that are referenced more than once in the view must be shielded from UPDATE.
• For a DELETE statement, if the join results in more than one key-preserved table, then Oracle Database deletes from the first table named in the FROM clause, whether or not the view was created WITH CHECK OPTION.

There is no way to rollback the generated sequence.

To restart the sequence at a different number, you must drop and re-create it. See http://docs.oracle.com/cd/B19306_01/server.102/b14200/statements_2011.htm.

If you change the INCREMENT BY value before the first invocation of NEXTVAL, some sequence numbers will be skipped. Therefore, if you want to retain the original START WITH value, you must drop the sequence and re-create it with the original START WITH value and the new INCREMENT BY value.