Rerunning script (DML and DDL) without ora-00001 and ora-00955 errors

Question

What is the best way to write a DML and DDL script so that it can be run multiple times without these ORA errors:
ORA-00955: name is already used by an existing object
ORA-00001: unique constraint (JILL.SYS_C00160247) violated
I have just joined a product development company using SQL Server as there primary database. They have just completed a port to Oracle.
Their product release upgrades (given to clients) include sql scripts with database changes (structure and data). They require that the client be able to rerun the scripts more than once with no errors. In SQL Server, the accomplish it this way.
For DDL:
if not exists (select * from dbo.sysobjects where id = object_id(N'[dbo].[MyTab]') and OBJECTPROPERTY(id, N'IsUserTable') = 1)
Begin
     CREATE TABLE [dbo].[MyTab] (
          [ID]int IDENTITY(1,1),
          [InvID] uniqueidentifier not null
For DML:
IF NOT EXISTS (SELECT 1 FROM [dbo].[mytab] WHERE [Name] = 'Smith' and [ID] = 3)
BEGIN
INSERT INTO [dbo].[mytab]
([ID] ,[Name]
VALUES (3,'Smith')
END
I am tasked with duplicating this logic on the Oracle side. The only way I can think of so far is using plsql and checking for existence before every insert and create statement. The other options I thought of cannot be used in this case
- "whenever sqlerror continue" - gives the same response for all errors. True errors should stop the code, so this is too risky.
- "log errors into ... reject limit unlimited" on the insert - I thought this was my best solution until I found out that it doesn't support lobs.
Do you know of any more elegant (and more efficient) solution other than plsql cursors to check for existence before running each insert/create?
Any suggestions would be greatly appreciated.

Answer 1

select table_name from user_tables will give you the table exist or not.
all_tables/dba_tables
http://download.oracle.com/docs/cd/B14117_01/server.101/b10755/statviews_1190.htm#i1592091