Oracle SQL template to create re-usable DDL/DML Scripts for Oracle database

Question

Hi,
I have a requirement to put together a Oracle SQL template to create re-usable DDL/DML Scripts for Oracle databases.
Only the Oracle DBA will be running the scripts so permissions is not an issue.
The workflow for any DDL is as follows:-
1) New Table
a. Check if the table exists from the system/admin views.
b. If table exists then give message "Table Exists"
c. If table does not exist then execute DDL code
2) Add Column
a. Check if Column exists for a given table from system/admin views
b. If column exists in the specified table,
b1. backup table.
b2. alter table to make changes to the column
b3. verify data or execute dml script convert from backup to the new change.
c. If Column does not exist
c1. backup table
c2. alter table to add column
c3. execute dml to populate column with default value.
The DML scripts are for populating base tables with data required for business operations.
3) Add new row
a. check if row exists by comparing old values of each column with new values to be added for the new record.
b. If exists, give message row exists
c. If not exists, add new record.
4) Update existing record (We have createtime columns in these tables so changes can be tracked)
a. check if row exists using primary key.
b. If exists,
b1. deactivate the record using the "active" column of the table
b2. Add new record with the changes required.
c. If does not exist, add new record with the changes required.
Could you please help with some ideas which can get this done accurately?
I have tried several ways, but I am not able to put together something that fulfills all requirements.
Thank you,

Answer 1

First let me address your question. (This is the easy part.)
1. The existence of tables can be found in DBA_TABLES. Query it and and then use conditional logic and execute immediate to process the DDL.
2. The existence of table columns is found in DBA_TAB_COLUMNS. Query it and then conditionally execute your DDL. You can copy the "before picture" of the table using that same dba view, or even better, use DBMS_METADATA.
As for your DML scripts, they should be restartable, reversible, and re-run-able. They should "fail gracefully" on error, be written in such a way that they can run twice in a row without creating duplicate changes.
3. Adding appropriate constraints can prevent invalid duplicate rows. Also, you can usually add to the where clause so that the DML does only what it needs to do without even relying on the constraint (but the constraint is there as a safeguard). Look up the MERGE statement to learn how to do an UPSERT (update/insert), which will let you conditionally "deactivate" (update) or insert a record. Anything that you cannot do in SQL can be done with simple procedural code.
Now, to the heart of the matter...
You think I did not understand your requirements?
Please be respectful of people's comments. Many of us are professionals with decades of experience working with databases and Oracle technology. We volunteer our valuable time and knowledge here for free. It is extremely common for someone to post what they feel is an easy SQL or PL/SQL question without stating the real goal--the business objective. Experienced people will spot that the "wrong question" has been asked, and then cut to the chase.
We have some good questions for you. Not questions we need answers from, but questions you need to ask yourself and your team. You need to reexamine this post and deduce what those questions are. But I'll give you some hints: Why do you need to do what you are asking? And will this construct you are asking for even solve the root cause of your problems?
Then ponder the following quotations about asking the right question:
Good questions outrank easy answers.
— Paul Samuelson
The only interesting answers are those which destroy the questions.
— Susan Sontag
The scientific mind does not so much provide the right answers as ask the right questions.
— Claude Levi-Strauss
You can tell whether a man is clever by his answers. You can tell whether a man is wise by his questions.
— Mahfouz Naguib
One hears only those questions for which one is able to find answers.
— Friedrich Nietzsche
Be patient towards all that is unresolved in your heart and try to love the questions themselves.
— Rainer Maria Rilke
What people think of as the moment of discovery is really the discovery of the question.
— Jonas Salk
Judge a man by his questions rather than his answers.
— Voltaire
The ability to ask the right question is more than half the battle of finding the answer.
— Thomas J. Watson