How to CREATE TABLE AS adding an identity? - oracle

Question

In such way I can create table from a table adding a column number:
drop table A_TEST
/
CREATE TABLE A_TEST AS
SELECT CAST( null as NUMBER ) as ROW_ID,
C_CODE,B_CODE
FROM A
However I want to add the column as an identity how to do that ? I tried the below, but throwing an error:
CREATE TABLE A_TEST AS
SELECT CAST( null as NUMBER GENERATED BY DEFAULT AS IDENTITY ) as ROW_ID,
C_CODE,B_CODE
FROM A

Answer 1

You can not create the table using CTAS including the IDENTITY column.
But, You can simply create a table without an identity column using CTAS and then ALTER the table to include IDENTITY columns as following:
CREATE TABLE A_TEST
AS
SELECT
C_CODE,
B_CODE
FROM
A;
ALTER TABLE A_TEST ADD ROW_ID NUMBER
GENERATED BY DEFAULT AS IDENTITY;
Cheers!!

Answer 2

Do it in multiple steps: create the table from the other table without data; then alter the table to add the identity column; and finally insert the data.
Oracle Setup:
CREATE TABLE A ( A_CODE, B_CODE, C_CODE ) AS
SELECT 999, 'BBB', SYSDATE FROM DUAL UNION ALL
SELECT 0, NULL, DATE '1970-01-01' FROM DUAL;
Create Table:
Create the table without the IDENTITY column and with no rows:
CREATE TABLE A_TEST AS
SELECT C_CODE, B_CODE
FROM A
WHERE 1 = 0;
Then alter the table to add the IDENTITY column:
ALTER TABLE A_TEST ADD (
ROW_ID NUMBER
GENERATED ALWAYS AS IDENTITY
CONSTRAINT A_TEST__ROW_ID__PK PRIMARY KEY
);
Then insert the rows:
INSERT INTO A_TEST ( C_CODE, B_CODE )
SELECT C_CODE, B_CODE FROM A;
(Or you can create the table and insert the rows in the first step; and alter the table to add the identity column without a NOT NULL/PRIMARY KEY constraint in the second step; and, if you want to add a NOT NULL/PRIMARY KEY constraint afterwards then it must be done in a separate subsequent ALTER TABLE statement. db<>fiddle)
Output:
SELECT * FROM A_TEST;
C_CODE | B_CODE | ROW_ID
:------------------ | :----- | -----:
2019-12-19 09:06:27 | BBB | 1
1970-01-01 00:00:00 | null | 2
db<>fiddle here