Autopopulating a column with a Trigger SQL - sql

Question

I'm trying to start a sequence that automatically populates a column when an insertion has been made. It should start from 500 and increment by 1. Any idea how would I go about doing this? So far I have something like this, but it seems to crash
CREATE TRIGGER ADD_TRIGGER ON tablename
AFTER INSERT
AS
BEGIN
ADD colname NUMBER IDENTITY(500,1);
END
GO

Answer 1

You can create a sequence
CREATE SEQUENCE mySeq
START WITH 500
INCREMENT BY 1
CACHE 100;
and then use it in your trigger
CREATE OR REPLACE TRIGGER myTrigger
AFTER INSERT ON table_name
FOR EACH ROW
BEGIN
SELECT mySeq.nextval
INTO :new.colname
FROM dual;
END;

Answer 2

Oracle 12c introduces IDENTITY COLUMNS.
SQL> CREATE TABLE new_identity_table
2 (
3 ID NUMBER GENERATED ALWAYS AS IDENTITY,
4 text VARCHAR2(50)
5 );
Table created.
SQL>
SQL> INSERT
2 INTO new_identity_table
3 (
4 text
5 )
6 VALUES
7 (
8 'This table has an identity column'
9 );
1 row created.
SQL> column text format A40;
SQL>
SQL> select * from new_identity_table;
ID TEXT
---------- ----------------------------------------
1 This table has an identity column
Oracle creates a sequence to populate the identity column. You can find it named as ISEQ$$
SQL> select sequence_name, min_value, max_value, increment_by from user_sequences;
SEQUENCE_NAME MIN_VALUE MAX_VALUE INCREMENT_BY
-------------------- ---------- ---------------------------- ------------
ISEQ$$_93199 1 9999999999999999999999999999 1
More more information about the identity columns, use the ALL_TAB_IDENTITY_COLS view.
SQL> SELECT table_name,
2 column_name,
3 generation_type,
4 identity_options
5 FROM all_tab_identity_cols
6 WHERE owner = 'LALIT'
7 ORDER BY 1, 2;
TABLE_NAME COLUMN_NAME GENERATION IDENTITY_OPTIONS
-------------------- --------------- ---------- --------------------------------------------------
NEW_IDENTITY_TABLE ID ALWAYS START WITH: 1, INCREMENT BY: 1, MAX_VALUE: 9999999
999999999999999999999, MIN_VALUE: 1, CYCLE_FLAG: N
, CACHE_SIZE: 20, ORDER_FLAG: N
For pre 12c releases, see Auto-increment primary key in Pre 12c releases (Identity functionality)