I was explaining to a developer today how Oracle doesn’t support auto-increment columns for IDs and how you have to implement your own. I wanted to point him to a tutorial but I found that most of the tutorials have PL/SQL code with unexplained side effects. So I thought I’d post some Oracle PL/SQL code that behaves just like a MySQL or PostgreSQL auto-increment column, but which also fixes the sequence number when an application provides its own ID number for a record, which is one place where MySQL and PostgreSQL autoincrement columns fail.

Step 1: Know the names of your tablespaces. Know which tablespace you want to use for your tables and which tablespace you want to use for your indexes. Don’t assume that Oracle is going to put the table in the right place. To get a list of available tablespaces:

select tablespace_name from user_tablespaces;

Step 2: Create your table. In this example the table is built in the tablespace TDATA and the primary key index is built in the tablespace INDX.

create table example (
    id number not null,
    name varchar2(30),
    constraint example_pk primary key(id) using index tablespace INDX
) tablespace TDATA;

Note that by declaring the id column “not null” you can’t add a record with an empty ID field, ever.

Creating the primary key constraint on a separate line allows us to name the constraint, which can be handy when you’re looking through the data dictionary later on trying to figure out which constraint does what.

Step 3: Create a sequence. In this case I create an ID sequence that starts at 1 and goes on from there. If you want to start your ID sequence with a different number, or increment by some other amount, change these values.

create sequence example_id_seq start with 1 increment by 1;

Step 4: Create a trigger. This is what does the actual work setting the ID number. There are many ways to do this, here are two:

Method 1: Use a sequence-generated ID for all IDs.

create or replace trigger example_insert
before insert on example
for each row
begin
    select example_id_seq.nextval into :new.id from dual;
end;
/

Method 1 will always use a sequence-generated ID no matter what. For example:

SQL> insert into example (id, name) values (100, 'Earl');
1 row created.
SQL> insert into example (name) values ('Cleopatra');
1 row created.
SQL> insert into example (name) values ('Frankenstein');
1 row created.
SQL> select * from example;
        ID NAME
---------- ------------------------------
         1 Earl
         2 Cleopatra
         3 Frankenstein

In this case the trigger-supplied ID 1 overwrote the user-supplied id 100.

Method 2: Allow users to supply their own IDs when they want to.

create or replace trigger example_insert
before insert on example
for each row
declare
    max_id number;
    cur_seq number;
begin
    if :new.id is null then
        -- No ID passed, get one from the sequence
        select example_id_seq.nextval into :new.id from dual;
    else
        -- ID was set via insert, so update the sequence
        select greatest(nvl(max(id),0), :new.id) into max_id from example;
        select example_id_seq.nextval into cur_seq from dual;
        while cur_seq < max_id
        loop
            select example_id_seq.nextval into cur_seq from dual;
        end loop;
    end if;
end;
/

Method 2 will use the sequence-generated ID if the insert statement doesn’t supply an ID, which is what MySQL and PostgreSQL do. It also updates the sequence so that the next value of the sequence won’t collide with IDs supplied by an insert statement.

For example, let’s say I’m using Trigger Method 2 and just loaded the table with a bunch of old data, including old ID numbers.

SQL> delete from example;
3 rows deleted.
SQL> insert into example (id, name) values (200, 'Cleopatra');
1 row created.
SQL> insert into example (id, name) values (300, 'Frankenstein');
1 row created.
SQL> insert into example (name) values ('Earl');
1 row created.
SQL> select * from example;

        ID NAME
---------- ------------------------------
       200 Cleopatra
       300 Frankenstein
       301 Earl

The last record added gets an ID of 301.

Hope you find this useful.