Question

I have a to write a insert trigger on a tableA. which will perform update with same table but different column. I am getting error while doing this. My trigger is

create or replace trigger trigger_A
after insert on table_A
begin
  update table_A set col1=1 where col1 is null;
end;

I have an application will perform col2 alone will be inserted and col1 will be kept null. so my trigger will give value for col1 once the row is inserted. But i am getting error saying “Trigger is failed and invalid” when a row is inserted.
How to do this. TIA.

Answer 1

If you want to assign a simple default value, the easiest way is to declare it on the table, using the DEFAULT clause.

SQL> create table t42
  2    ( col1 number default 1 not null
  3      , col2 date)
  4  /

Table created.

SQL> insert into t42 (col2) values (sysdate)
  2  /

1 row created.

SQL> select * from t42
  2  /

      COL1 COL2
---------- ---------
         1 03-AUG-11

SQL>

This works with literals or pseudocolumns such as SYSDATE or USER. If you want to derive a more complicated value with a user-defined function or a sequence, you will need to use
a trigger.

Here is a new version of the table…

SQL> create table t42
  2    ( col1 number default 1 not null
  3      , col2 date default sysdate
  4      , col3 varchar2(30) default user
  5      , col4 number )
  6  /

Table created.

SQL>

… with a trigger:

SQL> create or replace trigger t42_trg
  2      before insert or update
  3      on t42
  4      for each row
  5  begin
  6      if :new.col4 is null
  7      then
  8          :new.col4 := my_seq.nextval;
  9      end if;
 10  end;
 11  /

Trigger created.

SQL> insert into t42 (col1, col2, col3)
  2  values (99, sysdate, 'MR KNOX')
  3  /

1 row created.

SQL> select * from t42
  2  /

      COL1 COL2      COL3                                 COL4
---------- --------- ------------------------------ ----------
        99 03-AUG-11 MR KNOX                               161

SQL>

Note that although every column on the table is defaultable, I have to populate at least one column to make the SQL valid:

SQL> insert into t42 values ()
  2  /
insert into t42 values ()
                        *
ERROR at line 1:
ORA-00936: missing expression


SQL>

But I can pass in NULL to COL4 to get a completely defaulted record:

SQL> insert into t42 (col4) values (null)
  2  /

1 row created.

SQL> select * from t42
  2  /

      COL1 COL2      COL3                                 COL4
---------- --------- ------------------------------ ----------
        99 03-AUG-11 MR KNOX                               161
         1 03-AUG-11 APC                                   162

SQL>

---

Caveat lector: my trigger uses the new 11g syntax. In previous versions we have to assign the sequence value using a SELECT statement:

select my_seq.nextval
into :new.col4
from dual;