sum value in before oracle trigger - oracle

Question

I have a table : Table1 (field1, field2, field3).
I want to validate values of updating.
If Sum(field1) group by field2 > 10 then raise error.
CREATE OR REPLACE TRIGGER HDB_TSGH_REVISE
BEFORE UPDATE OF field1
ON Table1
FOR EACH ROW
DECLARE
v_sum_amt NUMBER := 0;
BEGIN
SELECT SUM(field1)
INTO v_sum_amt
FROM Table1
WHERE field2 = 'VND';
IF v_sum_amt > 10 THEN
RAISE_APPLICATION_ERROR(-20000, 'ERROR');
END IF;
END;
Error 4091 at:
SELECT SUM(field1)
INTO v_sum_amt
FROM Table1
WHERE field2 = 'VND';
Please help me

Answer 1

It is caused because of ORA-04091: table name is mutating, trigger/function may not see it
As per the suggestion given, try using AFTER UPDATE trigger instead of BEFORE UPDATE
Since your case is that, you should not update the value if error, maybe you can re-update to the old value in case of error in the after update trigger.
You can also consider using AUTONOMOUS TRANSACTION.

Answer 2

You cannot select from the same table you are manipulating in a row level trigger.
Do NOT use an Autonomous Transaction to get round this error. Although an Autonomous Transaction would cause the error to disappear, the error indicates a fundamental mistake with your methodology and so this needs to change.
I would suggest something like:
CREATE OR REPLACE TRIGGER hdb_tsgh_revise
FOR INSERT OR UPDATE OR DELETE ON table1
COMPOUND TRIGGER
-- Flag to indicate if VND total has been increased
g_vnd_total_increased BOOLEAN;
BEFORE STATEMENT
IS
BEGIN
-- Reset VND total flag
g_vnd_total_increased := FALSE;
END BEFORE STATEMENT;
AFTER EACH ROW
IS
BEGIN
-- If inserting VND record with positive value,
-- Or updating to a VND record with a positive value,
-- Or updating from a VND record with a negative value,
-- Or updating a VND record to a larger value,
-- Or deleting VND record with negative value;
-- Then set the VND total flag
IF ( ( INSERTING
AND :new.field2 = 'VND'
AND :new.field1 > 0)
OR ( UPDATING
AND ( ( nvl(:new.field2, '?') <> nvl(:old.field2, '?')
AND ( ( :new.field2 = 'VND'
AND :new.field1 > 0)
OR ( :old.field2 = 'VND'
AND :old.filed1 < 0)))
OR ( :new.field2 = 'VND'
AND :old.field2 = 'VND'
AND nvl(:new.field1, 0) > nvl(:old.field1, 0)) ) )
OR ( DELETING
AND :old.field2 = 'VND'
AND :old.field1 < 0) )
THEN
g_vnd_total_increased := TRUE;
END IF;
END AFTER EACH ROW;
AFTER STATEMENT
IS
l_sum_field1 NUMBER;
BEGIN
-- If VND total has been increased;
-- Serialise the constraint so concurrent transactions do not affect each other
-- Ensure VND total does not exceed allowed value
IF g_vnd_total_increased THEN
dbms_lock.request
(id => 12345 -- A unique id for this constraint
,lockmode => dbms_lock.x_mode
,release_on_commit => TRUE);
SELECT sum(field1)
INTO l_sum_field1
FROM table1
WHERE field1 = 'VND';
IF l_sum_field1 > 10 THEN
raise_application_error(-20001, 'Field1 total for VND greater than 10 in Table1');
END IF;
END IF;
END AFTER STATEMENT;
END;