The error message will include the name of the constraint that was violated (there may be more than one unique constraint on a table). You can use that constraint name to identify the column(s) that the unique constraint is declared on
SELECT column_name, position
FROM all_cons_columns
WHERE constraint_name = <<name of constraint from the error message>>
AND owner = <<owner of the table>>
AND table_name = <<name of the table>>
Once you know what column(s) are affected, you can compare the data you're trying to INSERT
or UPDATE
against the data already in the table to determine why the constraint is being violated.