After and before____oracle in triggers in Oracle

1. What's the difference between after and before in a trigger in Oracle?
One is triggered before the record operation, and one is triggered after the record operation.

2. What occasions are used separately
For example, the definition between the table has a foreign key, delete the primary key, you must first delete the foreign key table, then there are successively points.

3, there is also a statement-level trigger and row-level trigger what is the difference.
If defined as the statement level, when the delete from T executes, the trigger runs only once, and if it is defined as a row level, the above delete operation causes the stylus to run multiple times (several records are run a few times).

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Inserting action: New
Update operation: New,:old, if updating a field triggers the use of If updating (' col ') then ...
Delete operation: Old