You can write a trigger which queries the table that forced the trigger to fire. So that part is not a problem. You get a Mutating table error when you attempt to modify the table that caused the trigger to fire. What happens is a change (insert/update/delete) occurs on that table. So a trigger gets fired. In that trigger, you perform a change on that table. The trigger will want to fire again. But the system detects that the change is caused by the trigger itself, not something else. This is called a mutating table error. So don't modify the table in the table's trigger and you won't see this error.
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This was first published in April 2002