If you try to change or select the data of the same table on which the trigger is firing , then the the table is said to be mutating. this is because you are trying to alter the data in the table which is undergoing a change.
A related issue is that you cannot access the key columns in a table with a defined primary/foreign key relationship to the mutating table. This is called a constraining table.
Thus if your trigger in table "child" has a FK of "ID", you cannot query the related table "parent" by "ID" to update some field in that table, since the "ID" is the related field.
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