Triggers are Apex code working in concert with the Force.com database engine, automatically invoked by Force.com when database records are modified. Trigger code can perform any necessary processing on the modified data before or after Force.com completes its own work. The following list describes scenarios commonly implemented with triggers:
A validation rule is required that is too complex to define on the database object using formula expressions.
Two objects must be kept synchronized. When a record in one object is updated, ...
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