Multirow considerations are particularly important when the function of a trigger is to recalculate summary values, or provide ongoing tallies.
Triggers used to maintain summary values should contain group by clauses or subqueries that perform implicit grouping. This creates summary values when more than one row is being inserted, updated, or deleted. Since a group by clause imposes extra overhead, the following examples are written to test whether @@rowcount = 1, meaning that only one row in the trigger table was affected. If @@rowcount = 1, the trigger actions take effect without a group by clause.
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