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A. displaying the total order value for sales representatives 161 and 163
B. displaying the order total for sales representative 161 in the year 1999
C. displaying the number of orders that have order mode online and order date in 1999
D. displaying the number of orders whose order total is more than the average order total for all online orders

A. When a table is dropped， the corresponding indexes are automatically dropped.
B. For each DML operation performed， the corresponding indexes are automatically updated.
C. Indexes should be created on columns that are frequently referenced as part of an expression.
D. A non-deferrable PRIMARY KEY or UNIQUE KEY constraint in a table automatically creates a uniqueindex.