单项选择题
In which scenario would an index be most useful?（）
A.The indexed column is declared as NOT NULL.
B.The indexed columns are used in the FROM clause.
C.The indexed columns are part of an expression.
D.The indexed column contains a wide range of values.
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单项选择题
Click the Exhibit button to examine the data of the EMPLOYEES table. Evaluate this SQL statement: SELECT e.employee_id "Emp_id", e.emp_name "Employee", e.salary, m.employee_id "Mgr_id", m.emp_name "Manager" FROM employees e JOIN employees m ON (e.mgr_id = m.employee_id) AND e.salary > 4000; What is its output?（）
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