Examine the data of the EMPLOYEES table. EMPLOYEES (EMPLOYEE_ID is the primary key. MGR_ID is the ID of managers and refers to the EMPLOYEE_ID) 
Evaluate this SQL statement: 
What is its output?（）
- A、
- B、
- C、
- D、
- E、 The SQL statement produces an error.
正确答案:B
答案解析:有
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