Consider the index information for the dept_emp table in the employee’s schema:

Which two conclusions can be made based on the output of the query?
A. There are three indexes on the table.
B. There is a redundant index on the dept_no column.
C. The secondary indexes are optimized for unique key look-ups.
D. The values on the emp_no column must be unique.
E. The selectivity of the dept_no column is the best of the indexed columns.
F. There is a redundant index on the emp_no column.
Answer: AF

下面哪两个结论是正确的？
A． 表上有3个索引
B． Dept_no字段上存在一个冗余索引
C． 辅助索引针对唯一键查询存在优化
D． Emp_no字段必须是唯一的
E． Dept_no字段的选择性是最好的
F． Emp_no字段上存在一个冗余索引http://www.cndba.cn/hbhe0316/article/22783http://www.cndba.cn/hbhe0316/article/22783

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MYSQL