解法1:使用group by去重
SELECT salary FROM salaries WHERE to_date='9999-01-01' GROUP BY salary ORDER BY salary DESC;
解法2:使用distinct去重
SELECT distinct salary FROM salaries WHERE to_date='9999-01-01' ORDER BY salary DESC;
SELECT salary FROM salaries WHERE to_date='9999-01-01' GROUP BY salary ORDER BY salary DESC;
SELECT distinct salary FROM salaries WHERE to_date='9999-01-01' ORDER BY salary DESC;