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<>1.使用 DISTINCT 关键词
在表中，可能会包含重复值。这并不成问题，不过，有时您也许希望仅仅列出不同（distinct）的值。
关键词 DISTINCT 用于返回唯一不同的值。
<>语法：
SELECT DISTINCT 列名称 FROM 表名称
其中的一种使用方式：
select t.a A,count(distinct t.b) B from table_name t group by t.a
<>2.使用row_number() over (partition by col1 order by col2)
row_number() over (partition by col1 order by col2)
表示根据col1分组，在分组内部根据col2排序，而此函数计算的值就表示每组内部排序后的顺序编号（组内连续的唯一的)
select t.* from ( select a.*, row_number() over(partition by a.col1 order by
a.col2 desc) rn from table_name a) t where t.rn = 1
思路：
通过col1字段分组后再以col2字段组内降序排列并添加序号，然后再查询出组内序号等于1的数据实现去重操作。
<>3.使用group by
可以使用group by根据指定字段进行分组
select col1 from group by col1
