sql count distinct

在SQL中，使用COUNT(DISTINCT 列名)可以统计某个列中不重复的唯一值的数量。例如，对于表格fromtable，可以使用以下查询语句统计userType为1和2的不重复userid的数量:

SELECT COUNT(DISTINCT case when userType=1 then userid else null end) as 类型1, COUNT(DISTINCT case when userType=2 then userid else null end) as 类型2 FROM fromtable

这个查询语句会返回两个列，分别是类型1和类型2对应的不重复userid的数量。<span class="em">1</span><span class="em">2</span><span class="em">3</span>
#### 引用[.reference_title]
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