Hive count(*) count(1) count(c1) 区别

1 count(c1) 不统计c1=NULL的记录，但统计c1='' 和 c1=' ' 空格的记录！

2 count(*) = count(1) = count(10000) = count(c1) + count(*) where c1 is null

3 count(数字)，这个数字可以设置任意的正整数，不影响结果。

          select count(1)
            from (
            
            select 1 as c1 
            
            union all
            select '' as c1
                 
            union all
            select null as c1
            ) t1  -- 3 行
 
 
            select count(1)
            from (
            
            select 1 as c1 
            
            union all
            select '' as c1
                       
            union all
            select ' ' as c1 -- 空格
            
            union all
            select null as c1
            ) t1  -- 4 行
 
--------------
 
 
            select count(*)
            from (
            
            select 1 as c1 
            
            union all
            select '' as c1
                 
            union all
            select null as c1
            ) t1  -- 3 行
 
 
            select count(*)
            from (
            
            select 1 as c1 
            
            union all
            select '' as c1
                       
            union all
            select ' ' as c1 -- 空格
            
            union all
            select null as c1
            ) t1  -- 4 行
 
--------------
 
      select count(c1)
            from (
            
            select 1 as c1 
            
            union all
            select '' as c1
                 
            union all
            select null as c1
            ) t1  -- 2行
 
 
            select count(c1)
            from (
            
            select 1 as c1 
            
            union all
            select '' as c1
                       
            union all
            select ' ' as c1 -- 空格
            
            union all
            select null as c1
            ) t1  -- 3行
 
--------------
 

 
Hive2.0.0 版本

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