在尝试隔离下面第一个查询中的7行时，作为一个新手，我得到了出乎意料的结果。我仔细阅读了此SQLite document，但不明白为什么在第一个查询中，7行NULL在GROUP BY中被分隔，但在!= ''、not in ('A','H')和=''上进行的测试都排除了NULL行。
看起来好像这些测试是互斥的，例如NULL不是=''就是!=''，或者是in ('A','H')就是not in ('A','H')。所有这些测试似乎都忽略了NULL，但它在GROUP BY中被分隔开。
你能解释一下为什么会这样吗？
谢谢你。

sqlite> select substr(trim(grammarCode),1,1) as c, count(indexRow) as cnt
        from tbl
        group by c
        order by cnt;
c  cnt   
-  ------
   7     
A  4828  
   20046 
H  300679

sqlite> select substr(trim(grammarCode),1,1) as c, count(indexRow) as cnt
        from tbl
        where c != ''
        group by c
        order by cnt;
c  cnt   
-  ------
A  4828  
H  300679

sqlite> select substr(trim(grammarCode),1,1) as c, count(indexRow) as cnt
        from tbl
        where c not in ('A', 'H')
        group by c
        order by cnt;
c  cnt   
-  ------
   20046 

sqlite> select substr(trim(grammarCode),1,1) as c, count(indexRow) as cnt
        from tbl
        where c = ''
        group by c
        order by cnt;
c  cnt   
-  ------
   20046 

sqlite> select substr(trim(grammarCode),1,1) as c, count(indexRow) as cnt
        from tbl
        where c is null
        group by c
        order by cnt;
c  cnt   
-  ------
   7