mysql查询问题：“like”未按预期工作

我试图选择列“cats”不包含字符串“people and activities”的行。字符串“cats”可以是、NULL或非空值

select ref from images where (url like 'parks%')  and ((not (cats like '%people-and-activities%')));
10634 rows in set

先前的答复是不正确的。这个似乎是正确的

 select ref,caption,cats from images where (url like 'parks%')  and  (cats like '%people-and-activities%') ; 
 647 rows in set 

我不明白的是，数字加起来是不正确的：

 select ref,caption,cats from images where (url like 'parks%');
 13600 rows in set

 select ref,caption,cats from images where (url like 'parks%')  and ((not (cats like '%people-and-activities%')) or (cats like '%people-and-activities%')) ;
 11281 rows in set

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有人能给我解释一下这个谜吗？谢谢

猫是一个可空字段吗？数据中有空字段