exercise bug fixed

data/2.MySQL中阶/5.子查询/4.EXISTS/exists.md

@@ -40,21 +40,13 @@ where exists(select * from employee as i where o.dept = i.dept and post='assista
 ```
 
 ### B
-
-```sql
-select id, name, dept
-from employee as o
-where not exists(select * from employee as i where o.dept = i.dept and post='assistant'); 
-```
-
-### C
 ```sql
 select id, name, dept 
 from employee as o
 where 'assistant' != exists(select post from employee as i); 
 ```
 
-### D
+### C
 
 ```sql
 select id, name, dept