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SQL> select * from dual;
DUMMY
-----
X
SQL>
SQL> select * from dual;
D
-
X
SQL> select count(*) from dual;
COUNT(*)
----------
1
SQL> insert into dual values('h'
1 row created.
SQL> select * from dual;
D
-
X
SQL> select count(*) from dual;
COUNT(*)
----------
1
SQL>
1.条件不足,没有答案。这个SQL等同于
select * from a where a.column1 not is null
2.2
3.问题都错了。