在mysql中有一张表，内容如下： mysql> select * from t1; +------+ | c1 | +------+ | 5 | | NULL | | 5 | | NULL | | 5 | +------+ 5 rows in set (0.00 sec) 请给出以下sql语句的结果 553 mysql> select avg(c1) as avg_num from t1; mysql> select count(*) from t1; mysql> select count(c1) from t1;