这种说法是错的：”MySql 中是不能用 null 作索引的，任何包含null值的列都将不会被包含在索引中“

我们做个实验如下，可以看到b字段建有索引，b 字段中有 null，但是

select count(*) from a group by b

用了b上的覆盖索引。 如果索引里不包含null的话，只用覆盖索引是 不能满足要求的(无法计算出b中null的个数)。

mysql> show create table a;

+-------+-------------------------------------------------------------------------------------------

----------------------------------------+

| Table | Create Table

|

+-------+-------------------------------------------------------------------------------------------

----------------------------------------+

| a | CREATE TABLE `a` (

`a` int(11) DEFAULT NULL,

`b` int(11) DEFAULT NULL,

KEY `k` (`b`)

) ENGINE=InnoDB DEFAULT CHARSET=latin1 |

+-------+-------------------------------------------------------------------------------------------

----------------------------------------+

1 row in set (0.03 sec)

mysql> select * from a;

+------+------+

| a | b |

+------+------+

| 1 | NULL |

| 2 | NULL |

| 2 | 23 |

| 2 | 45 |

| 2 | 4 |

| 2 | 43 |

+------+------+

6 rows in set (0.00 sec)

mysql> explain select count(*) from a group by b;

+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+

| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |

+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+

| 1 | SIMPLE | a | index | NULL | k | 5 | NULL | 6 | Using index |

+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+

1 row in set (0.00 sec)

再来："任何在where子句中使用is null或is not null的语句优化器是不允许使用索引的。"

mysql> show index from t3;

+-------+------------+----------+--------------+-------------+-----------+-------------+----------+-

-------+------+------------+---------+---------------+

| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part |

Packed | Null | Index_type | Comment | Index_comment |

+-------+------------+----------+--------------+-------------+-----------+-------------+----------+-

-------+------+------------+---------+---------------+

| t3 | 1 | isd | 1 | uid | A | 200 | NULL |

NULL | YES | BTREE | | |

+-------+------------+----------+--------------+-------------+-----------+-------------+----------+-

-------+------+------------+---------+---------------+

1 row in set (0.00 sec)

mysql> select count(*) from t3 where uid is null;

+----------+

| count(*) |

+----------+

| 3009 |

+----------+

1 row in set (0.03 sec)

mysql> explain select count(*) from t3 where uid is null;

+----+-------------+-------+------+---------------+------+---------+-------+------+-----------------

---------+

| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra

|

+----+-------------+-------+------+---------------+------+---------+-------+------+-----------------

---------+

| 1 | SIMPLE | t3 | ref | isd | isd | 5 | const | 3008 | Using where; Usi

ng index |

+----+-------------+-------+------+---------------+------+---------+-------+------+-----------------

---------+

1 row in set (0.00 sec)