有时我们想统计某种类型有多少个，会用这个SQL。全表扫描之余，还要filesort，耗时1.34秒。

mysql> select country,count(*) from t1 group by country;

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

| country | count(*) |

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

| NULL | 32 |

| africa | 524288 |

| america | 524288 |

| china | 524288 |

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

4 rows in set (1.34 sec)

mysql> desc select country,count(*) from t1 group by country;

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

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

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

| 1 | SIMPLE | t1 | ALL | NULL | NULL | NULL | NULL | 1573382 | Using temporary; Using filesort |

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

1 row in set (0.00 sec)

以下是两种优化方法，都是全表扫描，但估计count()要比sum()耗的CPU少点，方法更佳。

mysql> select count(country='africa' or null) as africa,count(country='america' or null) as america, count(country='china' or null) as china from t1;

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

| africa | america | china |

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

| 524288 | 524288 | 524288 |

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

1 row in set (0.78 sec)

mysql> desc select count(country='africa' or null) as africa,count(country='america' or null) as america, count(country='china' or null) as china from t1;

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

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

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

| 1 | SIMPLE | t1 | ALL | NULL | NULL | NULL | NULL | 1573382 | |

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

1 row in set (0.00 sec)

mysql> select sum(country='africa') as africa ,sum(country='america') as america,sum(country='china') from t1;

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

| africa | america | sum(country='china') |

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

| 524288 | 524288 | 524288 |

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

1 row in set (0.86 sec)

mysql> desc select sum(country='africa') as africa ,sum(country='america') as america,sum(country='china') from t1;

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

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

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

| 1 | SIMPLE | t1 | ALL | NULL | NULL | NULL | NULL | 1573382 | |

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

1 row in set (0.00 sec)

版本mysql5.5.30