平常开发中，咱们写sql语句时可能会使用到count()聚合函数，可是经常使用的count(*),count(1),count(字段)之间有何区别呢，对应用的影响如何？mysql
- 测试用例
- 分析
- 总结
测试用的表结构：
+----------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra | +----------+--------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment | | name | varchar(128) | NO | | NULL | | | price | double | NO | | NULL | | | sellerId | bigint(20) | NO | | NULL | | | menu_url | varchar(128) | NO | | | |
| detail | varchar(128) | NO | | | | +----------+--------------+------+-----+---------+----------------+
执行的语句：
mysql> explain select count(*) from t_menu where price = 2011; +----+-------------+--------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | t_menu | ALL | NULL | NULL | NULL | NULL | 2 | Using where | +----+-------------+--------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)
mysql> explain select count(*) from t_menu; +----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
| 1 | SIMPLE | t_menu | index | NULL | PRIMARY | 4 | NULL | 2 | Using index | +----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)
mysql> explain select count(id) from t_menu; +----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
| 1 | SIMPLE | t_menu | index | NULL | PRIMARY | 4 | NULL | 2 | Using index | +----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)
mysql> explain select count(id) from t_menu where price = 2011; +----+-------------+--------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | t_menu | ALL | NULL | NULL | NULL | NULL | 2 | Using where | +----+-------------+--------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)
mysql> explain select count(1) from t_menu; +----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
| 1 | SIMPLE | t_menu | index | NULL | PRIMARY | 4 | NULL | 2 | Using index | +----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)
mysql> explain select count(1) from t_menu where price = 2011; +----+-------------+--------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | t_menu | ALL | NULL | NULL | NULL | NULL | 2 | Using where | +----+-------------+--------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)
mysql> explain select count(price) from t_menu where price = 2011; +----+-------------+--------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | t_menu | ALL | NULL | NULL | NULL | NULL | 2 | Using where | +----+-------------+--------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)
mysql> explain select count(price) from t_menu; +----+-------------+--------+------+---------------+------+---------+------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+------+---------------+------+---------+------+------+-------+
| 1 | SIMPLE | t_menu | ALL | NULL | NULL | NULL | NULL | 2 | | +----+-------------+--------+------+---------------+------+---------+------+------+-------+
1 row in set (0.00 sec)
在sql语句前加“explain”，能够显示执行计划。
能够发现，使用count()聚合函数后，如有where条件，且where条件的字段未创建索引，则查询不会走索引，直接扫描了全表。
因为表内数据较少，没法得出比较准确的效率比较。
根据资料所查：
有主见或联合主键的状况下，count(*)略比count(1)快一些。
没有主键的状况下count(1)比count(*)快一些。
若是表只有一个字段，则count(*)是最快的。
当在给表添加几个索引后，count(*)有时候会走到其余索引，能够说明，mysql系统会自动优化到一个合适的索引项。web
其次，为什么说count(主键) 不必定比count(其他索引) 快？
索引是一种B+树的结构，以块为单位进行存储。假设块大小是1K，主键索引大小为4B，有一个字段A的索引大小为2B。
一样一个块，能存放256个主键索引，可是能存放512个字段A的索引。
假设总数据是2K条，意味着主键索引占用了8个块，而A字段索引占用了4个块，统计时用主键索引须要经历的块多，IO次数多。效率也比A字段索引慢。sql
1.若是在开发中确实须要用到count()聚合，那么优先考虑count(*)，由于mysql数据库自己对于count(*)作了特别的优化处理。
2.使用count()聚合函数后，最好不要跟where age = 1；这样的条件，会致使不走索引，下降查询效率。除非该字段已经创建了索引。
3.count(字段),非主键字段，这样的使用方式最好不要出现。由于它不会走索引。数据库