前言

count函数是用来统计表中或数组中记录的一个函数，count(*) 它返回检索行的数目， 不论其是否包含 NULL值。最近感觉大家都在讨论count的区别，那么我也写下吧：欢迎留言讨论，话不多说了，来一起看看详细的介绍吧。

1、表结构：

dba_jingjing@3306>[rds_test]>CREATE TABLE `test_count` (

-> `c1` varchar(10) DEFAULT NULL,

-> `c2` varchar(10) DEFAULT NULL,

-> KEY `idx_c1` (`c1`)

-> ) ENGINE=InnoDB DEFAULT CHARSET=utf8;

Query OK, 0 rows affected (0.11 sec)

2、插入测试数据：

dba_jingjing@3306>[rds_test]>insert into test_count values(1,10);

Query OK, 1 row affected (0.03 sec)

dba_jingjing@3306>[rds_test]>insert into test_count values(abc,null);

ERROR 1054 (42S22): Unknown column 'abc' in 'field list'

dba_jingjing@3306>[rds_test]>insert into test_count values('abc',null);

Query OK, 1 row affected (0.04 sec)

dba_jingjing@3306>[rds_test]>insert into test_count values(null,null);

Query OK, 1 row affected (0.04 sec)

dba_jingjing@3306>[rds_test]>insert into test_count values('368rhf8fj',null);

Query OK, 1 row affected (0.03 sec)

dba_jingjing@3306>[rds_test]>select * from test_count;

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

| c1 | c2 |

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

| 1 | 10 |

| abc | NULL |

| NULL | NULL |

| 368rhf8fj | NULL |

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

4 rows in set (0.00 sec)

测试：

dba_jingjing@3306>[rds_test]>select count(*) from test_count;

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

| count(*) |

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

| 4 |

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

1 row in set (0.00 sec)

EXPLAIN: {

"query_block": {

"select_id": 1,

"message": "Select tables optimized away"

1 row in set, 1 warning (0.00 sec)

dba_jingjing@3306>[rds_test]>select count(1) from test_count;

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

| count(1) |

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

| 4 |

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

1 row in set (0.00 sec)

EXPLAIN: {

"query_block": {

"select_id": 1,

"message": "Select tables optimized away"

1 row in set, 1 warning (0.00 sec)

dba_jingjing@3306>[rds_test]>select count(c1) from test_count;

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

| count(c1) |

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

| 3 |

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

1 row in set (0.00 sec)

"table": {

"table_name": "test1",

"access_type": "index",

"key": "idx_c1",

"used_key_parts": [

"c1"

],

"key_length": "33",

那么这里面的"key_length": "33",为什么是33呢，什么是二级索引？见下节

count(*) 和count(1) 是没有区别的，而count(col) 是有区别的

执行计划有特点：可以看出它没有查询索引和表，有时候会出现select tables optimized away 不会查表，速度会很快

Extra有时候会显示“Select tables optimized away”，意思是没有更好的可优化的了。

官方解释For explains on simple count queries (i.e. explain select count(*) from people) the extra