mysql count(*),count(n),count(filed)区别

count(*),count(1),count(filed)区别

• 参考链接-高性能MySQL——Count(1) OR Count(*)

• 表结构

tb_user2 CREATE TABLE `tb_user2` (
  `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT COMMENT 'id',
  `name` varchar(30) COLLATE utf8mb4_unicode_ci DEFAULT NULL,
  `age` int(11) DEFAULT NULL,
  `created_on` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci COMMENT='用户测试2'

• 记录数据

+----+------+------+------------+
| id | name | age  | created_on |
+----+------+------+------------+
|  1 | a    |   10 |       NULL |
|  2 | b    | NULL |       NULL |
|  3 | c    | NULL |       NULL |
+----+------+------+------------+

count(*)与count（n）的区别

select count(*),count(1),count(2) from tb_user2;
+----------+----------+----------+
| count(*) | count(1) | count(2) |
+----------+----------+----------+
|        3 |        3 |        3 |
+----------+----------+----------+

explain select count(*),count(1),count(2) from tb_user2;
+------------+-------+---------------+---------+---------+-
| table    | partitions | type  | possible_keys | key     |
+----+-------------+----------+------------+-------+-------
| tb_user2 | NULL       | index | NULL          | PRIMARY |
+----+-------------+----------+------------+-------+------

• 结论

表中只有一个主键，则采用主键来统计行数,count(*)和count(n)没有什么区别

给tb_user2添加二级索引在次统计

alter table tb_user2 add index idx_age (age);

explain select count(*),count(1),count(2) from tb_user2;
+------------+-------+---------------+---------+---------+-
| table    | partitions | type  | possible_keys | key     |
+----+-------------+----------+------------+-------+-------
| tb_user2 | NULL       | index | NULL          | idx_age |
+----+-------------+----------+------------+-------+------

• 结论

如果表中有二级索引，则采用占用空间最小的二级索引， 因为二级索引比主键索引(聚簇索引))占用的空间要小。

count(field)

select count(age) from tb_user2;
+------------+
| count(age) |
+------------+
|          1 |
+------------+

• 结论

count(field)则统计非空字段的数据记录数

结论

• count(*)和count(1),count(n)

1、都是统计数据的行数
2、如果表中只有一个主键索引，没有二级索引，那么则通过主键索引来统计行数，如果表中有二级索引，则采用占用空间最小的二级索引，来统计行数

• count(filed)

统计非空字段的数据行数