一、count(1) and count(*)
当表的数据量大些时,对表做分析以后,使用count(1)还要比使用count()用时多了!
从执行计划来看,count(1)和count()的效果是同样的。可是在表作过度析以后,count(1)会比count(*)的用时少些(1w之内数据量),不过差不了多少。mysql
若是count(1)是聚索引,id,那确定是count(1)快,可是差的很小的。
由于count(),自动会优化指定到那一个字段。因此不必去count(1),用count(),sql会帮你完成优化的,所以:count(1)和count(*)基本没有差异!web
二、count(1) and count(字段)
二者的主要区别是
count(1) 会统计表中的全部的记录数,包含字段为null 的记录。
count(字段) 会统计该字段在表中出现的次数,忽略字段为null 的状况。即不统计字段为null 的记录。sql
count(*) 和 count(1)和count(列名)区别
执行效果上:svg
执行效率上:优化
mysql> create table counttest(name char(1), age char(2)); Query OK, 0 rows affected (0.03 sec) mysql> insert into counttest values -> ('a', '14'),('a', '15'), ('a', '15'), -> ('b', NULL), ('b', '16'), -> ('c', '17'), -> ('d', null), ->('e', ''); Query OK, 8 rows affected (0.01 sec) Records: 8 Duplicates: 0 Warnings: 0 mysql> select * from counttest; +------+------+ | name | age | +------+------+ | a | 14 | | a | 15 | | a | 15 | | b | NULL | | b | 16 | | c | 17 | | d | NULL | | e | | +------+------+ 8 rows in set (0.00 sec) mysql> select name, count(name), count(1), count(*), count(age), count(distinct(age)) -> from counttest -> group by name; +------+-------------+----------+----------+------------+----------------------+ | name | count(name) | count(1) | count(*) | count(age) | count(distinct(age)) | +------+-------------+----------+----------+------------+----------------------+ | a | 3 | 3 | 3 | 3 | 2 | | b | 2 | 2 | 2 | 1 | 1 | | c | 1 | 1 | 1 | 1 | 1 | | d | 1 | 1 | 1 | 0 | 0 | | e | 1 | 1 | 1 | 1 | 1 | +------+-------------+----------+----------+------------+----------------------+ 5 rows in set (0.00 sec)