mysql中count()语句的用法介绍

本篇内容主要讲解"mysql中count()语句的用法介绍"，感兴趣的朋友不妨来看看。本文介绍的方法操作简单快捷，实用性强。下面就让小编来带大家学习"mysql中count()语句的用法介绍"吧!

查看表结构：

mysql> show create table coupon_use_test \G*************************** 1. row ***************************       Table: coupon_use_testCreate Table: CREATE TABLE `coupon_use_test` (  `id` int(11) NOT NULL DEFAULT '0',  `user_id` varchar(40) CHARACTER SET utf8 COLLATE utf8_bin DEFAULT NULL,  `coupon_code` varchar(40) CHARACTER SET utf8 COLLATE utf8_bin NOT NULL DEFAULT '',  `status` varchar(2) CHARACTER SET utf8 COLLATE utf8_bin DEFAULT '00',  `use_time` datetime DEFAULT NULL,  `remark1` varchar(200) CHARACTER SET utf8 COLLATE utf8_bin DEFAULT NULL,  `remark2` varchar(200) CHARACTER SET utf8 COLLATE utf8_bin DEFAULT NULL,  `remark3` varchar(200) CHARACTER SET utf8 COLLATE utf8_bin DEFAULT NULL,  `create_time`  timestamp NULL DEFAULT CURRENT_TIMESTAMP,  `create_user_id` varchar(128) CHARACTER SET utf8 COLLATE utf8_bin DEFAULT NULL,  `update_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,  KEY `idx_create_time` (`create_time`)) ENGINE=InnoDB DEFAULT CHARSET=utf81 row in set (0.00 sec)

查看create_time字段为空的行数

mysql> select * from coupon_use_test where create_time is null;Empty set (0.00 sec)

把id为1的记录create_time改为空

mysql> update coupon_use_test set create_time = null where id = 1;Query OK, 1 row affected (6.56 sec)Rows matched: 1  Changed: 1  Warnings: 0mysql> select count(*) from coupon_use_test where create_time is null;+----------+| count(*) |+----------+|        1 |+----------+1 row in set (0.00 sec)

count(*)

mysql> select count(*) from coupon_use_test;+----------+| count(*) |+----------+|  1800000 |+----------+1 row in set (0.69 sec)mysql> explain select count(*) from coupon_use_test;+----+-------------+-----------------+------------+-------+---------------+-----------------+---------+------+---------+----------+-------------+| id | select_type | table           | partitions | type  | possible_keys | key             | key_len | ref  | rows    | filtered | Extra       |+----+-------------+-----------------+------------+-------+---------------+-----------------+---------+------+---------+----------+-------------+|  1 | SIMPLE      | coupon_use_test | NULL       | index | NULL          | idx_create_time | 5       | NULL | 1771323 |   100.00 | Using index |+----+-------------+-----------------+------------+-------+---------------+-----------------+---------+------+---------+----------+-------------+1 row in set, 1 warning (0.00 sec)

可以看到count(*)走了create_time字段的索引idx_create_time

count(1)

mysql> select count(1) from coupon_use_test;+----------+| count(1) |+----------+|  1800000 |+----------+1 row in set (0.63 sec)mysql> explain select count(1) from coupon_use_test;+----+-------------+-----------------+------------+-------+---------------+-----------------+---------+------+---------+----------+-------------+| id | select_type | table           | partitions | type  | possible_keys | key             | key_len | ref  | rows    | filtered | Extra       |+----+-------------+-----------------+------------+-------+---------------+-----------------+---------+------+---------+----------+-------------+|  1 | SIMPLE      | coupon_use_test | NULL       | index | NULL          | idx_create_time | 5       | NULL | 1771323 |   100.00 | Using index |+----+-------------+-----------------+------------+-------+---------------+-----------------+---------+------+---------+----------+-------------+1 row in set, 1 warning (0.00 sec)

count(create_time)

mysql> select count(create_time) from coupon_use_test;+--------------------+| count(create_time) |+--------------------+|            1799999 |+--------------------+1 row in set (0.73 sec)mysql> explain select count(create_time) from coupon_use_test;+----+-------------+-----------------+------------+-------+---------------+-----------------+---------+------+---------+----------+-------------+| id | select_type | table           | partitions | type  | possible_keys | key             | key_len | ref  | rows    | filtered | Extra       |+----+-------------+-----------------+------------+-------+---------------+-----------------+---------+------+---------+----------+-------------+|  1 | SIMPLE      | coupon_use_test | NULL       | index | NULL          | idx_create_time | 5       | NULL | 1771323 |   100.00 | Using index |+----+-------------+-----------------+------------+-------+---------------+-----------------+---------+------+---------+----------+-------------+1 row in set, 1 warning (0.00 sec)

count(*)和count(1)可以查询全表总行数，count(create_time)查询到的行数不包括null。

count(1) 与 count(*) 比较 :

1> 如果数据表没有主键，那么 count(1) 比 count(*) 快

2> 如果有主键的话，那主键 (联合主键) 作为 count条件也比 count(*) 要快

3> 如果你的表只有一个字段的话那 count(*) 就是最快

在不加 WHERE 限制条件的情况下，COUNT(*) 与 COUNT(COL) 基本可以认为是等价的，但是在有 WHERE 限制条件的情况下，COUNT(*) 会比 COUNT(COL) 快非常多

COUNT(*) 通常是对主键进行索引扫描，而COUNT(COL)就不一定了，另外前者是统计表中的所有符合的纪录总数，而后者是计算表中所有符合的COL的纪录数

count(*) 与 count(1) 两者比较，主要还是要取决于 count(1) 所相对应的数据字段，如果count(1)是聚索引 id 那肯定是count(1)快，但是差的很小，因为 count(*) 自动会优化指定到那一个字段，所以没必要去count(?)用count(*)sql会自动完成优化

1> 任何情况下 SELECT COUNT(*) FROM tablename 是最优选择

2> 尽量减少 SELECT COUNT(*) FROM tablename WHERE COL = 'value' 这种查询

3> 杜绝 SELECT COUNT(COL) FROM tablename 的出现

到此，相信大家对"mysql中count()语句的用法介绍"有了更深的了解，不妨来实际操作一番吧！这里是网站，更多相关内容可以进入相关频道进行查询，关注我们，继续学习！