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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。下面就让丸趣 TV 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_test
Create 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=utf8
1 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: 0
mysql 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！这里是丸趣 TV 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！
