mysql数据库max()函数的作用是什么

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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 NOT 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) ENGINE=InnoDB DEFAULT CHARSET=utf81 row in set (0.00 sec)

测试查询速度：

mysql> select max(create_time) from coupon_use_test;+---------------------+| max(create_time)    |+---------------------+| 2016-06-25 16:44:25 |+---------------------+1 row in set (2.01 sec)

查看执行计划：

mysql> explain select max(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       | ALL  | NULL          | NULL | NULL    | NULL | 1706101 |   100.00 | NULL  |+----+-------------+-----------------+------------+------+---------------+------+---------+------+---------+----------+-------+1 row in set, 1 warning (0.00 sec)

创建create_time字段索引

mysql> alter table coupon_use_test add index idx_create_time(create_time);Query OK, 0 rows affected (17.49 sec)Records: 0  Duplicates: 0  Warnings: 0

再次查询：

mysql> select max(create_time) from coupon_use_test;+---------------------+| max(create_time)    |+---------------------+| 2016-06-25 16:44:25 |+---------------------+1 row in set (0.00 sec)

查看执行计划：

mysql> explain select max(create_time) from coupon_use_test;+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra                        |+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+|  1 | SIMPLE      | NULL  | NULL       | NULL | NULL          | NULL | NULL    | NULL | NULL |     NULL | Select tables optimized away |+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+1 row in set, 1 warning (0.00 sec)

索引是有序的，create_time字段加完索引之后取max（create_time）速度变快。

看到其他优化方法，通过转变SQL查询方式实现

mysql> select create_time from coupon_use_test order by create_time desc limit 1;+---------------------+| create_time         |+---------------------+| 2016-06-25 16:44:25 |+---------------------+1 row in set (0.00 sec)

查看执行计划：

mysql> explain select create_time from coupon_use_test order by create_time desc limit 1;+----+-------------+-----------------+------------+-------+---------------+-----------------+---------+------+------+----------+-------------+| 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 | 4       | NULL |    1 |   100.00 | Using index |+----+-------------+-----------------+------------+-------+---------------+-----------------+---------+------+------+----------+-------------+1 row in set, 1 warning (0.01 sec)

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