mysql数据库max 函数的作用是什么
发布时间：2022-01-18 13:29:48 所属栏目：编程 来源：互联网
导读：本篇内容介绍了mysql数据库max()函数的作用是什么的有关知识，在实际案例的操作过程中，不少人都会遇到这样的困境，接下来就让小编带领大家学习一下如何处理这些情况吧！希望大家仔细阅读，能够学有所成！ 查看表结构： mysql show create table coupon_use_
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本篇内容介绍了“mysql数据库max()函数的作用是什么”的有关知识，在实际案例的操作过程中，不少人都会遇到这样的困境，接下来就让小编带领大家学习一下如何处理这些情况吧！希望大家仔细阅读，能够学有所成！ 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 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=utf8 1 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) “mysql数据库max()函数的作用是什么”的内容就介绍到这里了，感谢大家的阅读。 （编辑：广西网） 【声明】本站内容均来自网络，其相关言论仅代表作者个人观点，不代表本站立场。若无意侵犯到您的权利，请及时与联系站长删除相关内容! |
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