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！
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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。如果想了解更多行业相关的知识可以关注创新互联-成都网站建设公司网站，小编将为大家输出更多高质量的实用文章！
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