88 lines
3.4 KiB
Markdown
88 lines
3.4 KiB
Markdown
---
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title: 一条SQL查询今年每月统计信息
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author: 查尔斯
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date: 2021/11/22 18:22
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categories:
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- 方案春秋志
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tags:
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- SQL
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---
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# 一条SQL查询今年每月统计信息
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## 前言
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**C：** 前不久，笔者介绍过一种统计近 N 天记录数的需求解决方案。今天，笔者再介绍一种也很常见的统计需求。
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::: info 示例需求： 统计今年每月的注册用户数。
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你可以基于这个示例需求，去完成各种类似的月统计需求。而且啊，笔者今天这个需求解决方案的重点是在 SQL 上，这类需求问题在 SQL 语句笔试上也挺常见，所以下回再见到类似的需求，你可以好好回想下本篇实现。
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:::
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## 涉及技术栈
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- Spring Boot 2.3.1.RELEASE
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- MyBatis Plus 3.1.0（使用了 MyBatis Plus 的代码生成器）
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- MySQL 5.6
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## Controller层
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略
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## Service层
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略
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## DAO层
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::: warning 笔者说
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记得要采用 LinkedHashMap，这样可以保证结果集的有序，即：1月、2月、......。
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:::
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```java
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@Select({
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"SELECT",
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"SUM(CASE MONTH(`create_time`) WHEN '1' THEN 1 ELSE 0 END) AS `1月`,",
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"SUM(CASE MONTH(`create_time`) WHEN '2' THEN 1 ELSE 0 END) AS `2月`,",
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"SUM(CASE MONTH(`create_time`) WHEN '3' THEN 1 ELSE 0 END) AS `3月`,",
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"SUM(CASE MONTH(`create_time`) WHEN '4' THEN 1 ELSE 0 END) AS `4月`,",
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"SUM(CASE MONTH(`create_time`) WHEN '5' THEN 1 ELSE 0 END) AS `5月`,",
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"SUM(CASE MONTH(`create_time`) WHEN '6' THEN 1 ELSE 0 END) AS `6月`,",
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"SUM(CASE MONTH(`create_time`) WHEN '7' THEN 1 ELSE 0 END) AS `7月`,",
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"SUM(CASE MONTH(`create_time`) WHEN '8' THEN 1 ELSE 0 END) AS `8月`,",
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"SUM(CASE MONTH(`create_time`) WHEN '9' THEN 1 ELSE 0 END) AS `9月`,",
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"SUM(CASE MONTH(`create_time`) WHEN '10' THEN 1 ELSE 0 END) AS `10月`,",
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"SUM(CASE MONTH(`create_time`) WHEN '11' THEN 1 ELSE 0 END) AS `11月`,",
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"SUM(CASE MONTH(`create_time`) WHEN '12' THEN 1 ELSE 0 END) AS `12月`,",
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"FROM `t_user`",
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"WHERE YEAR(`create_time`)= YEAR(NOW())"
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})
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LinkedHashMap<String, Integer> countRegisterByMonth();
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```
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## SQL语句
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::: tip 笔者说
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这条 SQL 的思路就是将每条记录的 create_time（创建时间）求一下月份信息，求出的月份如果是对应的月份，那么就记为 1，否则记为 0，这样每月最后再做个 SUM 求和，就可以快速得到对应月份的记录数量了，不用 COUNT 依然可以计数。
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SQL语句单独放在下面，方便各位同学复制。:smile:
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:::
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```sql
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# 统计今年每月的注册用户数
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SELECT
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SUM(CASE MONTH(`create_time`) WHEN '1' THEN 1 ELSE 0 END) AS `1月`,
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SUM(CASE MONTH(`create_time`) WHEN '2' THEN 1 ELSE 0 END) AS `2月`,
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SUM(CASE MONTH(`create_time`) WHEN '3' THEN 1 ELSE 0 END) AS `3月`,
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SUM(CASE MONTH(`create_time`) WHEN '4' THEN 1 ELSE 0 END) AS `4月`,
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SUM(CASE MONTH(`create_time`) WHEN '5' THEN 1 ELSE 0 END) AS `5月`,
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SUM(CASE MONTH(`create_time`) WHEN '6' THEN 1 ELSE 0 END) AS `6月`,
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SUM(CASE MONTH(`create_time`) WHEN '7' THEN 1 ELSE 0 END) AS `7月`,
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SUM(CASE MONTH(`create_time`) WHEN '8' THEN 1 ELSE 0 END) AS `8月`,
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SUM(CASE MONTH(`create_time`) WHEN '9' THEN 1 ELSE 0 END) AS `9月`,
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SUM(CASE MONTH(`create_time`) WHEN '10' THEN 1 ELSE 0 END) AS `10月`,
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SUM(CASE MONTH(`create_time`) WHEN '11' THEN 1 ELSE 0 END) AS `11月`,
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SUM(CASE MONTH(`create_time`) WHEN '12' THEN 1 ELSE 0 END) AS `12月`
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FROM `t_user` # 根据自身需要确定实际业务表
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WHERE YEAR(`create_time`)= YEAR(NOW());
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```
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