MySQL部分查询写法

1.根据a列进行分组，然后取b列最新的一条数据，b列是时间类型
使用子查询和GROUP BY

SELECT yt.*  
FROM your_table yt  
INNER JOIN (  
    SELECT a, MAX(b) AS latest_b  
    FROM your_table  
    GROUP BY a  
) AS latest_times ON yt.a = latest_times.a AND yt.b = latest_times.latest_b;

2.查看group_concat返回值的最大长度，默认1024

SHOW VARIABLES LIKE "group_concat_max_len";

2.1修改group_concat返回值的最大长度，永久生效，需要重启
在mysqld下加入

group_concat_max_len = 2048

2.2修改group_concat返回值的最大长度，临时修改，重启失效

SET GLOBAL group_concat_max_len=2048; 
SET SESSION group_concat_max_len=2048; 

3.MySQL + mybatis plus批量插入配置

spring:
  datasource:
    url: jdbc:mysql://ip:3306/demo?useUnicode=true&characterEncoding=utf-8&rewriteBatchedStatements=true&allowMultiQueries=true
    username: root
    password: 'xxx'

不加insert-strategy: ignored会导致批量插入失败

mybatis-plus:
 global-config:
   db-config:
     insert-strategy: ignored

4.有一张表，有四个字段，医院，就诊，处方，是否合理，一个医院有多个就诊，每个就诊有多个处方，每个处方可能合理可能不合理，现在要按医院维度进行统计有多少个就诊合理，如果一个就诊既有合理，又有不合理，那就算不合理，这种三级关联统计的查询需要使用子表来实现，举例

SELECT
    hospital,
    SUM(is_reasonable_visit) AS reasonable_visits_count
FROM (
    SELECT
        hop.hsp_id AS hospital,
        hop.visit,
        CASE
            WHEN SUM(CASE WHEN hopd.is_reasonable = 0 THEN 1 ELSE 0 END) > 0 THEN 0
            ELSE 1
        END AS is_reasonable_visit
    FROM
        his_outpatient_prescription hop
    INNER JOIN his_outpatient_prescription_detail hopd ON hop.prescription_number = hopd.prescription_number
        AND hop.hsp_id = hopd.hsp_id
    GROUP BY
        hop.hsp_id,
        hop.visit
) AS visit_reasonability
GROUP BY
    hospital;