mysql 使用union all时碰到的一个奇怪现象：

问题描述

写了一个稍微有一点复杂的sql：先根据where条件查询出结果集，然后union all结果集中的各个列的sum。简单来说，就是在结果的最后一行加上一个总和。只不过这一步是放在sql中执行，而不是程序
然后返回让两个union all 前后的内容交换就报错：

Illegal mix of collations (utf8mb4_0900_ai_ci,COERCIBLE) and (gbk_chinese_ci,IMPLICIT) for operation

原来能正确执行的sql：

SELECT gbk_office                                                               as office,leader_name   ,leader_id                                                                as leaderId,count(DISTINCT (opp.id))                                                 as sjzs,count(DISTINCT (case when foll.id is null then null else opp.id end))    as ygjsj,count(DISTINCT (foll.creator_id))                                        as gjrs,COUNT(foll.id)                                                            as gjjls,max(opp.last_follow_date)                                                as zhgjsj,COUNT(DISTINCT (case when foll.type = 1 then opp.id else null end))      as smbfs,COUNT(DISTINCT (case when opp.stage_id = 2 then opp.id else null end))    as xsjd,COUNT(DISTINCT (case when opp.stage_id = 3 then opp.id else null end))    as yxjd,COUNT(DISTINCT (case when opp.stage_id = 4 then opp.id else null end))     as bfjd,COUNT(DISTINCT (case when opp.stage_id = 5 then opp.id else null end))    as swjd,COUNT(DISTINCT (case when opp.stage_id = 6 then opp.id else null end))    as wcqysfrom channel_partner_opportunity oppLEFT JOIN channel_partner_opportunity_follow follon opp.id = foll.cpo_idGROUP BY gbk_office, leader_name
union allselect '合计'    as office,null        as leader_name,null        as leaderId,sum(sjzs)   as sjzs,sum(ygjsj)  as ygjsj,sum(gjrs)   as gjrs,sum(gjjls)  as gjjls,max(zhgjsj) as zhgjsj,sum(smbfs)  as smbfs,sum(xsjd)   as xsjd,sum(yxjd)   as yxjd,sum(bfjd)   as bfjd,sum(swjd)   as swjd,sum(wcqys)  as wcqysfrom (SELECT gbk_office                                                             as office,leader_name,leader_id                                                              as leaderId,count(DISTINCT (opp.id))                                               as sjzs,count(DISTINCT (case when foll.id is null then null else opp.id end))  as ygjsj,count(DISTINCT (foll.creator_id))                                      as gjrs,COUNT(foll.id)                                                         as gjjls,max(opp.last_follow_date)                                              as zhgjsj,COUNT(DISTINCT (case when foll.type = 1 then opp.id else null end))    as smbfs,COUNT(DISTINCT (case when opp.stage_id = 2 then opp.id else null end)) as xsjd,COUNT(DISTINCT (case when opp.stage_id = 3 then opp.id else null end)) as yxjd,COUNT(DISTINCT (case when opp.stage_id = 4 then opp.id else null end)) as bfjd,COUNT(DISTINCT (case when opp.stage_id = 5 then opp.id else null end)) as swjd,COUNT(DISTINCT (case when opp.stage_id = 6 then opp.id else null end)) as wcqysfrom channel_partner_opportunity oppLEFT JOIN channel_partner_opportunity_follow follon opp.id = foll.cpo_idGROUP BY gbk_office, leader_name) tableA

修改后的sql（仅交换了union all 前后的内容位置）：

select '合计'    as office,null        as leader_name,null        as leaderId,sum(sjzs)   as sjzs,sum(ygjsj)  as ygjsj,sum(gjrs)   as gjrs,sum(gjjls)  as gjjls,max(zhgjsj) as zhgjsj,sum(smbfs)  as smbfs,sum(xsjd)   as xsjd,sum(yxjd)   as yxjd,sum(bfjd)   as bfjd,sum(swjd)   as swjd,sum(wcqys)  as wcqysfrom (SELECT gbk_office                                                             as office,leader_name,leader_id                                                              as leaderId,count(DISTINCT (opp.id))                                               as sjzs,count(DISTINCT (case when foll.id is null then null else opp.id end))  as ygjsj,count(DISTINCT (foll.creator_id))                                      as gjrs,COUNT(foll.id)                                                         as gjjls,max(opp.last_follow_date)                                              as zhgjsj,COUNT(DISTINCT (case when foll.type = 1 then opp.id else null end))    as smbfs,COUNT(DISTINCT (case when opp.stage_id = 2 then opp.id else null end)) as xsjd,COUNT(DISTINCT (case when opp.stage_id = 3 then opp.id else null end)) as yxjd,COUNT(DISTINCT (case when opp.stage_id = 4 then opp.id else null end)) as bfjd,COUNT(DISTINCT (case when opp.stage_id = 5 then opp.id else null end)) as swjd,COUNT(DISTINCT (case when opp.stage_id = 6 then opp.id else null end)) as wcqysfrom channel_partner_opportunity oppLEFT JOIN channel_partner_opportunity_follow follon opp.id = foll.cpo_idGROUP BY gbk_office, leader_name) tableAunion allSELECT gbk_office                                                               as office,leader_name   ,leader_id                                                                as leaderId,count(DISTINCT (opp.id))                                                 as sjzs,count(DISTINCT (case when foll.id is null then null else opp.id end))    as ygjsj,count(DISTINCT (foll.creator_id))                                        as gjrs,COUNT(foll.id)                                                            as gjjls,max(opp.last_follow_date)                                                as zhgjsj,COUNT(DISTINCT (case when foll.type = 1 then opp.id else null end))      as smbfs,COUNT(DISTINCT (case when opp.stage_id = 2 then opp.id else null end))    as xsjd,COUNT(DISTINCT (case when opp.stage_id = 3 then opp.id else null end))    as yxjd,COUNT(DISTINCT (case when opp.stage_id = 4 then opp.id else null end))     as bfjd,COUNT(DISTINCT (case when opp.stage_id = 5 then opp.id else null end))    as swjd,COUNT(DISTINCT (case when opp.stage_id = 6 then opp.id else null end))    as wcqysfrom channel_partner_opportunity oppLEFT JOIN channel_partner_opportunity_follow follon opp.id = foll.cpo_idGROUP BY gbk_office, leader_name;

报错信息：

Illegal mix of collations (utf8mb4_0900_ai_ci,COERCIBLE) and (gbk_chinese_ci,IMPLICIT) for operation

原因分析：

有某列的字符集不一致导致

执行sql1：show full columns from 表名;
查出当前表的全部列信息

执行结果：
其中collation是字符集列。在出问题前，有一列的coolation不是utf8_general_ci，而是gbk_chinese_ci

执行sql2：SHOW COLLATION;

执行结果：
可以在其中搜到，发生问题的：gbk_chinese_ci 字符集是gbk。而其他列的：utf8_general_ci字符集utf8。两个是不一样的

因此，得出结论：是 gbk_offic 列的字符集与其他列的不同，导致的错误。

解决方案：

修改与其他列字符集不同的列，与其他列保持一致

新增：alter table 表名 add column 列名 varchar (50) character set utf8;
修改：alter table 表名 modify column 列名 varchar(50) character set utf8;

小tip：

因为是解决了问题后，才来记录的这个bug，所以可能有一些信息截图中与保存信息不一致的存在。

报错信息是不知道修改调试到哪一个版本进行的。所以可能和解决方案中描述的不太一样，但是不影响解决思路，
解决方案中，我先用了修改，然后报错：‘Changing the STORED status’ is not supported for generated columns. 所以我直接使用了新增，然后删除了这个旧列，然后重命名新列。这样一个方法，因为时间有限
之所以换位置就会报错。是因为老版本中，将字符集不同的gbk_office当做输出结果，新版本中：将null 作为 offIce的结果，去关联字符集不同的列。故而报错

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mysql使用union all时字符集不一致