Illegal mix of collations (utf8mb4_unicode_ci,IMPLICIT) and (utf8mb4_general_ci,IMPLICIT) for operation '='

执行类似如下SQL时：

select a.xxx
from a bjdm, b
where a.id=b.id and a.yyy='20000007' ;

报错如下
Illegal mix of collations (utf8mb4_unicode_ci,IMPLICIT) and (utf8mb4_general_ci,IMPLICIT) for operation '='

解决办法：
alter table a convert to character set utf8mb4 collate utf8mb4_general_ci;

alter table b convert to character set utf8mb4 collate utf8mb4_general_ci;