drop table if exists T_Test;

create table T_Test

  select 'A' parent, 'A1' child union all

  select 'A', 'A1' union all

  select 'A', '' union all

  select 'A', null union all

  select 'A', 'A2' union all

  select 'B', 'B1' union all

  select 'B', 'B2' union all

  select 'C', 'C1' union all

  select 'C', 'C2' ;

select * from t_test;

select parent , group_concat(child) as child from t_test group by parent;                      #直接拼接

select parent , group_concat(distinct child) as child from t_test group by parent;             #去重 但未去空字符串

select parent , group_concat(distinct nullif(child, '')) as child from t_test group by parent; #去重且去空字符串