MySQL 8.0

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select version();

version()

8.0.30

create table table_A as SELECT 'A001' as transnumber, 0 as lpost;
insert into table_A
values('A002', 	1),
('A003', 	1),
('A004', 	1),
('A005', 	0),
('A006', 	1);

Records: 1  Duplicates: 0  Warnings: 0

Records: 5  Duplicates: 0  Warnings: 0

set @var1 = (select group_concat(transnumber) from table_A where lpost = 0);
select @var1;

@var1

A001,A005