我的代码返回错误错误代码：1172.当我运行下面的代码时，结果包含了不止一行：

delimiter //
create function purchase_status(customernumber int) returns varchar(100) reads sql data
begin
    declare purchase_state varchar(100);
    select 
        case
            when amount < 25000 then "Silver" 
            when amount between 25000 and 50000 then "Gold"
            else "Platinum"
        end as purchase_state2 into purchase_state
    from payments;
    return purchase_status;
end//

#drop function purchase_status;

select customernumber,customername,purchase_status(customernumber) as purchase_status
from customers;

我不知道该怎么称呼它。