MySQL Group Concat one cell when other cells also match
You need to divide the rows into groups using GROUP BY. Something like:

select group_concat(product_no), date_sold, shift_start 
from sales 
group by date_sold, shift_start;


I assume you did something like:

select group_concat(product_no), date_sold, shift_start 
from sales 


The latter is invalid in most (all but MySQL?) DBMS products, but MySQL allows it by default.