If you meant just ABC as simple value, answer above is the one that works fine.
If you meant concatenation of values of rows that are not selected by your main query, you will need to use a subquery.
Something like this may work:
SELECT t1.col1,
t1.col2,
(SELECT GROUP_CONCAT(col2 SEPARATOR '') FROM Table1 t2 WHERE t2.col1 != 0) as col3
FROM Table1 t1
WHERE t1.col1 = 0;
Actual syntax maybe a bit off though