GROUP_CONCAT mysql statement error, MySQL

GROUP_CONCAT mysql statement error

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SET @sql = ' someSQL ';
SET @sql = CONCAT('SELECT id_c, students,', @sql, '
        FROM(
            SELECT A.id_a, D.id_c id_c,
                C.students students,
                CONCAT(B.`code`, ''_'', A.id_a) col,
                CONCAT(D.value_m, '','', D.value_n) val
            FROM table_a A
                INNER JOIN table_d D ON A.id_a =D.id_a
                INNER JOIN table_b B ON D.id_b=B.id_b 
                INNER JOIN table_c C ON D.id_c=C.id_c
        )dd 
        GROUP BY id_c'
    );    
select @sql;

SET @sql = ' someSQL ';
SET @sql = CONCAT('SELECT id_c, students,', @sql, "
        FROM(
            SELECT A.id_a, D.id_c id_c,
                C.students students,
                CONCAT(B.`code`, '_', A.id_a) col,
                CONCAT(D.value_m, ',', D.value_n) val
            FROM table_a A
                INNER JOIN table_d D ON A.id_a =D.id_a
                INNER JOIN table_b B ON D.id_b=B.id_b 
                INNER JOIN table_c C ON D.id_c=C.id_c
        )dd 
        GROUP BY id_c"
    );
    
select @sql;


absolute service time: 0,32 sec

	@sql
1	SELECT id_c, students, someSQL FROM( SELECT A.id_a, D.id_c id_c, C.students students, CONCAT(B.`code`, '_', A.id_a) col, CONCAT(D.value_m, ',', D.value_n) val FROM table_a A INNER JOIN table_d D ON A.id_a =D.id_a INNER JOIN table_b B ON D.id_b=B.id_b INNER JOIN table_c C ON D.id_c=C.id_c )dd GROUP BY id_c

	@sql
1	SELECT id_c, students, someSQL FROM( SELECT A.id_a, D.id_c id_c, C.students students, CONCAT(B.`code`, '_', A.id_a) col, CONCAT(D.value_m, ',', D.value_n) val FROM table_a A INNER JOIN table_d D ON A.id_a =D.id_a INNER JOIN table_b B ON D.id_b=B.id_b INNER JOIN table_c C ON D.id_c=C.id_c )dd GROUP BY id_c