双查询注入问题

Question

下面是双查询注入语句，会正常报错。

select count(*),concat(((select schema_name from information_schema.schemata limit 0,1)),floor (rand(0)*2))x from information_schema.tables group by x;

但改成下面为啥就可以正常执行，不会报错了。

select count(*),concat(((select group_concat(schema_name) from information_schema.schemata limit 0,1)),floor (rand(0)*2))x from information_schema.tables group by x