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I'm trying to validate a challenge on Error-based SQL injection: and there is this SQL statement in the documentation which procude an error, but i really don't see why (also i'm not sure to understand the statement itself):

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

Error :

ERROR 1062 (23000): Duplicate entry '5.0.841' for key 1

marked as duplicate by tim, Polynomial, Xander, Matthew, Neil Smithline Mar 30 '16 at 22:12

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