SQL Injection attack - What does this do?

I have detected some failed SQL injection attacks on my website. The failed queries are of the form:

SELECT 6106 FROM(SELECT COUNT(*),':sjw:1:ukt:1'x FROM information_schema.tables GROUP BY x)

The ':sjw:1:ukt:1' part is specially constructed with variables concatenated together to give random 0s or 1s etc.

I would like to know what these queries do?

The database is MySQL.

Update: Here is the original injected SQL:

(SELECT 6106
 FROM  (SELECT COUNT(*),
               CONCAT(
                        CHAR(58, 115, 106, 119, 58), 
                        (SELECT ( CASE WHEN ( 6106 = 6106 ) THEN 1 ELSE 0 END )), 
                        CHAR(58, 117, 107, 116, 58), 
                        FLOOR(RAND(0) * 2)
                      ) x
        FROM   INFORMATION_SCHEMA.TABLES
        GROUP  BY x)a) 

It fails with message

Duplicate entry ':sjw:1:ukt:1' for key 'group_key'