select
Random_String =
substring(x,(abs(checksum(newid()))%36)+1,1)+
substring(x,(abs(checksum(newid()))%36)+1,1)+
substring(x,(abs(checksum(newid()))%36)+1,1)+
substring(x,(abs(checksum(newid()))%36)+1,1)+
/* and so on for as many characters as needed */
substring(x,(abs(checksum(newid()))%36)+1,1)
from
(select x='0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ') a
Results:
Random_String
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T7TAR