>What do I have to do to have the following results
>
>Letter 'a' to letter 'z'
>or
>letter 'A' to letter 'A'
>or
>digit '0' to digit '9'
>
>While using the function RAND()?

You have to use CHR() to get characters

RAND(-1) && Seed
?Chr(64+rand()*26) (A-Z)
?Chr(96+rand()*26) (a-z)
?RAND()*9 && 0-9