Most Efficient Way to… Unique Random String

Question

I need to efficently insert a 5 character RANDOM string into a database while also ensuring that it is UNIQUE. Generating the random string is not the problem, but currently what I am doing is generating the string and then checking the DB if it exists already... if it does, I start over.

Is there a more efficient way to do this process?

Please note, I do NOT want to use GUID or anything else that is more than 5 Characters.... I MUST stick to 5 Characters.

PS: I don't think it makes a difference, but my strings are all case sensitive.

Here is the "Random String" portion

    Public Function GetRandomNumbers(ByVal numChars As Integer) As String
    Dim chars As String() = { _
     "A", "B", "C", "D", "E", "F", _
     "G", "H", "I", "J", "K", "L", _
     "M", "N", "O", "P", "Q", "R", _
     "S", "T", "U", "V", "W", "X", _
     "Y", "Z", "0", "1", "2", "3", _
     "4", "5", "6", "7", "8", "9", _
     "a", "b", "c", "d", "e", "f", _
     "g", "h", "i", "j", "k", "l", _
     "m", "n", "o", "p", "q", "r", _
     "s", "t", "u", "v", "w", "x", _
     "y", "z"}
    Dim rnd As New Random()
    Dim random As String = String.Empty
    Dim i As Integer = 0
    While i < numChars
        random += chars(rnd.[Next](0, 62))
        System.Math.Max(System.Threading.Interlocked.Increment(i), i - 1)
    End While
    Return random
End Function

Answer 1

If you are inserting the string to an existing, populated, table then you will always need to check if the string doesn't exist there (it doesn't have to be an explicit SELECT). You can either to it manually, or have an UNIQUE constraint on the column and let the database to do it. So if the database returns an error because the string is already there, generate another one.

Note that if you have an empty table and want to populate it with multiple random strings, it's a different problem.