select *
from table_name
where 30.2 between cast(substr(items, 1, instr(items, ':') - 1) as real)
               and cast(substr(items, instr(items, ':') + 1) as real)

Module Module1

    Sub Main()

        Dim SQLCn As New SQLite.SQLiteConnection("data source=:memory:")
        SQLCn.Open()

        Dim SQLCmd As SQLite.SQLiteCommand = SQLCn.CreateCommand()
        SQLCmd.CommandText = "create table table_name (" &
                                "id        integer primary key," &
                                "items      char(128) not null," &
                                "decsr      char(128) null" &
                            ");" &
                            "insert into table_name values ( 1, '40:100,300:900,1000:1100', 'fish' );" &
                            "insert into table_name values ( 2, '20:30.5,200:300', 'tv' );"
        SQLCmd.ExecuteNonQuery()

        SQLCmd = SQLCn.CreateCommand()
        SQLCmd.CommandText = "select decsr from table_name where InRange(items, 300) = 1" 'Используем нашу функцию

        Dim SQLRead As SQLite.SQLiteDataReader = SQLCmd.ExecuteReader()
        Do While SQLRead.Read()
            Console.WriteLine(SQLRead.GetString(0))
        Loop

        Console.ReadLine()
    End Sub



End Module

'Классу добавляем аттрибут указывающий что это SQLite функция, также наследуем абстрактный класс и переопределяем
<SQLite.SQLiteFunction("InRange", 2, SQLite.FunctionType.Scalar)>
Public Class SQLiteInRange
    Inherits SQLite.SQLiteFunction

    Public Overrides Function Invoke(args() As Object) As Object
        Dim sItems As String = Convert.ToString(args(0))
        Dim iValue As Integer = Convert.ToInt32(args(1))

        'Здесь закладываем логику
        For Each xItem In sItems.Split(",")
            Dim arrNumbers() As String = xItem.Split(":")

            If iValue >= Convert.ToDouble(arrNumbers(0).Replace(".", ",")) And iValue <= Convert.ToDouble(arrNumbers(1).Replace(".", ",")) Then
                Return True
            End If
        Next

        Return False
    End Function
End Class