Hello,
In API versions prior to 2.0, the ID of a record is always a 15-character case-sensitive ID and should not be compared in a case-insensitive manner.
15 digit Id number will have numeric digits range from (0-9), a Lowercase letter(a-z) or a Uppercase letters(A-Z). 15 digit ID in salesforce.com is case sensitive. Example 100000000000ABC is different from 100000000000abc. When using external productive tools like Microsoft Excel, MS Access and SQL Server external ID’s are not case sensitive and they don’t recognize the difference between 100000000000ABC and 100000000000abc.
Thanks.