See the question and my original answer on StackOverflow

The algorithm is documented by the SQL Server guys here: How are GUIDs compared in SQL Server 2005? I am quoting it here (since it's an old article that may be gone forever in a few years)

In general, equality comparisons make a lot of sense with uniqueidentifier values. However, if you find yourself needing general ordering, then you might be looking at the wrong data type and should consider various integer types instead.

If, after careful thought, you decide to order on a uniqueidentifier column, you might be surprised by what you get back.

Given these two uniqueidentifier values:

@g1= '55666BEE-B3A0-4BF5-81A7-86FF976E763F' @g2 = '8DD5BCA5-6ABE-4F73-B4B7-393AE6BBB849'

Many people think that @g1 is less than @g2, since '55666BEE' is certainly smaller than '8DD5BCA5'. However, this is not how SQL Server 2005 compares uniqueidentifier values.

The comparison is made by looking at byte "groups" right-to-left, and left-to-right within a byte "group". A byte group is what is delimited by the '-' character. More technically, we look at bytes {10 to 15} first, then {8-9}, then {6-7}, then {4-5}, and lastly {0 to 3}.

In this specific example, we would start by comparing '86FF976E763F' with '393AE6BBB849'. Immediately we see that @g2 is indeed greater than @g1.

Note that in .NET languages, Guid values have a different default sort order than in SQL Server. If you find the need to order an array or list of Guid using SQL Server comparison semantics, you can use an array or list of SqlGuid instead, which implements IComparable in a way which is consistent with SQL Server semantics.

Plus, the sort follows byte groups endianness (see here: Globally unique identifier). The groups 10-15 and 8-9 are stored as big endian (corresponding to the Data4 in the wikipedia article), so they are compared as big endian. Other groups are compared using little endian.