Casting binary(16) to a bigint will truncate it. bigint only has 8 bytes of storage.

You must convert both the first and second sets of 8 bytes to a bigint to get the equivalent value.

[font="Courier New"]DECLARE @uuid AS UNIQUEIDENTIFIER = NEWID()

DECLARE @bits AS VARBINARY(16) = CAST(@uuid AS VARBINARY(16))

SELECT

CAST(@bits AS BIGINT) 'your answer',

@bits 'all16bits',

CAST(SUBSTRING(@bits, 1,8) AS BIGINT) leftBits,

CAST(SUBSTRING(@bits, 9,8) AS BIGINT) rightBits

guidaschar: 9E021A10-CD60-4275-A5E0-B208196B07B3

your answer: -6493994914812328013

all16bits: 0x101A029E60CD7542A5E0B208196B07B3

leftBits: 1160242733253489986

rightBits: -6493994914812328013

[/font]