Question 1

what is the nil uuid for

Accepted Answer

To state “no identifier” explicitly where a field must be filled: a NOT NULL column, a mandatory protocol field, a default in a structure. Nil is a known constant of all zeros, so absence is recognised at once. In .NET it is Guid.Empty, in Java new UUID(0, 0), and in RFC 9562 the Nil UUID.

Question 2

nil uuid or null which is better

Accepted Answer

If the column allows NULL, NULL is usually better: it honestly means absence and stays out of unique indexes. Nil is a real value: two rows holding it cannot coexist in a unique column, and a foreign key will demand a row with that ID. Nil is chosen when NULL is unavailable — in a format, a protocol or a language without null references.

Question 3

what is the max uuid

Accepted Answer

Nil's counterpart from RFC 9562: all 128 bits set, ffffffff-ffff-ffff-ffff-ffffffffffff. It serves as the upper bound of a range or as an “any value” marker — for example, in queries like “all identifiers up to the maximum”. Like nil, it has neither version nor variant: it is a placeholder, not an identifier.

Question 4

does a nil uuid pass validation

Accepted Answer

A format check, yes: 32 hexadecimal digits in their places. A strict version check, no: the version position holds zero while 1–8 are allowed, so some validators handle nil separately. The Check field on the generator page calls it the zero identifier rather than a non-standard version, so a placeholder is not mistaken for an error.

Question 5

how to write a nil uuid in postgresql

Accepted Answer

As a string with a type cast: '00000000-0000-0000-0000-000000000000'::uuid. It can serve as a column default or be compared against in queries. PostgreSQL has no dedicated nil function. In application code a constant is handier — Guid.Empty in C#, uuid.UUID(int=0) in Python — than typing thirty-two zeros by hand each time.

Question 6

can a random uuid turn out to be all zeros

Accepted Answer

No. Versions 4 and 7 have some bits fixed: 4 or 7 in the version position and 8, 9, a or b in the variant position. An all-zero value is impossible for them in principle, not merely unlikely. That is why nil unambiguously means a placeholder and cannot be confused with a real identifier.

Format	Length	Randomness	Time inside	Sortable	Standard
UUID v4	36	122 bits	no	no	RFC 9562
UUID v7	36	74 bits	ms, 48 bits	yes	RFC 9562
UUID v1	36	node and counter	100 ns, 60 bits	no	RFC 9562, legacy
Nil UUID	36	none	no	—	RFC 9562
NanoID	21	126 bits	no	no	a library, not an RFC
ULID	26	80 bits	ms, 48 bits	yes	ULID spec, not an RFC

UUID generator · formatNil UUID — the zero identifier

Generator Nil UUID

How it is built: Nil UUID

When to use it

How it differs from its neighbours

Format comparison

Frequently asked questions

what is the nil uuid for

nil uuid or null which is better

what is the max uuid

does a nil uuid pass validation

how to write a nil uuid in postgresql

can a random uuid turn out to be all zeros

UUID generator · formatNil UUID — the zero identifier

Generator Nil UUID

How it is built: Nil UUID

When to use it

How it differs from its neighbours

Format comparison

Frequently asked questions

what is the nil uuid for

nil uuid or null which is better

what is the max uuid

does a nil uuid pass validation

how to write a nil uuid in postgresql

can a random uuid turn out to be all zeros

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