Newid

Newid, in the context of Microsoft SQL Server, refers to the NEWID function, which generates a new value of the uniqueidentifier data type. A uniqueidentifier is commonly known as a GUID (globally unique identifier) and is intended to be unique across databases and servers. NEWID produces a 128-bit value typically displayed in the standard GUID format, such as xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx.

The NEWID function generates a random GUID rather than a deterministic sequence. It is non-deterministic, making it suitable for creating surrogate keys or identifiers for rows that require uniqueness without relying on an incremental numeric sequence. Because GUIDs are large, they are often used as primary keys or as keys in distributed systems, though they can have implications for index size and performance when used as clustered keys.

Usage and examples are straightforward. The function is invoked as SELECT NEWID(); to return a new GUID value. It can also be used as a default constraint to populate a column automatically, for example: CREATE TABLE MyTable (Id uniqueidentifier DEFAULT NEWID() PRIMARY KEY, Name nvarchar(100)); Inserting rows without specifying Id will assign a new GUID automatically.

Related alternatives include NEWSEQUENTIALID, which returns sequential GUIDs that are designed to reduce index fragmentation when GUIDs are used as primary keys. While NEWID produces random GUIDs, NEWSEQUENTIALID aims for improved index performance in certain scenarios.

In other database systems, similar functionality exists under different names, such as UUID generation functions in MySQL and PostgreSQL.