UniqueRandomID in SQL

Generate new Random ID, unique not duplicated in two columns in two tables.
This was created by AI, then I just copied and pasted with minor modifications to fit my needs.

CodeFunctionName

What is this?

Public

Tested

Original Work

CREATE PROCEDURE [dbo].[GenerateUniqueRandomID]
AS
BEGIN
    DECLARE @RandomID NVARCHAR(12)
    DECLARE @ExistingCount INT

    SET @RandomID = ( SELECT LEFT(CONVERT(NVARCHAR(36), CONVERT(NVARCHAR(36), Replace(NEWID(), '-', ''))), 12) ) -- Generate a random ID

    SELECT @ExistingCount = COUNT(*)    -- Check if the ID exists in both tables
    FROM (
        SELECT PostID As ID FROM Posts
        UNION ALL
        SELECT UserID As ID FROM Users
    ) AS CombinedIDs
    WHERE ID = @RandomID

    WHILE @ExistingCount > 0 -- If the ID already exists, regenerate
    BEGIN
        SET @RandomID = ( SELECT LEFT(CONVERT(NVARCHAR(36), CONVERT(NVARCHAR(36), Replace(NEWID(), '-', ''))), 12) )
        SELECT @ExistingCount = COUNT(*) FROM (
                SELECT PostID as ID FROM Posts
                UNION ALL
                SELECT UserID As ID FROM Users
        ) AS CombinedIDs
        WHERE ID = @RandomID
    END

    SELECT @RandomID AS UniqueRandomID    -- Return the unique random ID
    Union All
    SELECT 'U' + Convert(nvarchar, Convert(int , RAND( (DATEPART(mm, GETDATE()) * 100000 ) + (DATEPART(ss, GETDATE()) * 1000 ) + DATEPART(ms, GETDATE()) )    * 100000000 )) As UniqueRandomID
END