How to automatically generate unique id in SQL like UID12345678?

The only viable solution in my opinion is to use

• an ID INT IDENTITY(1,1) column to get SQL Server to handle the automatic increment of your numeric value
• a computed, persisted column to convert that numeric value to the value you need

So try this:

CREATE TABLE dbo.tblUsers
  (ID INT IDENTITY(1,1) NOT NULL PRIMARY KEY CLUSTERED,
   UserID AS 'UID' + RIGHT('00000000' + CAST(ID AS VARCHAR(8)), 8) PERSISTED,
   .... your other columns here....
  )

Now, every time you insert a row into tblUsers without specifying values for ID or UserID:

INSERT INTO dbo.tblUsersCol1, Col2, ..., ColN)
VALUES (Val1, Val2, ....., ValN)

then SQL Server will automatically and safely increase your ID value, and UserID will contain values like UID00000001, UID00000002,...... and so on - automatically, safely, reliably, no duplicates.

Update: the column UserID is computed - but it still OF COURSE has a data type, as a quick peek into the Object Explorer reveals:

CREATE TABLE dbo.tblUsers
(
    ID INT IDENTITY(1,1) NOT NULL PRIMARY KEY CLUSTERED,
    UserID AS 'UID' + RIGHT('00000000' + CAST(ID AS VARCHAR(8)), 8) PERSISTED, 
    [Name] VARCHAR(50) NOT NULL,
)

marc_s's Answer Snap