Generate a Unique AlphaNumeric number with Fixed Alphabet characters in SQL Server

The function LEFT returns the count leftmost characters / bytes in an expression.

Make 3 characters to 20

DECLARE @AlphaId VARCHAR(10)
DECLARE @Count
INT SELECT @Count = COUNT(*) + 1 FROM tbl_UD
SET @AlphaID = '301A' + LEFT(CAST(@Count AS VARCHAR(10)), 15)
SELECT @AlphaID
insert into tbl_UD (id) values (@AlphaID)

Now till fifteen character length of number it will generate.