insert username with letters only (using triggers) sql

CREATE DATABASE sqlprofiles

USE sqlprofiles

CREATE TABLE users(
    id INT  PRIMARY KEY ,
    username VARCHAR(20)

)


CREATE TRIGGER addusers
ON users
instead OF INSERT,UPDATE
AS
BEGIN
    DECLARE @username VARCHAR(20)
    SET @username=(SELECT username FROM inserted)
    IF PATINDEX('%[0-9]%', @username) =0
        INSERT INTO users (id,username) SELECT id,username FROM inserted
    ELSE
        print 'num detected'

END




INSERT INTO users (id,username) VALUES(1,'blaba')
INSERT INTO users (id,username) VALUES(2,'blab0da')


SELECT * FROM users