[sql-server] How to convert int to char with leading zeros?

This work for me in MYSQL:

FUNCTION leadingZero(format VARCHAR(255), num VARCHAR(255))
  RETURNS varchar(255) CHARSET utf8
BEGIN
  return CONCAT(SUBSTRING(format,1,LENGTH(format)-LENGTH(num)),num);
END

For example:

leadingZero('000',999); returns '999'
leadingZero('0000',999); returns '0999'
leadingZero('xxxx',999); returns 'x999'

Hope this will help. Best regards