code: func builtinCoalesce(args []types.Datum, ctx context.Context) (d types.Datum, err error) {
  	for _, d = range args {
  		if !d.IsNull() {
  			return d, nil
  		}
  	}
  	return d, nil
  }
docs: // See http://dev.mysql.com/doc/refman/5.7/en/comparison-operators.html#function_coalesce