code: func (b *builtinSubDateAndStringSig) evalString(row chunk.Row) (string, bool, error) {
  	arg0, isNull, err := b.args[0].EvalDuration(b.ctx, row)
  	if isNull || err != nil {
  		return "", isNull, err
  	}
  	s, isNull, err := b.args[1].EvalString(b.ctx, row)
  	if isNull || err != nil {
  		return "", isNull, err
  	}
  	if !isDuration(s) {
  		return "", true, nil
  	}
  	arg1, err := types.ParseDuration(b.ctx.GetSessionVars().StmtCtx, s, getFsp4TimeAddSub(s))
  	if err != nil {
  		return "", true, err
  	}
  	result, err := arg0.Sub(arg1)
  	if err != nil {
  		return "", true, err
  	}
  	return result.String(), false, nil
  }
docs: // evalString evals a builtinSubDateAndStringSig.
  // See https://dev.mysql.com/doc/refman/5.7/en/date-and-time-functions.html#function_subtime