code: func (p *preprocessor) checkUnionSelectList(stmt *ast.UnionSelectList) {
  	for _, sel := range stmt.Selects[:len(stmt.Selects)-1] {
  		if sel.IsInBraces {
  			continue
  		}
  		if sel.Limit != nil {
  			p.err = ErrWrongUsage.GenWithStackByArgs("UNION", "LIMIT")
  			return
  		}
  		if sel.OrderBy != nil {
  			p.err = ErrWrongUsage.GenWithStackByArgs("UNION", "ORDER BY")
  			return
  		}
  	}
  }
docs: // checkUnionSelectList checks union's selectList.
  // refer: https://dev.mysql.com/doc/refman/5.7/en/union.html
  // "To apply ORDER BY or LIMIT to an individual SELECT, place the clause inside the parentheses that enclose the SELECT."