code: func (a *havingWindowAndOrderbyExprResolver) Enter(n ast.Node) (node ast.Node, skipChildren bool) {
  	switch n.(type) {
  	case *ast.AggregateFuncExpr:
  		a.inAggFunc = true
  	case *ast.WindowFuncExpr:
  		a.inWindowFunc = true
  	case *ast.WindowSpec:
  		a.inWindowSpec = true
  	case *driver.ParamMarkerExpr, *ast.ColumnNameExpr, *ast.ColumnName:
  	case *ast.SubqueryExpr, *ast.ExistsSubqueryExpr:
  		// Enter a new context, skip it.
  		// For example: select sum(c) + c + exists(select c from t) from t;
  		return n, true
  	default:
  		a.inExpr = true
  	}
  	return n, false
  }
docs: // Enter implements Visitor interface.