SqlServer uses a common table expression (CTE) to build an infinite tree structure. sqlservercte

SqlServer uses a common table expression (CTE) to build an infinite tree structure. sqlservercte

Starting from SQL Server 2005, we can use CTE to support recursive queries. CTE is the common table expression.

A common table expression (CTE) is a temporary naming result set defined in the query and will be used in the from clause. Each CTE is defined only once (but can be referenced any time in its scope) and will survive during the query period. You can use CTE to perform recursive operations.

DECLARE @Level INT=3;WITH cte_parent(CategoryID,CategoryName,ParentCategoryID,Level)AS(  SELECT category_id,category_name,parent_category_id,1 AS Level  FROM TianShenLogistic.dbo.ProductCategory WITH(NOLOCK) WHERE category_id IN ( SELECT category_id  FROM TianShenLogistic.dbo.ProductCategory  WHERE parent_category_id=0 )  UNION ALL  SELECT b.category_id,b.category_name,b.parent_category_id,a.Level+1 AS Level  FROM TianShenLogistic.dbo.ProductCategory b  INNER JOIN cte_parent a  ON a.CategoryID = b.parent_category_id)SELECT  CategoryID AS value, CategoryName as label, ParentCategoryID As parentId, LevelFROM cte_parent WHERE Level <=@Level;public static List<LogisticsCategoryTreeEntity> GetLogisticsCategoryByParent(int? level)    {      if (level < 1) return null;      var dataResult = CategoryDA.GetLogisticsCategoryByParent(level);      var firstlevel = dataResult.Where(d => d.level == 1).ToList();      BuildCategory(dataResult, firstlevel);      return firstlevel;    }    private static void BuildCategory(List<LogisticsCategoryTreeEntity> allCategoryList, List<LogisticsCategoryTreeEntity> categoryList)    {      foreach (var category in categoryList)      {        var subCategoryList = allCategoryList.Where(c => c.parentId == category.value).ToList();        if (subCategoryList.Count > 0)        {          if (category.children == null) category.children = new List<LogisticsCategoryTreeEntity>();          category.children.AddRange(subCategoryList);          BuildCategory(allCategoryList, category.children);        }      }    }