SqlServer使用公用表表达式（CTE）实现无限级树形构建

SQL Server 2005开始，我们可以直接通过CTE来支持递归查询,CTE即公用表表达式

公用表表达式（CTE），是一个在查询中定义的临时命名结果集将在from子句中使用它。每个CTE仅被定义一次（但在其作用域内可以被引用任意次），并且在该查询生存期间将一直生存。可以使用CTE来执行递归操作。

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);        }      }    }

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