SqlServer如何使用公用表表达式（CTE）如何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,
 Level
FROM 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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