Question

如何在sql中创建一个树，何时能以这种方式获取数据？

category_id |category_parent |category_code      |category_name        
------------|----------------|-------------------|---------------------
70          |0               |Vege               |Vegetable            
71          |0               |Frui               |Fruit                
72          |0               |Fish               |Fishery              
73          |71              |Butc               |Butchery             
74          |72              |Poul               |Poultry              
75          |73              |Dair               |Dairy                
76          |75              |Deli               |Delicatessen         
77          |75              |Frofod             |Frozen Food          
78          |76              |Foserv             |Food Service         
79          |77              |Pla Flo            |Plant & Flower       
80          |79              |ConFresh           |Consightment Fresh   
81          |79              |Café               |Café

Answer 1

您想要查找的是公用表表达式（CTE）。 Here是CTE的一个很好的例子。

所以你想做的事情看起来像那样：

WITH Tree(child_category_id, child_name, child_category, parent_id, Parents)
AS
(
--1st level rows (parent)
    SELECT category_id, category_name, category_code, category_parent, CAST('' AS VARCHAR(MAX))
        FROM YourTable AS FirtGeneration
        WHERE category_parent = 0
    UNION ALL
--children 
    SELECT NextGeneration.category_id, NextGeneration.category_name, NextGeneration.category_code, Parent.child_category_id,
    CAST(CASE WHEN Parent.Parents = ''
        THEN(CAST(NextGeneration.category_parent AS VARCHAR(MAX)))
        ELSE(Parent.Parents + '.' + CAST(NextGeneration.category_parent AS VARCHAR(MAX)))
    END AS VARCHAR(MAX))
        FROM YourTable AS NextGeneration
        INNER JOIN Tree AS Parent ON NextGeneration.category_id = Parent.child_category_id    
)
SELECT *
    FROM Tree
--not needed here because you wont do that many recursions
--OPTION(MAXRECURSION 32767)

如何在SQL中创建一个树？

1 个答案: