如何从下表中选择数据？

Question

表数据为：

如何获得结果：

Answer 1

您的预期输出导致混淆。我认为你期望以下格式作为输出。

DECLARE @CategoriesDetails TABLE (category_code INT, parent_code INT, category_name VARCHAR(100));

INSERT INTO @CategoriesDetails (category_code, parent_code, category_name) VALUES
(1, 0, 'Food'),
(2, 1, 'Bakery'),
(3, 2, 'Snaks');

SELECT  C1.category_name AS [Sub category], 
        ISNULL(C2.category_name, '') AS [Parent category]
FROM @CategoriesDetails C1
LEFT JOIN @CategoriesDetails C2 ON C2.category_code = C1.parent_code

输出：

Sub category    Parent category
--------------------------------
Food            
Bakery          Food
Snaks           Bakery

Answer 2

您需要LEFT JOIN自己，如下所示。

SELECT  
    T.categroy_name,
    COALESCE(Parent.category_name, '') AS 'parent category'
FROM
    Tbl T LEFT JOIN 
    Tbl Parent ON T.parent_code = Parent.categrory_code