Question

我有一个这样的表（这个表是我的产品类别）：

id   title   parent_id 
 1     A         0
 2     A1        1
 3     A2        1
 4     B         0
 5     A11       2
 6     B1        4

现在，我怎样才能获得最后一个子类别？例如：

B1
A11
A2

Answer 1

您似乎想要的行不是另一行的父级：

select pc.*
from productcategories pc
where not exists (select 1
                  from productcategories pc2
                  where pc2.parent_id = pc.id
                 );

Answer 2

尝试下面的内容;

我的SQL版

SELECT  parent_id
       ,id
       ,title
  FROM
     (
      SELECT  @row_num := IF(@prev_value=p.parent_id,@row_num+1,1) AS RowNumber
             ,p.parent_id 
             ,p.id
             ,p.title
             ,@prev_value := p.parent_id
        FROM Product p,
             (SELECT @row_num := 1) x,
             (SELECT @prev_value := 0) y
       ORDER BY  p.parent_id,p.id DESC
     ) subquery
 WHERE RowNumber = 1 AND parent_id>0 
 ORDER BY parent_id DESC

MSSQL版本：

 SELECT id,Title,parent_id 
 FROM ( SELECT id,Title,parent_id,ROW_NUMBER() OVER(PARTITION BY parent_id ORDER BY id DESC) LocalId 
        FROM [dbo].[Product] ) AS P 
WHERE P.LocalId=1 
AND parent_id>0 
ORDER BY parent_id DESC

如何获取自联接表的最后一个类别

2 个答案: