问题
Given the simple data structure:
ID | Category_Name | Parent_ID
Example:
1 Cars 0
2 Boxes 0
3 Lamborghinis 1
4 VW Camper Vans 1
5 Big Boxes 2
6 Small Boxes 2
7 Cereal Boxes 2
8 Broken Lambos 3
9 Yellow Ones 3
10 Rusty 8
11 Milkshake Stained 8
12 Chocolate Flavour 11
13 Strawberry 11
14 Indiscernible Solution 11
Representing a simple tree navigation structure, what would programatically be the best way to retrieve the tree in a presentable format? Can we create an SQL statement to retrieve them 'in order'?
Thanks for any help! If my approach is wrong, feel free to comment also.
I'm using SQL-Server 2000.
回答1:
If you're using SQL Server 2008 you might want to try out the new hierarchyid data type.
If you're not then another way is to look into the nested sets model which works on all databases.
If you're using SQL Server 2005 and up you can use recursive CTEs to retreive the tree structure.
回答2:
I usually build the tree structure in my application code. Partially because I'm more confident with c# than SQL, but it also because I usually need to process the data into suitable c# structures anyway.
SQL is quite bad at recursive structures like lists and trees. If I had to put the tree building in my database I'd go for a stored procedure. But there might be a smart way I don't know about.
If you use Oracle you might be able to hack something up with Connect By.
回答3:
Not for SQL2000, but if you manage to upgrade to 2k5, you can do
WITH t AS(SELECT id, parent_id, category_name FROM mytable WHERE parent_id IS NULL
UNION ALL
SELECT c.id, c.parent_id, c.category_name FROM t p JOIN mytable c ON c.parent_id = p.id)
SELECT * FROM t
来源：https://stackoverflow.com/questions/3555101/sql-to-retrieve-tree-structure-nicely