SQL Query to extract nesting from a table

Question

I have a table in which references are made from one row to another. So basically something like this:

ID | Name | ParentId
1  | Root | 
2  | Top  |  
3  | Sub1 | 1
4  | Sub2 | 2

The third column [ParentId] denotes the parent of the current row. My question is how do I get this nesting from the table using an SQL query. The result I would like to see from the query is

ID | Name | ParentName
1  | Root | NULL
2  | Top  | NULL
3  | Sub1 | Root
4  | Sub2 | Top

It might seem trivial as it is just replacing the id of the third column by the name of the item but I can't seem to figure it out. Thanks for you help!

Answer 1

for a single level you could use a left join on the same table

select a.id, a.name, b.name 
from my_table a  
left join my_table b on b.parentId = a.id 

Answer 2

select t1.id, t1.name, t2.name as parentName
from  table t1
left join  table t2
on t2.id = t1.parentId

Answer 3

The following query will help you to achieve your aim:

SELECT
    t1.Id,
    t1.Name,
    (SELECT t2.Name FROM MyTable t2 WHERE t2.Id = t1.ParentId) ParentName 
FROM
    MyTable t1

If there is a lot of records in the table, by creating an index on the ParentId column you can run the above query faster than the JOIN .

Answer 4

SQL Fiddle: hierarchyid example fiddles

I decided to take on a slightly different approach to answering this question, using the hierarchyid datatype that is supported natively inside of SQL server. hierarchyid was created to specifically handle record selection/query capabilities for nested relationships, and once you get used to some of the minor annoyances that come with the datatype (such as inline queries/subqueries which are often needed to arrive at the actual id value for a record), its actually pretty slick.

I created a SQL fiddle showing step by step how to use the data-type for an example nested relationship of:

/Category One
    /Subcategory 1
        /Subcategory 1.1
        /Subcategory 1.2
        /Subcategory 1.3
    /Subcategory 2
    /Subcategory 3
        /Subcategory 3.1
            /Subcategory 3.1.1
    /Subcategory 4
/Category Two
    /Subcategory 1

NB: the tsql is rewritten and annotated MSDN example code, which was actually quite poorly made imo. I have rewritten and presented it in such a way that I found it much easier to wrap my head around.

Answer 5

All proposed solution work fine for the posted question. In my "real" situation the table is far more complicated and has already many APPLY statements (specific SQL Server, I guess) so I ended up using the "Outer Apply" variant of posted Join statement: With a AS ( SELECT id, name from Nesting )
SELECT id, name, ParentName from a OUTER APPLY ( SELECT name as ParentName from Nesting n where n.parentid = a.id
) AS FOO With a AS ( SELECT id, name from Nesting )
SELECT id, name, ParentName from a OUTER APPLY ( SELECT name as ParentName from Nesting n where n.parentid = a.id
) AS FOO

Thank you all so much!