递归查询SQL Server

Question

I'm stuck on this problem for a while now.

I've got one table called Category which looks like this:

number, name, parent
1     , Fruit, null
2     , Apples, 1
3     , Golden Apples, 2
4     , Pineapples, 1

Now, how am I supposed to select the main category and with its sub categories and if they exist, their sub categories.

I have to output it in PHP like this:

<div>Fruit
     <div>Apples
          <div>Golden Apples</div>
     </div>
     <div>Pineapples</div>
</div>

In theory there is no known end or amount of sub categories you can have. I've struggled with this problem for a while.

It is possible to do this with a loop and then run a query to select its sub categories again however this will be very resource intensive on the database side.

I hope someone can help me with this one.

Answer 1

The ideal approach is to normalize your data. Have a table for categories and a table that holds the members of each category. Then you can do a simple left join between the tables to get what you are looking for. Since you do not have that, you will need to select from the main table and then do a left join on the same table using aliases.

Something like this will work.

select number,name from Category where Parent is NULL Left Join Category as Items on Items.Parent = Category.number

Answer 2

try using a recursive CTE like this:

--this is just an old example I've used before, but it kind of matches your table
--go through a nested table supervisor - user table and display the chain
DECLARE @Contacts table (id int, first_name varchar(10), reports_to_id int)
INSERT @Contacts VALUES (1,'Jerome', NULL )  -- tree is as follows:
INSERT @Contacts VALUES (2,'Joe'   ,'1')     --                      1-Jerome
INSERT @Contacts VALUES (3,'Paul'  ,'2')     --                     /        \
INSERT @Contacts VALUES (4,'Jack'  ,'3')     --              2-Joe           9-Bill
INSERT @Contacts VALUES (5,'Daniel','3')     --            /       \              \
INSERT @Contacts VALUES (6,'David' ,'2')     --     3-Paul          6-David       10-Sam
INSERT @Contacts VALUES (7,'Ian'   ,'6')     --    /      \            /    \
INSERT @Contacts VALUES (8,'Helen' ,'6')     -- 4-Jack  5-Daniel   7-Ian    8-Helen
INSERT @Contacts VALUES (9,'Bill ' ,'1')     --
INSERT @Contacts VALUES (10,'Sam'  ,'9')     --

DECLARE @Root_id  int

--get complete tree---------------------------------------------------
SET @Root_id=null
PRINT '@Root_id='+COALESCE(''''+CONVERT(varchar(5),@Root_id)+'''','null')
;WITH StaffTree AS
(
    SELECT 
        c.id, c.first_name, c.reports_to_id, c.reports_to_id as Manager_id, cc.first_name AS Manager_first_name, 1 AS LevelOf
        FROM @Contacts                  c
            LEFT OUTER JOIN @Contacts  cc ON c.reports_to_id=cc.id
        WHERE c.id=@Root_id OR (@Root_id IS NULL AND c.reports_to_id IS NULL)
    UNION ALL
        SELECT 
            s.id, s.first_name, s.reports_to_id, t.id, t.first_name, t.LevelOf+1
        FROM StaffTree            t
            INNER JOIN @Contacts  s ON t.id=s.reports_to_id
    WHERE s.reports_to_id=@Root_id OR @Root_id IS NULL OR t.LevelOf>1
)
SELECT * FROM StaffTree ORDER BY LevelOf,first_name

OUTPUT:

@Root_id=null
         id first_name reports_to_id  Manager_id Manager_first_name     LevelOf
----------- ---------- ------------- ----------- ------------------ -----------
          1 Jerome              NULL        NULL NULL                         1
          9 Bill                   1           1 Jerome                       2
          2 Joe                    1           1 Jerome                       2
          6 David                  2           2 Joe                          3
          3 Paul                   2           2 Joe                          3
         10 Sam                    9           9 Bill                         3
          5 Daniel                 3           3 Paul                         4
          8 Helen                  6           6 David                        4
          7 Ian                    6           6 David                        4
          4 Jack                   3           3 Paul                         4

(10 row(s) affected)

I would think you could loop over the result set in PHP and build your DIVs.