Question

I am writing a query which needs to return all child entries (and their children etc…) under any given parent ID.

For example:

SELECT id
FROM table
WHERE parent_id IN

    (SELECT id
    FROM table
    WHERE parent_id IN

        (SELECT id
        FROM table
        WHERE parent_id IN

            (SELECT id
            FROM table
            WHERE code = 'A01')
        )
    )

As expected, this will only return a result set for the topmost SELECT query. I could write a UNION SELECT for each child level but that seems a tad clumsy…

Is there a way to add each individual result set from each nested query into my main set of results? Or should I take the clumsy option?

Answer 1

Your problem is best solved with a recursive query. A great way to perform recursive queries in SQL Server 2005 or later is to use Common Table Expressions

;with cte as (
    SELECT id, parent_id
    FROM [table]
    WHERE code = 'A01'

    UNION ALL

    SELECT t.id, t.parent_id
    FROM [table] t
        INNER JOIN cte 
            ON cte.id = t.parent_id
)
SELECT id 
FROM cte
OPTION (MAXRECURSION 0);

On a side note… I’m not sure I like the idea of the table name being table.