Expanding Hierarchies

Databases often store hierarchical information. For example, the following data is a hierarchical representation of regions of the world. This representation does not clearly show the structure implied by the data.

Parent Child

---------------------------------- ----------------------------------

World Europe

World North America

Europe France

France Paris

North America United States

North America Canada

United States New York

United States Washington

New York New York City

Washington Redmond

This example is easier to interpret:

World

North America

Canada

United States

Washington

Redmond

New York

New York City

Europe

France

Paris

The following Transact-SQL procedure expands an encoded hierarchy to any arbitrary depth. Although Transact-SQL supports recursion, it is more efficient to use a temporary table as a stack to keep track of all of the items for which processing has begun but is not complete. When processing is complete for a particular item, it is removed from the stack. New items are added to the stack as they are identified.

CREATE PROCEDURE expand (@current char(20)) as

SET NOCOUNT ON

DECLARE @level int, @line char(20)

CREATE TABLE #stack (item char(20), level int)

INSERT INTO #stack VALUES (@current, 1)

SELECT @level = 1

WHILE @level > 0

BEGIN

IF EXISTS (SELECT * FROM #stack WHERE level = @level)

BEGIN

SELECT @current = item

FROM #stack

WHERE level = @level

SELECT @line = space(@level - 1) + @current

PRINT @line

DELETE FROM #stack

WHERE level = @level

AND item = @current

INSERT #stack

SELECT child, @level + 1

FROM hierarchy

WHERE parent = @current

IF @@ROWCOUNT > 0

SELECT @level = @level + 1

END

ELSE

SELECT @level = @level - 1

END -- WHILE

The input parameter (@current) indicates the place in the hierarchy to start. It also keeps track of the current item in the main loop.

The two local variables used are @level, which keeps track of the current level in the hierarchy, and @line, which is a work area used to construct the indented line.

The SET NOCOUNT ON statement avoids cluttering up the output with ROWCOUNT messages from each SELECT.

The temporary table, #stack, is created and primed with the item identifier of the starting point in the hierarchy, and @level is set to match. The level column in #stack allows the same item to appear at multiple levels in the database. Although this situation does not apply to the geographic data in the example, it can apply in other examples.

In this example, when @level is greater than 0, the procedure follows several steps:

If there are any items in the stack at the current level (@level), the procedure chooses one and calls it @current.
Indents the item @level spaces, and then prints the item.
Deletes the item from the stack so it won’t be processed again, and then adds all its child items to the stack at the next level (@level + 1). This is the only place where the hierarchy table (#stack) is used.

Note With a conventional programming language, you would have to find each child item and add it to the stack individually. With Transact-SQL, you can find all child items and add them with a single statement, avoiding another nested loop.
If there are child items (IF @@ROWCOUNT > 0), descends one level to process them (@level = @level + 1); otherwise, continues processing at the current level.
Finally, if there are no items on the stack awaiting processing at the current level, goes back up one level to see if there are any awaiting processing at the previous level (@level = @level - 1). When there is no previous level, the expansion is complete.