# Recursion example: bill of materials

Bill of materials (BOM) applications are a common requirement in many business environments. To illustrate the capability of a recursive common table expression for BOM applications, consider a table of parts with associated subparts and the quantity of subparts required by the part.

For this example, create the table as follows:
``````  CREATE TABLE PARTLIST
(PART VARCHAR(8),
SUBPART VARCHAR(8),
QUANTITY INTEGER);``````
To give query results for this example, assume that the PARTLIST table is populated with the following values:
``````  PART     SUBPART  QUANTITY
-------- -------- -----------
00       01                 5
00       05                 3
01       02                 2
01       03                 3
01       04                 4
01       06                 3
02       05                 7
02       06                 6
03       07                 6
04       08                10
04       09                11
05       10                10
05       11                10
06       12                10
06       13                10
07       14                 8
07       12                 8``````

## Example 1: Single level explosion

The first example is called single level explosion. It answers the question, What parts are needed to build the part identified by '01'?. The list will include the direct subparts, subparts of the subparts and so on. However, if a part is used multiple times, its subparts are only listed once.
``````WITH RPL (PART, SUBPART, QUANTITY) AS
(  SELECT ROOT.PART, ROOT.SUBPART, ROOT.QUANTITY
FROM PARTLIST ROOT
WHERE ROOT.PART = '01'
UNION ALL
SELECT CHILD.PART, CHILD.SUBPART, CHILD.QUANTITY
FROM RPL PARENT, PARTLIST CHILD
WHERE  PARENT.SUBPART = CHILD.PART
)
SELECT DISTINCT PART, SUBPART, QUANTITY
FROM RPL
ORDER BY PART, SUBPART, QUANTITY;``````

The preceding query includes a common table expression, identified by the name RPL, that expresses the recursive part of this query. It illustrates the basic elements of a recursive common table expression.

The first operand (fullselect) of the UNION, referred to as the initialization fullselect, gets the direct children of part '01'. The FROM clause of this fullselect refers to the source table and will never reference itself (RPL in this case). The result of this first fullselect goes into the common table expression RPL (Recursive PARTLIST). As in this example, the UNION must always be a UNION ALL.

The second operand (fullselect) of the UNION uses RPL to compute subparts of subparts by having the FROM clause refer to the common table expression RPL and the source table with a join of a part from the source table (child) to a subpart of the current result contained in RPL (parent). The result goes back to RPL again. The second operand of UNION is then used repeatedly until no more children exist.

The SELECT DISTINCT in the main fullselect of this query ensures the same part/subpart is not listed more than once.

The result of the query is as follows:
``````  PART     SUBPART  QUANTITY
-------- -------- -----------
01       02                 2
01       03                 3
01       04                 4
01       06                 3
02       05                 7
02       06                 6
03       07                 6
04       08                10
04       09                11
05       10                10
05       11                10
06       12                10
06       13                10
07       12                 8
07       14                 8``````

Observe in the result that part '01' goes to '02' which goes to '06' and so on. Further, notice that part '06' is reached twice, once through '01' directly and another time through '02'. In the output, however, its subcomponents are listed only once (this is the result of using a SELECT DISTINCT) as required.

It is important to remember that with recursive common table expressions it is possible to introduce an infinite loop. In this example, an infinite loop would be created if the search condition of the second operand that joins the parent and child tables was coded as:
``   PARENT.SUBPART = CHILD.SUBPART``

This example of causing an infinite loop is obviously a case of not coding what is intended. Exercise care when determining what to code so that there is a definite end of the recursion cycle.

The result produced by this example query can be produced in an application program without using a recursive common table expression. However, this approach would require starting of a new query for every level of recursion. Furthermore, the application needs to put all the results back in the database to order the result. This approach complicates the application logic and does not perform well. The application logic becomes even harder and more inefficient for other bill of material queries, such as summarized and indented explosion queries.

## Example 2: Summarized explosion

The second example is a summarized explosion. The question posed here is, what is the total quantity of each part required to build part '01'. The main difference from the single level explosion is the requirement to aggregate the quantities. The first example indicates the quantity of subparts required for the part whenever it is required. It does not indicate how many of the subparts are needed to build part '01'.
``````WITH RPL (PART, SUBPART, QUANTITY) AS
(
SELECT ROOT.PART, ROOT.SUBPART, ROOT.QUANTITY
FROM PARTLIST ROOT
WHERE ROOT.PART = '01'
UNION ALL
SELECT PARENT.PART, CHILD.SUBPART, PARENT.QUANTITY*CHILD.QUANTITY
FROM RPL PARENT, PARTLIST CHILD
WHERE PARENT.SUBPART = CHILD.PART
)
SELECT PART, SUBPART, SUM(QUANTITY) AS "Total QTY Used"
FROM RPL
GROUP BY PART, SUBPART
ORDER BY PART, SUBPART;``````

In the preceding query, the select list of the second operand of the UNION in the recursive common table expression, identified by the name RPL, shows the aggregation of the quantity. To find out how much of a subpart is used, the quantity of the parent is multiplied by the quantity per parent of a child. If a part is used multiple times in different places, it requires another final aggregation. This is done by the grouping over the common table expression RPL and using the SUM aggregate function in the select list of the main fullselect.

The result of the query is as follows:
``````  PART     SUBPART  Total Qty Used
-------- -------- --------------
01       02                    2
01       03                    3
01       04                    4
01       05                   14
01       06                   15
01       07                   18
01       08                   40
01       09                   44
01       10                  140
01       11                  140
01       12                  294
01       13                  150
01       14                  144``````

Looking at the output, consider the line for subpart '06'. The total quantity used value of 15 is derived from a quantity of 3 directly for part '01' and a quantity of 6 for part '02' which is needed 2 times by part '01'.

## Example 3: Controlling depth

The question might come to mind, what happens when there are more levels of parts in the table than you are interested in for your query? That is, how is a query written to answer the question, What are the first two levels of parts needed to build the part identified by '01'? For the sake of clarity in the example, the level is included in the result.
``````WITH RPL (LEVEL, PART, SUBPART, QUANTITY) AS
(
SELECT 1,               ROOT.PART, ROOT.SUBPART, ROOT.QUANTITY
FROM PARTLIST ROOT
WHERE ROOT.PART = '01'
UNION ALL
SELECT PARENT.LEVEL+1, CHILD.PART, CHILD.SUBPART, CHILD.QUANTITY
FROM RPL PARENT, PARTLIST CHILD
WHERE PARENT.SUBPART = CHILD.PART
AND PARENT.LEVEL < 2
)
SELECT PART, LEVEL, SUBPART, QUANTITY
FROM RPL;``````

This query is similar to example 1. The column LEVEL was introduced to count the levels from the original part. In the initialization fullselect, the value for the LEVEL column is initialized to 1. In the subsequent fullselect, the level from the parent is incremented by 1. Then to control the number of levels in the result, the second fullselect includes the condition that the parent level must be less than 2. This ensures that the second fullselect only processes children to the second level.

The result of the query is:
``````  PART     LEVEL       SUBPART  QUANTITY
-------- ----------- -------- -----------
01                 1 02                 2
01                 1 03                 3
01                 1 04                 4
01                 1 06                 3
02                 2 05                 7
02                 2 06                 6
03                 2 07                 6
04                 2 08                10
04                 2 09                11
06                 2 12                10
06                 2 13                10``````