How to Execute a Recursive Query in DB2 on z/OS to Fetch Associated CSECTs

Discover how to use recursive queries in DB2 on z/OS to find all associated CSECTs for a given Program without encountering circular references.
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Solving the Problem of Fetching Associated CSECTs from a Program in DB2 on z/OS

When working with databases like DB2 on z/OS, you may find yourself needing to handle hierarchical data. A common scenario is querying a table to find all associated CSECTs (control sections) for a particular Program. This task can be tricky, especially when circular references are involved. Let’s dive into how to set up a recursive query to efficiently fetch all associated CSECTs without getting caught in an infinite loop.

Understanding the Data Structure

Let’s take a look at our source table, PROGRAM, which has the following structure:

ProgramCSECTXYBXMPALBDCDACBCABDBThe Goal

Our objective is to find all the associated CSECTs for a specific Program, such as 'A'. For this case, the expected output should be:

ProgramCSECTALACABBDBXXYThe Challenge of Circular References

In SQL, particularly in recursive queries, there’s a risk of creating circular references—where two or more records point to each other, thus causing an infinite loop. A program 'B' may point to 'D', and 'D' in turn points back to 'B', which can lead your query to run indefinitely.

Crafting the Recursive Query

To avoid these problems, we will enhance our recursive query by adding an additional column, Chain. This column will keep track of the path of Programs that have been processed so far. Here is a step-by-step breakdown of the query that accomplishes this:

Query Breakdown

[[See Video to Reveal this Text or Code Snippet]]

Explanation of the Query

Common Table Expression (CTE): We use a Common Table Expression (WITH clause) to define a recursive query named RPL.

Base Case: In the first part of the CTE, we select the root record (where Program = 'A') and initialize the Chain with the root Program.

Recursive Case: The second part of the query joins the RPL CTE with the Program table using a parent-child relationship:

It looks for children records (CHILD) where the Program matches the current CSECT of the parent.

It appends the CHILD.Program to the Chain, creating a running path of processed Programs.

The clause LOCATE('|' || CHILD.Program || '|', PARENT.Chain) = 0 ensures we don’t revisit any Program that has already been included in the path, thus preventing circular references.

Final Selection: We select distinct entries to remove duplicates and order the output by Program.

Conclusion

By implementing this revised recursive query, you can efficiently fetch all associated CSECTs for a specified Program while elegantly handling any circular references in the data structure. This approach not only simplifies the process but also enhances the reliability of your database queries in DB2 on z/OS.

By leveraging this technique, you can ensure that your database queries are both effective and resilient against common pitfalls like circular references. Happy querying!

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