Are your SQL queries getting too long and hard to manage?

Let’s make them cleaner, faster, and easier to understand—with Common Table Expressions (CTEs)!

What is a CTE?

A Common Table Expression (CTE) is like a temporary table created inside your SQL query. It helps you break down complex logic into smaller, readable parts using the WITH keyword.

Think of it as a named subquery that:

·       Makes your SQL easier to read

·       Can be reused in the same query

·       Helps with recursive logic (like hierarchies)

·       Improves performance and debugging

Why Use CTEs on IBM i?

In Db2 for i, CTEs are super useful for:

·       Simplifying joins and nested queries

·       Avoiding permanent views for temporary logic

·       Writing cleaner SELECT, INSERT, UPDATE, DELETE, and MERGE statements

·       Managing recursive data (like org charts or BOMs) that deal with hierarchical data.

·       CTEs only exist while the query runs—they’re not stored in the database.

Basic CTE Syntax

·       CTEs are defined by the WITH keyword.

·       CTE_NAME specifies the name of the CTE, which can later be referenced within SQL statements.

·       A column name list called COLUMN_LIST is an optional list of column names that can be used with CTE.

·       The CTE_DEFINITION statement defines the result set for the CTE.

·       sql_statement includes the main SQL statement that references and uses the CTE.

Example: Orders from Chennai

Let’s say you have three tables: ORDERS, CUSTOMERS, and PRODUCTS. You want to find all orders placed by customers in CHENNAI, along with product details.

Using a CTE makes this much easier to write and understand.

Result

Note: After IBM i 7.3, CTEs are now materialized once per query, rather than being merged or re-evaluated for each reference. This aligns with SQL standards and improves performance, especially for CTEs referenced multiple times.

Understanding Recursive CTEs on IBM i

Recursive Common Table Expressions (CTEs) are a smart way to handle complex or layered data—like organization charts, bill of materials (BOM), or folder structures.

What makes them special?
They can refer to themselves—which means they can loop through data step by step until all results are collected.

Two Parts of a Recursive CTE

·       Anchor Member: This is the starting point. It gives the first set of results.

·       Recursive Member: This part calls the CTE itself and builds on the anchor results. It keeps running until a condition is met.

In Db2 for i, you use WITH RECURSIVE to define a recursive CTE. The anchor and recursive parts are joined using UNION ALL.

Example: Factorials from 1 to 10

Let’s say you want to calculate the factorials of numbers from 1 to 10 using SQL on IBM i.

Here’s how you can do it with a recursive CTE:

What this does:

·       Starts with 1! = 1

·       Then calculates 2! = 2 × 1, 3! = 3 × 2, and so on…

·       Stops at 10! = 10 × 9 × ... × 1

Result

Why Use Recursive CTEs on IBM i?

·       Great for working with hierarchical data

·       Avoids writing complex loops in your application code

·       Keeps your SQL clean and efficient

·       Fully supported in Db2 for i

Performance Tips and Best Practices for CTEs on IBM i

Optimizing CTEs in DB2 for IBM i ensures efficient execution, especially for large datasets or recursive queries. Here are key tips:

·       Limit Recursion Depth: Include a WHERE condition in the recursive member (e.g., WHERE exponent < 10) to cap iterations and avoid runaway queries.

·       Use Indexes: Create indexes on columns used in joins or filters within the CTE (e.g., parent_id in hierarchies) to accelerate data retrieval and reduce I/O.

·       Minimize CTE Complexity: Simplify large CTEs by splitting them into multiple CTEs or using temporary tables for massive datasets. Exclude unnecessary columns from the CTE definition to improve performance.

When NOT to Use CTEs

·       Simple Queries: For straightforward queries, using a CTE can add unnecessary complexity. A basic SELECT or a simple subquery can be more readable and efficient.

·       Reusability: CTEs are defined and used within a single query. If you need to reuse the same logic across multiple queries or sessions, a view or a stored procedure is a better choice.

Conclusion

Common Table Expressions (CTEs) are a powerful tool for any SQL developer—especially on IBM i systems. They help you write cleaner, more efficient queries and manage complex logic with ease. Whether you are simplifying joints or building recursive queries, CTEs make your SQL code more elegant and maintainable.

So go ahead—start using CTEs in your IBM i projects and take your SQL skills to the next level!