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In the world of database management, recursive queries are an essential tool for handling hierarchical or tree-structured data. They allow us to perform complex operations and retrieve deeply nested data efficiently. This blog post will dive into the fundamentals of Common Table Expressions (CTEs) in MySQL 8.0, and how they can be used to execute recursive queries. We will also touch on similarities and differences in SQL Server 2019, DB2 11.5, Azure SQL, and Azure Synapse.

A Common Table Expression (CTE) provides a temporary result set that is defined within the execution scope of a single SELECT, INSERT, UPDATE, DELETE, or CREATE VIEW statement. A CTE can be thought of as a temporary view, which is only visible to the query that declares it. In MySQL, a CTE starts with the WITH clause, followed by the CTE name, an optional list of column names, and a query that defines the CTE. Here is a basic non-recursive CTE example:

WITH customer_cte AS (
 SELECT customerId, firstName, lastName
 FROM Customers
 WHERE city = 'New York'
)
SELECT * FROM customer_cte;

Recursive CTEs have a unique structure that involves two main components: the anchor member (base result set) and the recursive member (recursive query). The anchor member is typically a simple, non-recursive SELECT statement. The recursive member is unioned with the anchor member and references the CTE within its own query. Here is a simple example:

WITH RECURSIVE employee_hierarchy AS (
 SELECT employeeId, managerId, firstName, lastName
 FROM Employees
 WHERE managerId IS NULL
 UNION ALL
 SELECT E.employeeId, E.managerId, E.firstName, E.lastName
 FROM Employees E INNER JOIN employee_hierarchy EH ON E.managerId = EH.employeeId
)
SELECT * FROM employee_hierarchy;

This recursive CTE starts with the CEO (where managerId is NULL) and recursively fetches all employees under him. The ability to reference the CTE within the CTE itself is the key to its recursive nature.

Recursive CTEs are not only limited to MySQL. They are also supported in SQL Server 2019, DB2 11.5, Azure SQL, and Azure Synapse, with minor syntax differences. For instance, in SQL Server 2019 and Azure SQL, the UNION ALL keyword is replaced with UNION, while in DB2 11.5, the keyword RECURSIVE is optional.

In conclusion, using Common Table Expressions for recursive queries can be a powerful tool in your SQL arsenal. Whether you're using MySQL, SQL Server, DB2, Azure SQL, or Azure Synapse, the ability to use CTEs can greatly simplify the process of writing complex recursive queries. So, the next time you find yourself dealing with hierarchical data, remember to consider using a CTE.