Recursive Common Table Expressions (CTEs) are a type of SQL (Structured Query Language) statement that can be used to solve complex query problems in a relational database management system (RDBMS) such as MySQL, Oracle, Microsoft SQL Server, and PostgreSQL.
A CTE is used to reference itself as part of the data it is selecting from the database. The syntax for a CTE is similar to a standard SELECT statement, but allows for more advanced logic.
A CTE can be used to perform recursive computations on a table or set of tables. This type of recursive computation can be used to solve problems that would otherwise require nested subqueries, such as computing the number of leaves in a tree structure or calculating the Fibonacci sequence on a column of numbers.
In addition to making queries more efficient, using recursive CTEs can help reduce the development and maintenance effort required, as a single statement can be used to solve a complex problem.
One example of a recursive CTE can be seen in the widely used “Employee Hierarchy” example, which is used to compute the total number of subordinates an employee has. In this example, the CTE is used to recursively query the table, counting the number of subordinates for each manager.
Overall, recursive CTEs are a powerful tool in SQL that facilitate complex query processing and making development and maintenance easier.
