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Recursion In Sql
SQL: 1999 permit recursive view definition.
Example: Find all employeemanager pairs, where the employee reports to the manager directly or indirectly.
WITH RECURSIVE empl(employee_name, manager_name) AS (
SELECT employee_name, manager_name
SELECT manager.employee_name, empl.manager_name
FROM manager, empl
WHERE manager.manager_name = empl.employee_name)
This example view, empl, is called the transitive closure of the manager relation.
The Power Of Recursion
- Recursive views make it possible to write queries, such as transitive closure queries, that cannot be written without recursion or iteration.
- Intuition: Without recursion, a non¬recursive non¬iterative program can perform only a fixed number of joins of manager with itself
- This can give only a fixed number of levels of managers
- Given a program we can construct a database with a greater number of levels of managers on which the program will not work
- Computing transitive closure
- The next slide shows a manager relation
- Each step of the iterative process constructs an extended version of empl from its recursive definition.
- The final result is called the fixed point of the recursive view definition.
- Recursive views are required to be monotonic. That is, if we add tuples to manager the view contains all of the tuples it contained before, plus possibly more.
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