Page 40 - SQL

however, common table expressions can be used with recursion to emulate that type of function.

The following example generates a common table expression called Numbers with a column i which
has a row for numbers 1-5:

--Give a table name `Numbers" and a column `i` to hold the numbers
WITH Numbers(i) AS (
--Starting number/index
SELECT 1
--Top-level UNION ALL operator required for recursion
UNION ALL
--Iteration expression:
SELECT i + 1
--Table expression we first declared used as source for recursion
FROM Numbers
--Clause to define the end of the recursion
WHERE i < 5
)
--Use the generated table expression like a regular table
SELECT i FROM Numbers;

i

1

2

3

4

5

This method can be used with any number interval, as well as other types of data.

recursively enumerating a subtree

WITH RECURSIVE ManagedByJames(Level, ID, FName, LName) AS (
-- start with this row
SELECT 1, ID, FName, LName
FROM Employees
WHERE ID = 1

UNION ALL

-- get employees that have any of the previously selected rows as manager
SELECT ManagedByJames.Level + 1,
Employees.ID,
Employees.FName,
Employees.LName
FROM Employees
JOIN ManagedByJames
ON Employees.ManagerID = ManagedByJames.ID

ORDER BY 1 DESC -- depth-first search

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