The document discusses common table expressions (CTE), which allow the user to define temporary result sets that can be referenced within the scope of a SELECT, INSERT, UPDATE or DELETE statement. CTEs are useful for queries involving multiple aggregates or hierarchical data. The document also covers recursive CTEs, which allow querying hierarchical data by defining an initial seed subquery and a recursive subquery using UNION ALL.

1. *Property of STI K0019
Common Table
Expressions
 What is Common Table Expressions?
 Using Common Table Expressions
 Application of Common Table Expressions
 What is Recursive Common Table Expressions?
 Restrictions on Recursive Common Table
Expressions

2. *Property of STI K0019
What is Common Table
Expressions?
 Common table expressions (CTE) are temporary result set
that are known only within the scope of a single SELECT,
INSERT, UPDATE, DELETE or CREATE VIEW statement.
 Common table expressions are generally useful in a
query that involves multiple aggregate functions.

3. *Property of STI K0019
Using Common Table
Expressions
 The common table expressions are defined using the WITH clause.
 Example 1:
WITH CountEmployees(dept_id, n) AS
( SELECT dept_id, count(*) AS n
FROM employee GROUP BY dept_id )
SELECT dept_id, n
FROM CountEmployees
WHERE n = ( SELECT max(n)
FROM CountEmployees )

4. *Property of STI K0019
Using Common Table
Expressions
 Example 2:
WITH CountEmployees(dept_id, n) AS
( SELECT dept_id, count(*) AS n
FROM employee GROUP BY dept_id )
SELECT a.dept_id, a.n, b.dept_id, b.n
FROM CountEmployees AS a JOIN CountEmployees AS b
ON a.n = b.n AND a.dept_id < b.dept_id

5. *Property of STI K0019
Using Common Table
Expressions
 Example 3:
WITH
CountEmployees(dept_id, n) AS
( SELECT dept_id, count(*) AS n
FROM employee GROUP BY dept_id ),
DeptPayroll( dept_id, amt ) AS
( SELECT dept_id, sum(salary) AS amt
FROM employee GROUP BY dept_id )
SELECT count.dept_id, count.n, pay.amt
FROM CountEmployees AS count JOIN DeptPayroll AS pay
ON count.dept_id = pay.dept_id
WHERE count.n = ( SELECT max(n) FROM CountEmployees )
OR pay.amt = ( SELECT min(amt) FROM DeptPayroll )

6. *Property of STI K0019
Exercise
 Assume that you need to determine which class has the
most number of students. The student table lists all the
students and specifies in which class each belong. Using
common table expressions, find the following:
1. Extract the class with the most students. Extract the
class with the fewest students.
2. List the class that has the highest GPA of students.

7. *Property of STI K0019
Applications of Common
Table Expressions
 Common table expressions are useful whenever multiple
levels of aggregation must occur within a single query.
 Views within a procedure that must contain a reference
to a program variable.
 Queries that use temporary result set to store a set of
values.

8. *Property of STI K0019
What is Recursive Common
Table Expressions?
 Recursive common table expressions allow you to query
tables that represent hierarchical information.
 A recursive common table expression is composed of an
initial subquery or seed and a recursive subquery.

9. *Property of STI K0019
Example
WITH RECURSIVE
manager ( emp_id, manager_id,
emp_fname, emp_lname, mgmt_level ) AS
( ( SELECT emp_id, manager_id, -- initial subquery
emp_fname, emp_lname, 0
FROM employee AS e
WHERE manager_id = emp_id )
UNION ALL
( SELECT e.emp_id, e.manager_id, -- recursive subquery
e.emp_fname, e.emp_lname, m.mgmt_level + 1
FROM employee AS e JOIN manager AS m
ON e.manager_id = m.emp_id
AND e.manager_id <> e.emp_id
AND m.mgmt_level < 20 ) )
SELECT * FROM manager
ORDER BY mgmt_level, emp_lname, emp_fname

10. *Property of STI K0019
Restrictions on Recursive
Common Table Expression
 Recursive common table expressions cannot be mutually
recursive.
 The only set operator permitted between the initial
subquery and the recursive subquery is UNION ALL.
 Within the definition of a recursive subquery, a self-
reference to the recursive table expression can appear
only within the FROM clause of the recursive subquery.

11. *Property of STI K0019
Restrictions on Recursive
Common Table Expression
 The recursive subquery cannot contain DISTINCT, or a
GROUP BY or an ORDER BY clause.
 The recursive subquery can not make use of any
aggregate function.
 To prevent runaway recursive queries, an error is
generated if the number of levels of recursion exceeds
the current setting of the MAX_RECURSIVE_ITERATIONS
option.

12. *Property of STI K0019
Exercise
 Using the recursive common table expression, write a
query that displays the Fibonacci sequence.
TIP: The Fibonacci sequence is the sequence in which each
number is the sum of the two preceding numbers such as 1,
1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597,
2584, 4181, ... (each number is the sum of the previous two).
The Fibonacci sequence, generated by the rule f1 = f2 = 1 ,
fn+1 = fn + fn-1, is well known in many different areas of
mathematics and science.