ADDBASE prelim CTE notes 3rd year 1st sem

1. F0004
* Property of STI
Page 1 of 11
Common Table Expressions
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.
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F0004
* Property of STI
Page 3 of 11
Common Table Expressions
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
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F0004
* Property of STI
Page 2 of 11
Common Table Expressions
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 )
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F0004
* Property of STI
Page 4 of 11
Common Table Expressions
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 )
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2. F0004
* Property of STI
Page 5 of 11
Common Table Expressions
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.
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F0004
* Property of STI
Page 7 of 11
Common Table Expressions
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.
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F0004
* Property of STI
Page 6 of 11
Common Table Expressions
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.
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F0004
* Property of STI
Page 8 of 11
Common Table Expressions
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
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3. F0004
* Property of STI
Page 9 of 11
Common Table Expressions
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.
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F0004
* Property of STI
Page 11 of 11
Common Table Expressions
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.
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F0004
* Property of STI
Page 10 of 11
Common Table Expressions
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.
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