03 ohp slide handout 1
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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. 1 _________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ 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 3 __________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ 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 ) 2 _________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ 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 ) 4 __________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________
Transcript of 03 ohp slide handout 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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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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F0004
* Property of STI
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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
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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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