03 ohp slide handout 1
1 like238 views
Common table expressions (CTE) allow for the creation of temporary tables that exist only within a single SELECT, INSERT, UPDATE, DELETE or CREATE VIEW statement. CTEs can be used to simplify complex queries by breaking them into multiple logical blocks and are useful when multiple levels of aggregation are required. Recursive CTEs allow querying hierarchical data by using an initial subquery and a recursive subquery to iteratively build the results.
03 ohp slide handout 1
- 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. 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 __________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________
- 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. 5 _________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ 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. 7 __________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ 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. 6 _________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ 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 8 __________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________
- 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. 9 _________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ 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. 11 _________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ 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. 10 ________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________ ___________________