What Are Group Functions? Group functions operate on sets of rows to give one result per group. Reporting Aggregated Data Using the Group Functions Maximum salary in table Copyright 2004, Oracle. All rights reserved. Objectives 4-3 Copyright 2004, Oracle. All rights reserved. Types of Group Functions After completing this lesson, you should be able to do the following: Identify the available group functions Describe the use of group functions Group data by using the GROUP BY clause Include or exclude grouped rows by using the HAVING clause AVG COUNT MAX MIN STDDEV SUM VARIANCE Group functions 4-2 Copyright 2004, Oracle. All rights reserved. 4-4 Copyright 2004, Oracle. All rights reserved.
Group Functions: Syntax Using the MIN and MAX Functions SELECT [column,] group_function(column),... [GROUP BY column] You can use MIN and MAX for numeric, character, and date data types. SELECT MIN(hire_date), MAX(hire_date) ; 4-5 Copyright 2004, Oracle. All rights reserved. Using the AVG and SUM Functions 4-7 Copyright 2004, Oracle. All rights reserved. Using the COUNT Function You can use AVG and SUM for numeric data. SELECT AVG(salary), MAX(salary), MIN(salary), SUM(salary) WHERE job_id LIKE '%REP%'; 1 COUNT(*) returns the number of rows in a table: SELECT COUNT(*) WHERE department_id = 50; 2 COUNT(expr) returns the number of rows with nonnull values for the expr: SELECT COUNT(commission_pct) WHERE department_id = 80; 4-6 Copyright 2004, Oracle. All rights reserved. 4-8 Copyright 2004, Oracle. All rights reserved.
Using the DISTINCT Keyword Creating Groups of Data COUNT(DISTINCT expr) returns the number of distinct non-null values of the expr. To display the number of distinct department values in the table: SELECT COUNT(DISTINCT department_id) ; 4400 9500 3500 6400 10033 Average salary in table for each department 4-9 Copyright 2004, Oracle. All rights reserved. Group Functions and Null Values 4-11 Copyright 2004, Oracle. All rights reserved. Creating Groups of Data: GROUP BY Clause Syntax 1 2 Group functions ignore null values in the column: SELECT AVG(commission_pct) ; The NVL function forces group functions to include null values: SELECT AVG(NVL(commission_pct, 0)) ; SELECT column, group_function(column) [GROUP BY group_by_expression] You can divide rows in a table into smaller groups by using the GROUP BY clause. 4-10 Copyright 2004, Oracle. All rights reserved. 4-12 Copyright 2004, Oracle. All rights reserved.
Using the GROUP BY Clause Grouping by More Than One Column All columns in the SELECT list that are not in group functions must be in the GROUP BY clause. SELECT department_id, AVG(salary) GROUP BY department_id ; Add the salaries in the table for each job, grouped by department 4-13 Copyright 2004, Oracle. All rights reserved. Using the GROUP BY Clause 4-15 Copyright 2004, Oracle. All rights reserved. Using the GROUP BY Clause on Multiple Columns The GROUP BY column does not have to be in the SELECT list. SELECT AVG(salary) GROUP BY department_id ; SELECT department_id dept_id, job_id, SUM(salary) GROUP BY department_id, job_id ; 4-14 Copyright 2004, Oracle. All rights reserved. 4-16 Copyright 2004, Oracle. All rights reserved.
Illegal Queries Using Group Functions Restricting Group Results Any column or expression in the SELECT list that is not an aggregate function must be in the GROUP BY clause: SELECT department_id, COUNT(last_name) ; SELECT department_id, COUNT(last_name) * ERROR at line 1: ORA-00937: not a single-group group function Column missing in the GROUP BY clause The maximum salary per department when it is greater than $10,000 4-17 Copyright 2004, Oracle. All rights reserved. Illegal Queries Using Group Functions 4-19 Copyright 2004, Oracle. All rights reserved. Restricting Group Results with the HAVING Clause You cannot use the WHERE clause to restrict groups. You use the HAVING clause to restrict groups. You cannot use group functions in the WHERE clause. SELECT department_id, AVG(salary) WHERE AVG(salary) > 8000 GROUP BY department_id; WHERE AVG(salary) > 8000 * ERROR at line 3: ORA-00934: group function is not allowed here Cannot use the WHERE clause to restrict groups When you use the HAVING clause, the Oracle server restricts groups as follows: 1. Rows are grouped. 2. The group function is applied. 3. Groups matching the HAVING clause are displayed. SELECT column, group_function [GROUP BY group_by_expression] [HAVING group_condition] 4-18 Copyright 2004, Oracle. All rights reserved. 4-20 Copyright 2004, Oracle. All rights reserved.
Using the HAVING Clause Nesting Group Functions SELECT department_id, MAX(salary) GROUP BY department_id HAVING MAX(salary)>10000 ; Display the maximum average salary: SELECT MAX(AVG(salary)) GROUP BY department_id; 4-21 Copyright 2004, Oracle. All rights reserved. Using the HAVING Clause 4-23 Copyright 2004, Oracle. All rights reserved. Summary SELECT job_id, SUM(salary) PAYROLL WHERE job_id NOT LIKE '%REP%' GROUP BY job_id HAVING SUM(salary) > 13000 ORDER BY SUM(salary); In this lesson, you should have learned how to: Use the group functions COUNT, MAX, MIN, and AVG Write queries that use the GROUP BY clause Write queries that use the HAVING clause SELECT column, group_function [GROUP BY group_by_expression] [HAVING group_condition] 4-22 Copyright 2004, Oracle. All rights reserved. 4-24 Copyright 2004, Oracle. All rights reserved.
Practice 4 Determine the validity of the following three statements. Circle either True or False. 1. Group functions work across many rows to produce one result per group. True/False 2. Group functions include nulls in calculations. True/False 3. The WHERE clause restricts rows prior to inclusion in a group calculation. True/False Practice 4 (continued) 6. Write a query to display the number of people with the same job. The HR department needs the following reports: 4. Find the highest, lowest, sum, and average salary of all employees. Label the columns Maximum, Minimum, Sum, and Average, respectively. Round your results to the nearest whole number. Place your SQL statement in a text file named lab_04_04.sql. 5. Modify the query in lab_04_04.sql to display the minimum, maximum, sum, and average salary for each job type. Resave lab_04_04.sql as lab_04_05.sql. Run the statement in lab_04_05.sql. Generalize the query so that the user in the HR department is prompted for a job title. Save the script to a file named lab_04_06.sql. 7. Determine the number of managers without listing them. Label the column Number of Managers. Hint: Use the MANAGER_ID column to determine the number of managers. 8. Find the difference between the highest and lowest salaries. Label the column DIFFERENCE. If you have time, complete the following exercises: 9. Create a report to display the manager number and the salary of the lowest-paid employee for that manager. Exclude anyone whose manager is not known. Exclude any groups where the minimum salary is $6,000 or less. Sort the output in descending order of salary. Oracle Database 10g: SQL Fundamentals I 4-26 Oracle Database 10g: SQL Fundamentals I 4-27