Sql5

Editor's Notes

• #2: Schedule:TimingTopic 35 minutesLecture 40 minutesPractice 75 minutesTotal
• #3: Lesson Aim This lesson further addresses functions. It focuses on obtaining summary information, such as averages, for groups of rows. It discusses how to group rows in a table into smaller sets and how to specify search criteria for groups of rows.
• #4: Group Functions Unlike single-row functions, group functions operate on sets of rows to give one result per group. These sets may be the whole table or the table split into groups.
• #5: Group Functions (continued) Each of the functions accepts an argument. The following table identifies the options that you can use in the syntax:
• #6: Guidelines for Using Group Functions DISTINCT makes the function consider only nonduplicate values; ALL makes it consider every value including duplicates. The default is ALL and therefore does not need to be specified. The datatypes for the arguments may be CHAR, VARCHAR2, NUMBER, or DATE where expr is listed. All group functions except COUNT(*) ignore null values. To substitute a value for null values, use the NVL function. The Oracle Server implicitly sorts the result set in ascending order when using a GROUP BY clause. To override this default ordering, DESC can be used in an ORDER BY clause. Class Management Note Stress the use of DISTINCT and group functions ignoring null values. ALL is the default and is very rarely specified.
• #7: Group Functions You can use AVG, SUM, MIN, and MAX functions against columns that can store numeric data. The example on the slide displays the average, highest, lowest, and sum of monthly salaries for all salespeople.
• #8: Group Functions (continued) You can use MAX and MIN functions for any datatype. The slide example displays the most junior and most senior employee. The following example displays the employee name that is first and the employee name that is the last in an alphabetized list of all employees. Note: AVG, SUM, VARIANCE, and STDDEV functions can be used only with numeric datatypes.
• #9: The COUNT Function The COUNT function has two formats: COUNT(*) COUNT(expr) COUNT(*) returns the number of rows in a table, including duplicate rows and rows containing null values in any of the columns. If a WHERE clause is included in the SELECT statement, COUNT(*) returns the number of rows that satisfies the condition in the WHERE clause. In contrast, COUNT(expr) returns the number of nonnull rows in the column identified by expr. The slide example displays the number of employees in department 30. Class Management Note Demo: l5count1.sql, l5count2.sql Purpose: To illustrate using the COUNT(*) and COUNT(expr) functions.
• #10: The COUNT Function (continued) The slide example displays the number of employees in department 30 who can earn a commission. Notice that the result gives the total number of rows to be four because two employees in department 30 cannot earn a commission and contain a null value in the COMM column. Example Display the number of departments in the EMP table. Display the number of distinct departments in the EMP table.
• #11: Group Functions and Null Values All group functions except COUNT (*) ignore null values in the column. In the slide example, the average is calculated based only on the rows in the table where a valid value is stored in the COMM column. The average is calculated as total commission being paid to all employees divided by the number of employees receiving commission (4).
• #12: Group Functions and Null Values (continued) The NVL function forces group functions to include null values. In the slide example, the average is calculated based on all rows in the table regardless of whether null values are stored in the COMM column. The average is calculated as total commission being paid to all employees divided by the total number of employees in the company (14).
• #13: Groups of Data Until now, all group functions have treated the table as one large group of information. At times, you need to divide the table of information into smaller groups. This can be done by using the GROUP BY clause.
• #14: The GROUP BY Clause You can use the GROUP BY clause to divide the rows in a table into groups. You can then use the group functions to return summary information for each group. In the syntax: group_by_expressionspecifies columns whose values determine the basis forgrouping rows Guidelines If you include a group function in a SELECT clause, you cannot select individual results as well unless the individual column appears in the GROUP BY clause. You will receive an error message if you fail to include the column list. Using a WHERE clause, you can preexclude rows before dividing them into groups. You must include the columns in the GROUP BY clause. You cannot use the column alias in the GROUP BY clause. By default, rows are sorted by ascending order of the columns included in the GROUP BY list. You can override this by using the ORDER BY clause.
• #15: The GROUP BY Clause (continued) When using the GROUP BY clause, make sure that all columns in the SELECT list that are not in the group functions are included in the GROUP BY clause. The example on the slide displays the department number and the average salary for each department. Here is how this SELECT statement, containing a GROUP BY clause, is evaluated: The SELECT clause specifies the columns to be retrieved: Department number column in the EMP table The average of all the salaries in the group you specified in the GROUP BY clause The FROM clause specifies the tables that the database must access: the EMP table. The WHERE clause specifies the rows to be retrieved. Since there is no WHERE clause, by default all rows are retrieved. The GROUP BY clause specifies how the rows should be grouped. The rows are being grouped by department number, so the AVG function that is being applied to the salary column will calculate the average salary for each department.
• #16: The GROUP BY Clause (continued) The GROUP BY column does not have to be in the SELECT clause. For example, the SELECT statement on the slide displays the average salaries for each department without displaying the respective department numbers. Without the department numbers, however, the results do not look meaningful. You can use the group function in the ORDER BY clause. Class Management Note Demonstrate the query with and without the DEPTNO in the SELECT statement.
• #17: Groups Within Groups Sometimes there is a need to see results for groups within groups. The slide shows a report that displays the total salary being paid to each job title, within each department. The EMP table is grouped first by department number, and within that grouping, it is grouped by job title. For example, the two clerks in department 20 are grouped together and a single result (total salary) is produced for all salespeople within the group. Class Management Note Demo: l5order1.sql,l5order2.sql Purpose: To illustrate ordering columns that are grouped by DEPTNO first and ordering columns that are grouped by JOB first.
• #18: Groups Within Groups (continued) You can return summary results for groups and subgroups by listing more than one GROUP BY column. You can determine the default sort order of the results by the order of the columns in the GROUP BY clause. Here is how the SELECT statement on the slide, containing a GROUP BY clause, is evaluated: The SELECT clause specifies the column to be retrieved: Department number in the EMP table Job title in the EMP table The sum of all the salaries in the group that you specified in the GROUP BY clause The FROM clause specifies the tables that the database must access: the EMP table. The GROUP BY clause specifies how you must group the rows: First, the rows are grouped by department number. Second, within the department number groups, the rows are grouped by job title. So the SUM function is being applied to the salary column for all job titles within each department number group.
• #19: Illegal Queries Using Group Functions Whenever you use a mixture of individual items (DEPTNO) and group functions (COUNT) in the same SELECT statement, you must include a GROUP BY clause that specifies the individual items (in this case, DEPTNO). If the GROUP BY clause is missing, then the error message “not a single-group group function” appears and an asterisk (*) points to the offending column. You can correct the error on the slide by adding the GROUP BY clause. Any column or expression in the SELECT list that is not an aggregate function must be in the GROUP BY clause. Class Management Note Demo: l5error.sql Purpose: To illustrate executing a SELECT statement with no GROUP BY clause.
• #21: Restricting Group Results In the same way that you use the WHERE clause to restrict the rows that you select, you use the HAVING clause to restrict groups. To find the maximum salary of each department, but show only the departments that have a maximum salary of more than $2900, you need to do the following: Find the average salary for each department by grouping by department number. Restrict the groups to those departments with a maximum salary greater than $2900.
• #22: The HAVING Clause You use the HAVING clause to specify which groups are to be displayed. Therefore, you further restrict the groups on the basis of aggregate information. In the syntax: group_conditionrestricts the groups of rows returned to those groups for which the specified condition is TRUE The Oracle Server performs the following steps when you use the HAVING clause: Rows are grouped. The group function is applied to the group. The groups that match the criteria in the HAVING clause are displayed. The HAVING clause can precede the GROUP BY clause, but it is recommended that you place the GROUP BY clause first because it is more logical. Groups are formed and group functions are calculated before the HAVING clause is applied to the groups in the SELECT list.
• #23: The HAVING Clause (continued) The slide example displays department numbers and maximum salary for those departments whose maximum salary is greater than $2900. You can use the GROUP BY clause without using a group function in the SELECT list. If you restrict rows based on the result of a group function, you must have a GROUP BY clause as well as the HAVING clause. The following example displays the department numbers and average salary for those departments whose maximum salary is greater than $2900:
• #24: The HAVING Clause (continued) The slide example displays the job title and total monthly salary for each job title with a total payroll exceeding $5000. The example excludes salespeople and sorts the list by the total monthly salary. Class Management Note Demo: l5job1.sql, l5job2.sql Purpose: To illustrate using a WHERE clause to restrict rows by JOB and using a HAVING clause to restrict groups by SUM(SAL).
• #25: Nesting Group Functions Group functions can be nested to a depth of two. The slide example displays the maximum average salary.
• #26: Summary Seven group functions are available in SQL: AVG COUNT MAX MIN SUM STDDEV VARIANCE You can create subgroups by using the GROUP BY clause. Groups can be excluded using the HAVING clause. Place the HAVING and GROUP BY clauses after the WHERE clause in a statement. Place the ORDER BY clause last. The Oracle Server evaluates the clauses in the following order: If the statement contains a WHERE clause, the server establishes the candidate rows. The server identifies the groups specified in the GROUP BY clause. The HAVING clause further restricts result groups that do not meet the group criteria in the HAVING clause.
• #27: Practice Overview At the end of this practice, you should be familiar with using group functions and selecting groups of data. Paper-Based Questions For questions 1–3, circle either True or False. Note: Column aliases are used for the queries. Class Management Note Hint for Practice #7: Advise the students to think about the MGR column in EMP when determining the number of managers, rather than the JOB column.