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OCA/OCP Oracle Database 11g All-in-One Exam Guide
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The GROUP BY clause
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EXAM TIP A dataset is divided into groups using the GROUP BY clause The grouping attribute is the common key shared by members of each group The grouping attribute is usually a single column but may be multiple columns or an expression that cannot be based on group functions Note that only grouping attributes and group functions are permitted in the SELECT clause when using GROUP BY
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A powerful extension to the GROUP BY clause uses multiple grouping attributes Oracle permits datasets to be partitioned into groups and allows these groups to be further divided into subgroups using a different grouping attribute Consider the following two queries: Query 1: select department_id, sum(commission_pct) from employees where commission_pct is not null group by department_id; Query 2: select department_id, job_id, sum(commission_pct) from employees where commission_pct is not null group by department_id, job_id; Query 1 restricts the rows returned from the EMPLOYEES table to the 35 rows with non-null COMMISSION_PCT values These rows are then divided into two
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groups: 80 and NULL based on the DEPARTMENT_ID grouping attribute The result set contains two rows, which return the sum of the COMMISSION_PCT values for each group Query 2 is similar to the first one except it has an additional item: JOB_ID in both the SELECT and GROUP BY clauses This second grouping attribute decomposes the two groups by DEPARTMENT_ID into the constituent JOB_ID components belonging to the rows in each group The distinct JOB_ID values for rows with DEPARTMENT_ ID=80 are SA_REP and SA_MAN The distinct JOB_ID value for rows with null DEPARTMENT_ID is SA_REP Therefore, query 2 returns two groupings, one that consists of two subgroups, and the other with only one, as shown in Figure 11-4 Exercise 11-2: Group Data Based on Multiple Columns Analysis of staff turnover is a common reporting requirement You are required to create a report that contains the number of employees who left their jobs, grouped by the year in which they left The jobs they performed is also required The results must be sorted in descending order based on the number of employees in each group The report must list the year, the JOB_ID, and the number of employees who left a particular job in that year 1 Start SQL Developer and connect to the HR schema 2 The JOB_HISTORY table contains the END_DATE and JOB_ID columns, which constitute the source data for this report 3 The year component may be extracted using the TO_CHAR function The number of employees who quit a particular job in each year may be obtained using the COUNT(*) function
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OCA/OCP Oracle Database 11g All-in-One Exam Guide
4 Executing the following statement returns the staff turnover report as requested:
select to_char(end_date,'yyyy') "Quitting Year" ,job_id, count(*) "Number of Employees" from job_history group by to_char(end_date,'yyyy'), job_id order by count(*) desc;
Nested Group Functions
Recall that single-row functions may be nested or embedded to any level of depth Group functions may only be nested two levels deep Three formats using group functions are shown here: G1(group_item) = result G1(G2(group_item ) = result G1(G2(G3(group_item))) is NOT allowed Group functions are represented by the letter G followed by a number The first simple form contains no nested functions Examples include the SUM(group_item) or AVG(group_item) functions that return a single result per group The second form supports two nested group functions, like SUM(AVG(group_item)) In this case, a GROUP BY clause is necessary because the average value of the group_item per group is calculated before being aggregated by the SUM function The third form is disallowed by Oracle Consider an expression that nests three group functions If the MAX function is applied to the previous example, the expression MAX(SUM(AVG(group_item))) is formed The two inner group functions return a single value representing the sum of a set of average values This expression becomes MAX(single value), which is not sensible, since a group function cannot be applied to a single value Figure 11-5 demonstrates two queries Both restrict the rows returned to those with DEPARTMENT_ID values of null, 40, and 80 These are then partitioned by their DEPARTMENT_ID values into three groups The first query calculates the sum of the COMMISSION_PCT values for each group and returns the values 015, null, and 765 Query 2 contains the nested group functions, which may be evaluated as follows: AVG(SUM(COMMISSION_PCT)) = (015 + 765) /2 = 39 EXAM TIP Single-row functions may be nested to any level, but group functions may be nested to at most two levels deep The nested function call COUNT(SUM(AVG( X))) returns the error ORA-00935: group function is nested too deeply It is acceptable to nest single-row functions within group functions Consider the following query: SELECT SUM(AVG(LENGTH(LAST_ NAME))) FROM EMPLOYEES GROUP BY DEPARTMENT_ID It calculates the sum of the average length of LAST_NAME values per department
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