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RETRIEVAL: MULTIPLE TABLES AND AGGREGATION
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Note To be more precise, we should refer to multisets instead of sets in this context. Duplicate values are maintained, as you can see in Listing 8-15. We will discuss multisets in 12.
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You can also group on multiple-column expressions, separated by commas. For example, the query in Listing 8-16 produces an overview of the number of registrations per course. Listing 8-16. Grouping on Two Columns select r.course, r.begindate , count(r.attendee) as attendees from registrations r group by r.course, r.begindate; COURSE -----JAV JAV OAU OAU PLS SQL SQL SQL XML BEGINDATE ATTENDEES ----------- --------13-DEC-1999 5 01-FEB-2000 3 10-AUG-1999 3 27-SEP-2000 1 11-SEP-2000 3 12-APR-1999 4 04-OCT-1999 3 13-DEC-1999 2 03-FEB-2000 2
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This result shows one row for each different (COURSE, BEGINDATE) combination found in the REGISTRATIONS table.
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Note As you can see, the rows in Listing 8-16 are ordered on the columns of the GROUP BY clause. However, if you want a certain ordering of your query results, you should never rely on implicit DBMS behavior and always specify an ORDER BY clause.
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If a column expression on which you apply the GROUP BY clause contains null values, these null values end up together in a separate group. See Listing 8-17 for an example.
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Listing 8-17. GROUP BY and Null Values select e.comm, count(e.empno) from employees e group by e.comm; COMM COUNT(E.EMPNO) -------- -------------0 1 300 1 500 1 1400 1 10 Apparently, we have ten employees without commission.
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8.6 Group Functions
In the previous section, we used the COUNT function to count the number of employees per department and the number of registrations per course. COUNT is an example of a group function. All group functions have two important properties in common: They can be applied only to sets of values. They return a single aggregated value, derived from that set of values.
That s why group functions often occur in combination with GROUP BY (and optionally the HAVING clause, covered in Section 8.7) in SQL commands. The most important Oracle group functions are listed in Table 8-1. Table 8-1. Common Oracle Group Functions
Function
COUNT() SUM() MIN() MAX() AVG() MEDIAN() STATS_MODE()
Description
Number of values Sum of all values Minimum value Maximum value Average value Median (middle value) Modus (most frequent value)
Applicable To
All datatypes Numeric data All datatypes All datatypes Numeric data Numeric or date (time) data All datatypes
RETRIEVAL: MULTIPLE TABLES AND AGGREGATION
STDDEV() VARIANCE()
Standard deviation Statistical variance
Numeric data Numeric data
The last column in Table 8-1 shows the applicable datatypes for all group functions. The functions MIN and MAX are applicable to any datatype, including dates and alphanumeric strings. MIN and MAX need only an ordering (sorting) criterion for the set of values. Note also that you can apply the AVG function only to numbers, because the average is defined as the SUM divided by the COUNT, and the SUM function accepts only numeric data. Let s look at some group function examples in Listing 8-18. Listing 8-18. Some Examples of Group Functions select , , , , from group e.deptno count(e.job) sum(e.comm) avg(e.msal) median(e.msal) employees e by e.deptno;
DEPTNO COUNT(E.JOB) SUM(E.COMM) AVG(E.MSAL) MEDIAN(E.MSAL) -------- ------------ ----------- ----------- -------------10 3 2916.667 2450 20 5 2175 2975 30 6 2200 1541.667 1375
Group Functions and Duplicate Values
If you apply a group function to a set of column values, that set of values may contain duplicate values. By default, these duplicate values are all treated as individual values, contributing to the end result of all group functions applied to the set of values. For example, we have five employees in department 20, but we have only two different jobs in that department. Nevertheless, Listing 8-18 shows 5 as the result of COUNT(e.JOB) for department 20. If you want SQL group functions to ignore duplicate values (except one, of course), you must specify the keyword DISTINCT immediately after the first parenthesis. Although it is syntactically correct, the addition of DISTINCT is meaningless for the MIN and MAX functions. Look at Listing 8-19 for some examples. Listing 8-19. Using the DISTINCT Option for Group Functions select count(deptno), count(distinct deptno) , avg(comm), avg(coalesce(comm,0)) from employees;
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