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5 Aggregating Data Using Group Functions

5 Aggregating Data Using Group Functions. Important Legal Notice: Materials on this lecture are from a book titled “Oracle Education” by Kochhar, Gravina, and Nathan (1999), published by Oracle Corp. For further information, visit www.oracle.com

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5 Aggregating Data Using Group Functions

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  1. 5 Aggregating Data Using Group Functions Important Legal Notice: Materials on this lecture are from a book titled “Oracle Education” by Kochhar, Gravina, and Nathan (1999), published by Oracle Corp. For further information, visit www.oracle.com This presentation must be used for only education purpose for students at Central Washington University which is a member of Oracle Academic Initiatives (OAI) and has used Oracle systems for HRIS & Accounting Systems as a database platform embedded on its PeopleSoft ERP system, since 1999.

  2. Objectives 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 using the GROUP BY clause • Include or exclude grouped rows by using the HAVING clause

  3. What are Group Functions? Group functions operate on sets of rows to give one result per group. “maximum salary in the EMP table” MAX(SAL) ---------------- 5000

  4. Types of Group Functions • AVG • COUNT • MAX • MIN • STDDEV • SUM • VARIANCE

  5. Using Group Functions SELECT [column,] group_function(column) FROM table [WHERE condition] [GROUP BYcolumn] [HAVING ….] [ORDER BY column];

  6. Using AVG and SUM Functions You can use AVG and SUM for numeric data. SQL> SELECT AVG(sal), MAX(sal), MIN(sal), SUM(sal) 3 FROM emp 4 WHERE job LIKE ‘SALES%’; AVG(SAL) MAX(SAL) MIN(SAL) SUM(SAL) -------------- --------------- --------------- -------------- 1400 1600 1250 5600

  7. Using MIN and MAX Functions You can use MIN and MAX for any datatype. SQL> SELECT MIN(hiredate), MAX(hiredate) 2 FROM emp; MIN(HIRED MAX(HIRED ---------------- ----------------- 17-DEC-80 12-JAN-83

  8. Using the COUNT Function COUNT (*) returns the number of rows in a table. SQL> SELECT COUNT(*) 2 FROM emp 3 WHERE deptno = 30; COUNT(*) -------------- 6

  9. Using the COUNT Function COUNT(expr) returns the number of non-null rows. SQL> SELECT COUNT(comm) 2 FROM emp 3 WHERE deptno = 30; COUNT(COMM) ------------------- 4

  10. Group Functions and Null Values Group functions ignore null values in the column. SQL> SELECT AVG(comm) 2 FROM emp; AVG(COMM) --------------- 550

  11. Using the NVL Function with Group Functions The NVL function forces group functions to include null values. SQL> SELECT AVG(NVL(comm,0)) 2 FROM emp; AVG(NVL(COMM,0)) ---------------------------- 157.14286

  12. Creating Groups of Data DEPTNO SAL ------------ ------------------- 10 2450 10 5000 10 1300 20 800 20 1100 20 3000 20 3000 20 2975 30 1600 30 2850 30 1250 30 950 30 1500 30 1250 2649.6667 “average salary in EMP table for each department” DEPTNO AVG(SAL) ------------ --------------- 10 2916.6667 20 2175 30 1566.6667 2175 1566.6667

  13. Creating Groups of Data:GROUP BY Clause SELECT [column,] group_function(column) FROM table [WHERE condition] [GROUP BY group_by_expression] [ORDER BY column]; Divide rows in a table into smaller groups by using the GROUP BY clause.

  14. Using the GROUP BY Clause All columns in the SELECT list that are not in group functions must be in the GROUP BY clause. SQL> SELECT deptno, AVG(sal) 2 FROM emp 3 GROUP BY deptno; • DEPTNO AVG(SAL) • ------------ -------------- • 2916.6667 • 2175 • 30 1566.6667

  15. Using the GROUP BY Clause The GROUP BY column does not have to be in the SELECT list. SQL> SELECT AVG(sal) 2 FROM emp 3 GROUP BY deptno; AVG(SAL) -------------- 2916.6667 2175 1566.6667

  16. Grouping by More Than One Column EMP DEPTNO JOB SAL ---------- ---------- ---------10 MANAGER 2450 10 PRESIDENT 5000 10 CLERK 1300 20 CLERK 800 20 CLERK 1100 20 ANALYST 3000 20 ANALYST 3000 20 MANAGER 2850 30 SALESMAN 1250 30 MANAGER 2850 30 SALESMAN 1250 30 CLERK 950 30 SALESMAN 1500 30 SALESMAN 1250 . . . 14 rows selected. DEPTNO JOB SUM(SAL) ------------ ------------ -------------- 10 CLERK 1300 10 MANAGER 2450 10 PRESIDENT 5000 20 ANALYST 6000 20 CLERK 1900 20 MANAGER 2975 30 CLERK 950 30 MANAGER 2850 30 SALESMAN 5600 “sum salaries in the EMP table for each job, grouped by department.”

  17. Using the GROUP BY Clause on Multiple Columns SQL> SELECT deptno, job, sum(sal) 2 FROM emp 3 GROUP BY deptno, job; DEPTNO JOB ------------ -------------- 10 CLERK 10 MANAGER 10 PRESIDENT 20 ANALYST 20 CLERK . . . 9 rows selected. SUM(SAL) -------------- 1300 2450 5000 6000 1900

  18. Illegal Queries Using Group Functions Any column or expression in the SELECT list that is not an aggregate function must be in the GROUP BY clause. SQL> SELECT deptno, COUNT(ename) 2 FROM emp; Column is missing in the GROUP BY clause SELECT deptno, COUNT(ename) * ERROR at line 1: ORA-00937: not a single-group group function

  19. Illegal QueriesUsing Group Functions • You cannot use the WHERE clause to restrict groups. • You use the HAVING clause to restrict groups. SQL> SELECT deptno, AVG(sal) 2 FROM emp3 WHERE AVG(sal) > 20004 GROUP BY deptno; Cannot use the WHERE clause to restrict groups WHERE AVG (sal) > 2000 * ERROR at line 3: ORA-00934: group function is not allowed here

  20. Excluding Group Results DEPTNO SAL ----------- ------------------- 10 2450 10 5000 10 1300 20 800 20 1100 20 3000 20 3000 20 2975 30 1600 30 2850 30 1250 30 950 30 1500 30 1250 5000 “maximum salary per departmentgreater than $2900” DEPTNO MAX(SAL) ------------ --------------- 10 5000 20 3000 3000 2850

  21. Excluding Group Results: HAVING Clause • Use the HAVING clause to restrict groups • Rows are grouped. • The group function is applied. • Groups matching the HAVING clause are displayed. SELECT column, group_function FROM table [WHERE condition] [GROUP BY group_by_expression] [HAVINGgroup_condition] [ORDER BY column];

  22. Using the HAVING Clause SQL> SELECT deptno, max(sal) 2 FROM emp3 GROUP BY deptno4 HAVING max(sal)>2900; DEPTNO ------------ 10 20 MAX(SAL) -------------- 5000 3000

  23. Using the HAVING Clause SQL> SELECT job, SUM(sal) PAYROLL2 FROM emp3 WHERE job NOT LIKE ‘SALES%’4 GROUP BY job5 HAVING SUM(sal) > 50006 ORDER BY SUM(sal); JOB -------------- ANALYSTMANAGER PAYROLL ------------ 60008275

  24. Nesting Group Functions Display the maximum average salary. SQL> SELECT max(avg(sal))2 FROM emp3 GROUP BY deptno; MAX(AVG(SAL) ) ------------------------ 2916.6667

  25. Summary SELECT column, group_function(column) FROM table [WHERE condition] [GROUP BY group_by_expression] [HAVING group_condition] [ORDER BY column]; • Order of evaluation of the clauses: • WHERE clause • GROUP BY clause • HAVING clause

  26. Practice Overview • Showing different queries that use group functions • Grouping by rows to achieve more than one result • Excluding groups by using the HAVING clause

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