Presentation on theme: "Group Functions Using GROUP BY clause Week 5 – Chapter 5."— Presentation transcript:
Group Functions Using GROUP BY clause Week 5 – Chapter 5
Objectives Group data using the GROUP BY clause Include or exclude grouped rows results by using the HAVING clause
EMPLOYEES maximum salary in salary in the EMPLOYEES table ID SALARY … MAXSAL Group functions without a GROUP BY clause operate on one set of rows to give one row result Group Fns Without GROUP BY SELECT MAX(salary) MAXSAL FROM employees;
GROUP BY Clause SELECT[column], group_function(column),... FROMtable [WHEREcondition] [GROUP BYcolumn] [HAVING expression] [ORDER BYcolumn]; Divide rows into smaller subsets or groups by using the GROUP BY clause.
EMPLOYEES maximum salary in each dept in salary in each dept in EMPLOYEES table DEPT SALARY … 7000 DEPT MAX … 7000 Group functions with a GROUP BY clause operate on sets of rows to give 1 row result per set of rows Group Fns With GROUP BY SELECT department_id DEPT, MAX(salary) MAX FROM employees GROUP BY department_id;
Notes on GROUP BY Clause A SELECT statement with a group function(s) and no GROUP BY clause returns only one row as its result A SELECT statement with a GROUP BY clause can return multiple rows as its result If you include a group function in a SELECT clause, you cannot display values from individual rows unless the individual column is in the GROUP BY clause The GROUP BY column does not have to be in the SELECT list; eg: SELECT SUM(salary) FROM employees GROUP BY department_id
Group Data on More Than 1 Column Data can be grouped into subsets by more than column. For example, salary could be totaled by job_id within each department as follows: SELECT department_id,job_id,SUM(salary) FROM employees GROUP BY department_id, job_id NOTE: All columns not used with group functions that are in the SELECT clause must be listed in the GROUP BY clause DEPARTMENT_ID JOB_ID SUM(SALARY) SA_REP AD_ASST MK_MAN MK_REP ST_MAN5800 … 110AC_ACCOUNT8300
Group Data on more than 1 Column If the order of the columns is changed in the GROUP BY clause then the data is simply presented in a different order. For example, salary could be totaled by department_id within each job as follows: SELECT job_id, department_id,job_id,SUM(salary) FROM employees GROUP BY job_id, department_id JOB_IDDEPARTMENT_IDSUM(SALARY) AD_VP AC_MGR MK_MAN MK_REP SA_MAN SA_REP7000 SA_REP … AC_ACCOUNT
Group Function Error You will receive an error message if you include a group function and GROUP BY clause and an expression referencing a non-group column in a SELECT clause. Example: SELECTlast_name, department_id, AVG(salary) FROMemployees GROUP BY department_id; ERROR at line 1: ORA-00979: not a GROUP BY expression Query does not make sense: there is only 1 value of department_id and average salary to display for many values of last name
Excluding Group Results Use the HAVING clause to exclude set(s) of results from a GROUP BY clause (rows are grouped; group function is applied; groups matching the HAVING clause are displayed). Example, do not display departments having 2 or less employees: SELECT department_id DEPT, MAX(salary) MAX, COUNT(*) FROM employees GROUP BY department_id HAVING COUNT(*) > 2 DEPT MAXCOUNT(*)
HAVING Clause Error Cannot use the HAVING clause to rows - must reference a group function result in HAVING clause. Example, try to exclude IT_PROG from results: SELECT department_id DEPT, MAX(salary) MAX, COUNT(*) FROM employees GROUP BY department_id HAVING job_id LIKE 'IT_PROG' ERROR at line 4: HAVING job_id LIKE 'IT_PROG' ORA-00979: not a GROUP BY expression
HAVING Clause Notes HAVING clause excludes result(s) for sets of rows. Must be used with GROUP BY clause Should only reference group results (or columns data is grouped by)
Important Notes on Group Functions You cannot use the WHERE clause to restrict groups. You must use the HAVING clause to restrict groups. You cannot use group functions in the WHERE clause (WHERE clause used to limit rows, not groups of rows) Any column or expression in the SELECT list that is not an aggregate function must be in the GROUP BY clause.
Nesting Group Functions Group functions can be nested Example: find the highest of the average salary for each department: SELECT MAX(AVG(salary)) MAX FROM employees GROUP BY department_id MAX