Column Functions and Grouping

Transcription

1 Column Functions and Grouping Course materials may not be reproduced in whole or in part without the prior written permission of IBM

2 Unit Objectives After completing this unit, you should be able to: Describe the difference between scalar and column functions List the more common DB2 column functions Group rows into sets based on one or more columns

3 QL Functions Function_name(f_argument) Column calar Functions Functions AVG(ALARY) YEAR(HIREDATE) }

4 Column Functions Compute Total UM(expression) Calculate Average Find Minimum Value Find Maximum Value Determine the number of rows meeting the search condition Determine the number of unique non-null values in a column AVG(expression) MIN(expression) MAX(expression) COUNT(*) COUNT_BIG(*) COUNT(DITINCT col-name) COUNT_BIG(DITINCT colname)

5 ample Column Functions ELECT FROM UM(ALARY) A UM, AVG(ALARY) A AVG, MIN(ALARY ) A MIN, MAX(ALARY) A MAX, COUNT(*) A COUNT, COUNT (DITINCT WORKDEPT) A DEPT EMPLOYEE UM AVG MIN MAX COUNT DEPT

6 Beware of Nulls UM(ALARY)+UM(BONU)+UM(COMM) VERU UM(ALARY+BONU+COMM) EMPNO ALARY BONU COMM ALARY+BONU+COMM NULL 300 NULL NULL NULL um: > ===== ===== ==== ====

7 Column Functions Based on ubset I need the sum of all salaries for workdepts beginning with the letter D ELECT UM(ALARY) A UM FROM EMPLOYEE WHERE WORKDEPT LIKE 'D%' UM

8 GROUP BY I need a listing of the salaries for all employees in the departments A00, B01, and C01. In addition, for these departments, I want the totals spent for salaries. ELECT WORKDEPT, ALARY FROM EMPLOYEE WHERE WORKDEPT IN ('A00', 'B01', 'C01') ORDER BY WORKDEPT ELECT WORKDEPT, UM(ALARY) A UM FROM EMPLOYEE WHERE WORKDEPT IN ('A00', 'B01', 'C01') GROUP BY WORKDEPT ORDER BY WORKDEPT WORKDEPT ALARY A00 A00 A00 B01 C01 C01 C WORKDEPT A00 B01 C01 UM

9 GROUP BY more than one Column Find out the average salary per education level for each department group (given by the first character of the department number), for education levels 18 and higher ELECT UBTR(WORKDEPT,1,1) A DEPT_GROUP, EDLEVEL, DECIMAL(AVG(ALARY),9,2) A AVGAL FROM EMPLOYEE WHERE EDLEVEL >= 18 GROUP BY UBTR(WORKDEPT,1,1), EDLEVEL DEPT_GROUP EDLEVEL AVGAL A A B C C D

10 The Hardest to Remember Rule in All of QL! Only applies to queries with COLUMN FUNCTION in the ELECT Clause If a ELECT clause has COLUMN functions AND columns not in COLUMN functions ALL columns not in COLUMN functions must be included in the GROUP BY clause

11 GROUP BY, ORDER BY ELECT MAJPROJ, DEPTNO, AVG(PRTAFF) A "AVG(PRTAFF)", COUNT(*) A "COUNT(*)" FROM PROJECT GROUP BY MAJPROJ, DEPTNO ORDER BY MAJPROJ DEC MAJPROJ DEPTNO AVG(PRTAFF) COUNT(*) - C D E OP2010 E OP2000 E OP1000 E MA2110 D MA2100 B MA2100 D AD3110 D AD3100 D

12 GROUP BY, HAVING (1 of 2) Now, I just want to see departments with total spent for salaries higher than ELECT WORKDEPT, UM(ALARY) A UM FROM EMPLOYEE WHERE WORKDEPT IN ('A00', 'B01', 'C01') GROUP BY WORKDEPT ORDER BY WORKDEPT ELECT WORKDEPT, UM(ALARY) A UM FROM EMPLOYEE WHERE WORKDEPT IN ('A00', 'B01', 'C01') GROUP BY WORKDEPT HAVING UM(ALARY) > ORDER BY WORKDEPT WORKDEPT A00 B01 C01 UM WORKDEPT A00 C01 UM

13 GROUP BY, HAVING (2 of 2) By department, I need a listing of jobs, excluding managers, designer, and field representative, with an average salary higher than $25,000. ELECT WORKDEPT, JOB, AVG(ALARY) A AVG FROM EMPLOYEE WHERE JOB NOT IN ('MANAGER', 'DEIGNER', 'FIELDREP') GROUP BY WORKDEPT, JOB HAVING AVG(ALARY) > ORDER BY WORKDEPT, JOB WORKDEPT A00 A00 A00 C01 JOB CLERK PRE ALEREP ANALYT AVG

14 Examples with HAVING Display the departments with more than one employee ELECT WORKDEPT, COUNT(* ) A NUMB FROM EMPLOYEE GROUP BY WORKDEPT ORDER BY NUMB, WORKDEPT ELECT WORKDEPT, COUNT(* ) A NUMB FROM EMPLOYEE GROUP BY WORKDEPT HAVING COUNT(*) > 1 ORDER BY NUMB, WORKDEPT WORKDEPT NUMB B01 1 E01 1 A00 3 C01 3 E21 4 E11 5 D21 6 D11 9 WORKDEPT NUMB A00 3 C01 3 E21 4 E11 5 D21 6 D11 9

15 Restrictions Column functions may be specified only in ELECT HAVING ELECT may specify only Column functions Columns specified in 'GROUP BY' HAVING may specify Any column function on any column in a table being queried. This column need not be in the ELECT. Column functions may not be nested

16 ELECT tatement - ix Clauses ELECT DEP, JOB, AVG(AL) FROM EMPL WHERE JOB <> 'M' GROUP BY DEP, JOB HAVING AVG(AL) > ORDER BY 3 DEC

17 Conceptual Execution of a ELECT (1 of 2) BAE TABLE WHERE GROUP BY JOB AL DEP BLU M RED BLU C GRE GRE M BLU RED C GRE RED M GRE RED GRE C BLU RED BLU JOB AL DEP BLU C C C BLU GRE GRE RED GRE RED RED GRE BLU RED BLU JOB AL DEP C BLU C C BLU BLU BLU GRE GRE GRE GRE RED RED RED RED

18 Conceptual Execution of a ELECT (2 of 2) HAVING ELECT ORDER BY JOB AL DEP DEP JOB AVG(AL) DEP JOB AVG(AL) BLU BLU BLU BLU GRE RED BLU GRE GRE GRE RED RED RED RED RED 30500

19 Checkpoint 1. True or False? A scalar function produces a summary row for a set of rows. 2. A ELECT statement whose ELECT list includes a column function (UM, AVG, MIN, MAX, COUNT, and so forth) and three columns not in column functions does not require a GROUP BY clause. 3. Which clause qualifies groups for further processing? a. ELECT b. FROM c. WHERE d. GROUP BY e. HAVING f. ORDER BY 4. True or False? The following query is syntactically correct. ELECT WORKDEPT, AVG(ALARY) FROM EMPLOYEE WHERE AVG(ALARY) > GROUP BY WORKDEPT HAVING COUNT(*) > 3 ORDER BY 2 DEC

20 Checkpoint olutions 1. False. A column function produces a summary row for a set of rows. 2. False. A GROUP BY is required and all three columns not in column functions must be listed in the GROUP BY clause. 3. e 4. False. Column functions may not be used in a WHERE clause.

21 Unit ummary Having completed this unit, you should be able to: Describe the difference between scalar and column functions List the more common DB2 column functions Group rows into sets based on one or more columns