Application Programming and SQL Guide - Kmlinux

|
|
|
#
#
#
#
#
#
Summarizing group values: GROUP BY
Use GROUP BY to group rows by the values of one or more columns or by the
results of an expression. You can then apply aggregate functions to each group.
Except for the columns that are named in the GROUP BY clause, the SELECT
statement must specify any other selected columns as an operand of one of the
aggregate functions.
Example: GROUP BY clause using one column: The following SQL statement
lists, for each department, the lowest and highest education level within that
department:
SELECT WORKDEPT, MIN(EDLEVEL), MAX(EDLEVEL)
FROM DSN8810.EMP
GROUP BY WORKDEPT;
If a column that you specify in the GROUP BY clause contains null values, DB2
considers those null values to be equal. Thus, all nulls form a single group.
When it is used, the GROUP BY clause follows the FROM clause and any WHERE
clause, and precedes the ORDER BY clause.
You can group the rows by the values of more than one column.
Example: GROUP BY clause using more than one column: The following
statement finds the average salary for men and women in departments A00 and
C01:
SELECT WORKDEPT, SEX, AVG(SALARY) AS AVG_SALARY
FROM DSN8810.EMP
WHERE WORKDEPT IN (’A00’, ’C01’)
GROUP BY WORKDEPT, SEX;
The result table looks similar to the following output:
WORKDEPT SEX AVG_SALARY
======== === ==============
A00 F 49625.00000000
A00 M 35000.00000000
C01 F 29722.50000000
DB2 groups the rows first by department number and then (within each department)
by sex before it derives the average SALARY value for each group.
You can also group the rows by the results of an expression
Example: GROUP BY clause using a expression: The following statement groups
departments by their leading characters, and lists the lowest and highest education
level for each group:
SELECT SUBSTR(WORKDEPT,1,1), MIN(EDLEVEL), MAX(EDLEVEL)
FROM DSN8810.EMP
GROUP BY SUBSTR(WORKDEPT,1,1);
Chapter 1. Retrieving data 11