SQL5.ppt

Aggregating Data
Using Group Functions

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

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

Types of Group Functions
– AVG
– COUNT
– MAX
– MIN
– SUM

SELECT [column,] group_function(column)
FROM table
[WHERE condition]
[GROUP BY column]
[ORDER BY column];

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

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

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

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

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

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

Creating Groups of Data
EMP
“average
salary
in EMP
table
for each
department”
2916.6667
2175
1566.6667
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
DEPTNO AVG(SAL)
------- ---------
10 2916.6667
20 2175
30 1566.6667

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.

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)
--------- ---------
10 2916.6667
20 2175
30 1566.6667

• 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

Grouping by More
Than One Column
EMP
“sum salaries in
the EMP table
for each job,
grouped by
department”
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 2975
30 SALESMAN 1600
30 MANAGER 2850
30 SALESMAN 1250
30 CLERK 950
30 SALESMAN 1500
30 SALESMAN 1250
JOB SUM(SAL)
--------- ---------
CLERK 1300
MANAGER 2450
PRESIDENT 5000
ANALYST 6000
CLERK 1900
MANAGER 2975
CLERK 950
MANAGER 2850
SALESMAN 5600
DEPTNO
--------
10
10
10
20
20
20
30
30
30

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

Illegal Queries
• 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;
SELECT deptno, COUNT(ename)
*
ERROR at line 1:
ORA-00937: not a single-group group function

Illegal Queries
– You cannot use the WHERE clause to restrict groups.
– You use the HAVING clause to restrict groups.
SQL> SELECT deptno, AVG(sal)
2 FROM emp
3 WHERE AVG(sal) > 2000
4 GROUP BY deptno;
WHERE AVG(sal) > 2000
*
ERROR at line 3:
ORA-00934: group function is not allowed here

Excluding Group Results
“maximum
salary
per department
greater than
$2900”
EMP
5000
3000
2850
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
DEPTNO MAX(SAL)
--------- ---------
10 5000
20 3000

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]
[HAVING group_condition]
[ORDER BY column];

Using the HAVING Clause
SQL> SELECT deptno, max(sal)
2 FROM emp
3 GROUP BY deptno
4 HAVING max(sal)>2900;
DEPTNO MAX(SAL)
--------- ---------
10 5000
20 3000

Using the HAVING Clause
SQL> SELECT job, SUM(sal) PAYROLL
2 FROM emp
3 WHERE job NOT LIKE 'SALES%'
4 GROUP BY job
5 HAVING SUM(sal)>5000
6 ORDER BY SUM(sal);
JOB PAYROLL
--------- ---------
ANALYST 6000
MANAGER 8275

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

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

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

SQL5.ppt

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