Post on 03-Jan-2016
description
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Aggregating Data Using Group Functions
Aggregating Data Using Group Functions
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What Are Group Functions?What Are Group Functions?Group functions operate on sets of rows to give Group functions operate on sets of rows to give one result per group.one result per group.Group functions operate on sets of rows to give Group functions operate on sets of rows to give one result per group.one result per group.EMPEMP
““maximum maximum salary in salary in
the EMP table”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
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Types of Group FunctionsTypes of Group Functions
• AVG
• COUNT
• MAX
• MIN
• STDDEV
• SUM
• VARIANCE
• AVG
• COUNT
• MAX
• MIN
• STDDEV
• SUM
• VARIANCE
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Using Group FunctionsUsing Group Functions
SELECT [column,] group_function(column)FROM table[WHERE condition][GROUP BY column][ORDER BY column];
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Using AVG and SUM FunctionsUsing 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.You can use AVG and SUM for numeric data.You can use AVG and SUM for numeric data.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%';
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Using MIN and MAX FunctionsUsing MIN and MAX Functions
You can use MIN and MAX for any datatype.You can use MIN and MAX for any datatype.You can use MIN and MAX for any datatype.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
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Using the COUNT FunctionUsing the COUNT Function
COUNT(*)--------- 6
SQL> SELECT COUNT(*) 2 FROM emp 3 WHERE deptno = 30;
COUNT(*) returns the number of rows in a COUNT(*) returns the number of rows in a table.table.COUNT(*) returns the number of rows in a COUNT(*) returns the number of rows in a table.table.
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Using the COUNT FunctionUsing the COUNT Function
COUNT(COUNT(exprexpr) returns the number of nonn) returns the number of nonnull rows.ull rows.COUNT(COUNT(exprexpr) returns the number of nonn) returns the number of nonnull rows.ull rows.
SQL> SELECT COUNT(comm) 2 FROM emp 3 WHERE deptno = 30;
COUNT(COMM)----------- 4
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Creating Groups of Data Creating Groups of Data
EMPEMP
““averageaveragesalary salary in EMPin EMPtable table
for each for each department”department”
2916.66672916.6667
21752175
1566.66671566.6667
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
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Creating Groups of Data: GROUP BY Clause
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 Divide rows in a table into smaller groups by using the GROUP BY clause.by using the GROUP BY clause.Divide rows in a table into smaller groups Divide rows in a table into smaller groups by using the GROUP BY clause.by using the GROUP BY clause.
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Using the GROUP BY Clause Using the GROUP BY Clause
All columns in the SELECT list that are not All columns in the SELECT list that are not in group functions must be in the GROUP in group functions must be in the GROUP BY clause.BY clause.
All columns in the SELECT list that are not All columns in the SELECT list that are not in group functions must be in the GROUP in group functions must be in the GROUP BY clause.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
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Using the GROUP BY Clause Using the GROUP BY Clause
The GROUP BY column does not have to The GROUP BY column does not have to be in the SELECT list.be in the SELECT list.The GROUP BY column does not have to The GROUP BY column does not have to be in the SELECT list.be in the SELECT list.
SQL> SELECT AVG(sal) 2 FROM emp 3 GROUP BY deptno;
AVG(SAL)--------- 2916.6667 21751566.6667
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Grouping by More Than One ColumnGrouping by More Than One Column
EMPEMP
““sum salaries in sum salaries in the EMP tablethe EMP tablefor each job, for each job, grouped by grouped by department”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
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Using the GROUP BY Clause on Multiple Columns
Using the GROUP BY Clause 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.
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Illegal Queries Using Group Functions
Illegal Queries Using Group Functions
Any column or expression in the SELECT Any column or expression in the SELECT list that is not an aggregate function must list that is not an aggregate function must be in the GROUP BY clause.be in the GROUP BY clause.
Any column or expression in the SELECT Any column or expression in the SELECT list that is not an aggregate function must list that is not an aggregate function must be in the GROUP BY clause.be in the GROUP BY clause.
SQL> SELECT deptno, COUNT(ename) 2 FROM emp;
SQL> SELECT deptno, COUNT(ename) 2 FROM emp;
SELECT deptno, COUNT(ename) *ERROR at line 1:ORA-00937: not a single-group group function
SELECT deptno, COUNT(ename) *ERROR at line 1:ORA-00937: not a single-group group function
Column missing in the GROUP BY clause
Column missing in the GROUP BY clause
Column missing in the GROUP BY clause
Column missing in the GROUP BY clause
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Illegal Queries Using Group Functions
Illegal Queries Using Group Functions
• You cannot use the WHERE clause to restrict groups.
• You use the HAVING clause to restrict groups.
• 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;
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
WHERE AVG(sal) > 2000 *ERROR at line 3:ORA-00934: group function is not allowed here
Cannot use the WHERE clause
Cannot use the WHERE clause
to
restrict groups
to restrict groups
Cannot use the WHERE clause
Cannot use the WHERE clause
to
restrict groups
to restrict groups
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Excluding Group ResultsExcluding Group Results
““maximummaximumsalarysalary
per departmentper departmentgreater thangreater than
$2900”$2900”
EMPEMP
50005000
30003000
28502850
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
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Excluding Group Results: HAVING Clause
Excluding Group Results: HAVING Clause
Use the HAVING clause to restrict groupsUse the HAVING clause to restrict groups
– Rows are grouped.
– The group function is applied.
– Groups matching the HAVING clause are displayed.
Use the HAVING clause to restrict groupsUse the HAVING clause to restrict groups
– Rows are grouped.
– The group function is applied.
– Groups matching the HAVING clause are displayed.
SELECT column, group_functionFROM table[WHERE condition][GROUP BY group_by_expression][HAVING group_condition][ORDER BY column];
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Using the HAVING ClauseUsing 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
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Using the HAVING ClauseUsing 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 6000MANAGER 8275
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SubqueriesSubqueries
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Using a Subquery to Solve a ProblemUsing a Subquery to Solve a Problem
““Who has a salary greater than Jones’?”Who has a salary greater than Jones’?”““Who has a salary greater than Jones’?”Who has a salary greater than Jones’?”
“Which employees have a salary greater than Jones’ salary?”
Main Query
??
“What is Jones’ salary?”??
Subquery
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SubqueriesSubqueries
• The subquery (inner query) executes once before the main query.
• The result of the subquery is used by the main query (outer query).
• The subquery (inner query) executes once before the main query.
• The result of the subquery is used by the main query (outer query).
SELECT select_listFROM tableWHERE expr operator
(SELECT select_list FROM table);
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2975
SQL> SELECT ename 2 FROM emp 3 WHERE sal > 4 (SELECT sal 5 FROM emp 6 WHERE empno=7566);
Using a SubqueryUsing a Subquery
ENAME----------KINGFORDSCOTT
ENAME----------KINGFORDSCOTT
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Types of SubqueriesTypes of Subqueries• Single-row subquery• Single-row subquery
Main query
Subquery returnsreturns
CLERKCLERK
• Multiple-row subquery• Multiple-row subquery
CLERKCLERKMANAGERMANAGER
Main query
Subquery returnsreturns
• Multiple-column subquery• Multiple-column subquery
CLERK 7900CLERK 7900MANAGER 7698MANAGER 7698
Main query
Subquery returnsreturns
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Single-Row SubqueriesSingle-Row Subqueries
• Return only one row
• Use single-row comparison operators
• Return only one row
• Use single-row comparison operators
Operator
=
>
>=
<
<=
<>
Meaning
Equal to
Greater than
Greater than or equal to
Less than
Less than or equal to
Not equal to
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Executing Single-Row SubqueriesExecuting Single-Row Subqueries
CLERK
1100
ENAME JOB---------- ---------MILLER CLERK
ENAME JOB---------- ---------MILLER CLERK
SQL> SELECT ename, job 2 FROM emp 3 WHERE job = 4 (SELECT job 5 FROM emp 6 WHERE empno = 7369) 7 AND sal > 8 (SELECT sal 9 FROM emp 10 WHERE empno = 7876);
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Using Group Functions in a Subquery
Using Group Functions in a Subquery
800
ENAME JOB SAL---------- --------- ---------SMITH CLERK 800
ENAME JOB SAL---------- --------- ---------SMITH CLERK 800
SQL> SELECT ename, job, sal 2 FROM emp 3 WHERE sal = 4 (SELECT MIN(sal) 5 FROM emp);
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HAVING Clause with SubqueriesHAVING Clause with Subqueries
• The Oracle Server executes subqueries first.
• The Oracle Server returns results into the HAVING clause of the main query.
• The Oracle Server executes subqueries first.
• The Oracle Server returns results into the HAVING clause of the main query.
800
SQL> SELECT deptno, MIN(sal) 2 FROM emp 3 GROUP BY deptno 4 HAVING MIN(sal) > 5 (SELECT MIN(sal) 6 FROM emp 7 WHERE deptno = 20);
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What Is Wrong with This Statement?
What Is Wrong with This Statement?
ERROR:ORA-01427: single-row subquery returns more thanone row
no rows selected
ERROR:ORA-01427: single-row subquery returns more thanone row
no rows selected
SQL> SELECT empno, ename 2 FROM emp 3 WHERE sal = 4 (SELECT MIN(sal) 5 FROM emp 6 GROUP BY deptno);
Single-row operator with
Single-row operator with
multiple-row subquery
multiple-row subquery
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Will This Statement Work?Will This Statement Work?
no rows selectedno rows selected
Subquery returns no values
Subquery returns no values
SQL> SELECT ename, job 2 FROM emp 3 WHERE job = 4 (SELECT job 5 FROM emp 6 WHERE ename='SMYTHE');