sql: part 3

SQL: Part 3SQL: Part 3

Original materials supplied by the Oracle Academic Initiative (OAI). Edited for classroom use by Professor Laku Chidambaram.

Not for commercial use. Do not redistribute.

Original materials supplied by the Oracle Academic Initiative (OAI). Edited for classroom use by Professor Laku Chidambaram.

Not for commercial use. Do not redistribute.

2

OUTLINEOUTLINE

Group FunctionsGroup Functions

Sub-queriesSub-queries

Set OperationsSet Operations





Original materials supplied by the Oracle Academic Initiative (OAI); edited for classroom use by Professor Laku Chidambaram. Do not redistribute.Original materials supplied by the Oracle Academic Initiative (OAI); edited for classroom use by Professor Laku Chidambaram. Do not redistribute.

4

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

5

Types of Group FunctionsTypes of Group Functions

• AVG

• COUNT

• MAX

• MIN

• STDDEV

• SUM

• VARIANCE

• AVG

• COUNT

• MAX

• MIN

• STDDEV

• SUM

• VARIANCE

6

Using Group FunctionsUsing Group Functions

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

7

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%'

8

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

9

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.

10

Using the COUNT FunctionUsing the COUNT Function

COUNT(COUNT(exprexpr) returns the number of ) returns the number of nonnull rows.nonnull rows.COUNT(COUNT(exprexpr) returns the number of ) returns the number of nonnull rows.nonnull rows.

SQL> SELECT COUNT(comm) 2 FROM emp 3 WHERE deptno = 30

COUNT(COMM)----------- 4

11

Group Functions and Null ValuesGroup Functions and Null Values

Group functions ignore null values in the Group functions ignore null values in the column.column.Group functions ignore null values in the Group functions ignore null values in the column.column.

SQL> SELECT AVG(comm) 2 FROM emp

AVG(COMM)--------- 550

12

Using the ISNULL Function with Group Functions

Using the ISNULL Function with Group Functions

The ISNULL function forces group The ISNULL function forces group functions to include null values.functions to include null values.The ISNULL function forces group The ISNULL function forces group functions to include null values.functions to include null values.

SQL> SELECT AVG(ISNULL(comm,0)) 2 FROM emp

AVG(ISNULL(COMM,0))---------------- 157.14286

13

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

14

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.

15

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

16

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

17

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

18

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.

19

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: not a single-group group function

SELECT deptno, COUNT(ename) *ERROR at line 1: not a single-group group function

Column missing in the GROUP BY clause




20



• 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: group function is not allowed here

WHERE AVG(sal) > 2000 *ERROR at line 3: group function is not allowed hereCannot use the WHERE clause

Cannot use the WHERE clause

to

restrict groups

to restrict groups



to

restrict groups

to restrict groups

21

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

22

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]

23

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

24

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

26

What Is a Subquery?What Is a Subquery?

A subquery is a SELECT statement A subquery is a SELECT statement embedded in a clause of another SQL embedded in a clause of another SQL statement.statement.

A subquery is a SELECT statement A subquery is a SELECT statement embedded in a clause of another SQL embedded in a clause of another SQL statement.statement.

SELECT . . . FROM . . .WHERE . . .

(SELECT . . .FROM . . .WHERE . . .)

MainMainQueryQuery

SubquerySubquery

27

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)

28

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 ---------- KING FORD SCOTT

ENAME ---------- KING FORD SCOTT

29

Correlated SubqueriesCorrelated Subqueries

Used to affect row-by-row processing, Used to affect row-by-row processing, each subquery is executed once for every each subquery is executed once for every row of the outer query.row of the outer query.

Used to affect row-by-row processing, Used to affect row-by-row processing, each subquery is executed once for every each subquery is executed once for every row of the outer query.row of the outer query.

GETcandidate row

EXECUTEinner query using candidate row value

USEvalue(s) from inner query to qualify

candidate row

30

Correlated SubqueriesCorrelated Subqueries

SELECT outer1, outer2, ... FROM table1 alias1 WHERE outer1 operator

(SELECT inner1 FROM table2 alias2 WHERE alias1.outer2 =

alias2.inner1)

The subquery references a column from subquery references a column from a table in the parent query.a table in the parent query.The subquery references a column from subquery references a column from a table in the parent query.a table in the parent query.

31

Using Correlated SubqueriesUsing Correlated Subqueries

Each time the outer queryis processed the

inner query is evaluated.

SQL> SELECT empno, sal, deptno 2 FROM emp outr 3 WHERE sal > (SELECT AVG(sal) 4 FROM emp innr 5 WHERE outr.deptno = innr.deptno)

EMPNO SAL DEPTNO-------- --------- --------- 7839 5000 10 7698 2850 30 7566 2975 20 ... 6 rows selected.

EMPNO SAL DEPTNO-------- --------- --------- 7839 5000 10 7698 2850 30 7566 2975 20 ... 6 rows selected.

Find all employees who make more than Find all employees who make more than the average salary in their department.the average salary in their department.Find all employees who make more than Find all employees who make more than the average salary in their department.the average salary in their department.

32

Using the EXISTS OperatorUsing the EXISTS Operator

• If a subquery row value is found:

– The search does not continue in the inner query.

– The condition is flagged TRUE.

• If a subquery row value is not found:

– The condition is flagged FALSE.

– The search continues in the inner query.

• If a subquery row value is found:

– The search does not continue in the inner query.

– The condition is flagged TRUE.

• If a subquery row value is not found:

– The condition is flagged FALSE.

– The search continues in the inner query.

33

Find employees who have at least one person reporting to them.

Using the EXISTS OperatorUsing the EXISTS Operator

EMPNO ENAME JOB DEPTNO--------- ---------- --------- --------- 7839 KING PRESIDENT 10 7698 BLAKE MANAGER 30 7782 CLARK MANAGER 10 7566 JONES MANAGER 20...6 rows selected.

SQL> SELECT empno, ename, job, deptno 2 FROM emp outr 3 WHERE EXISTS (SELECT empno 4 FROM emp innr 5 WHERE innr.mgr = outr.empno)

34

Find all departments that do not have any employees.

Using the NOT EXISTS OperatorUsing the NOT EXISTS Operator

DEPTNO DNAME--------- ---------- 40 OPERATIONS

DEPTNO DNAME--------- ---------- 40 OPERATIONS

SQL> SELECT deptno, dname 2 FROM dept d 3 WHERE NOT EXISTS (SELECT * 4 FROM emp e 5 WHERE d.deptno = e.deptno)

35

Correlated UPDATECorrelated UPDATE

Use a correlated subquery to update rows Use a correlated subquery to update rows in one table based on rows from another in one table based on rows from another table.table.

Use a correlated subquery to update rows Use a correlated subquery to update rows in one table based on rows from another in one table based on rows from another table.table.

UPDATE table1 alias1SET column = (SELECT expression FROM table2 alias2 WHERE alias1.column = alias2.column)

UPDATE table1 alias1SET column = (SELECT expression FROM table2 alias2 WHERE alias1.column = alias2.column)

36

DELETE FROM table1 alias1 WHERE column operator (SELECT expression FROM table2 alias2 WHERE alias1.column = alias2.column)

DELETE FROM table1 alias1 WHERE column operator (SELECT expression FROM table2 alias2 WHERE alias1.column = alias2.column)

Use a correlated subquery to delete only Use a correlated subquery to delete only those rows that also exist in another those rows that also exist in another table.table.

Use a correlated subquery to delete only Use a correlated subquery to delete only those rows that also exist in another those rows that also exist in another table.table.

Correlated DELETECorrelated DELETE

37

SummarySummary

• Correlated subqueries are useful whenever a subquery must return a different result for each candidate row.

• The EXISTS operator is a Boolean operator, testing the presence of a value.

• Correlated subqueries can be used with SELECT, UPDATE, and DELETE statements.

• Correlated subqueries are useful whenever a subquery must return a different result for each candidate row.

• The EXISTS operator is a Boolean operator, testing the presence of a value.

• Correlated subqueries can be used with SELECT, UPDATE, and DELETE statements.

39

UNIONUNION

AA BB

40

Using the UNION OperatorUsing the UNION OperatorDisplay the name, job title, and department Display the name, job title, and department of all employees.of all employees.Display the name, job title, and department Display the name, job title, and department of all employees.of all employees.

ENAME JOB DEPTNO---------- --------- ---------ADAMS CLERK 30ALLEN SALESMAN 30ALLEN SALESMAN 20BALFORD CLERK 20BLAKE MANAGER 30...20 rows selected.

SQL> SELECT ename, job, deptno 2 FROM emp 3 UNION 4 SELECT name, title, deptid 5 FROM emp_history

41

UNION ALLUNION ALL

AA BB

42

Using the UNION ALL OperatorUsing the UNION ALL OperatorDisplay the names, employee numbers, and Display the names, employee numbers, and job titles of all employees. job titles of all employees. Display the names, employee numbers, and Display the names, employee numbers, and job titles of all employees. job titles of all employees. SQL> SELECT ename, empno, job 2 FROM emp 3 UNION ALL 4 SELECT name, empid, title 5 FROM emp_history

ENAME EMPNO JOB---------- --------- ---------KING 7839 PRESIDENTBLAKE 7698 MANAGERCLARK 7782 MANAGERCLARK 7782 MANAGERMARTIN 7654 SALESMAN...23 rows selected.

43

SET Operator RulesSET Operator Rules• The expressions in the SELECT lists

must match in number and datatype.

• Duplicate rows are automatically eliminated except in UNION ALL.

• Column names from the first query appear in the result.

• The output is sorted in ascending order by default except in UNION ALL.

• Parentheses can be used to alter the sequence of execution.

• The expressions in the SELECT lists must match in number and datatype.

• Duplicate rows are automatically eliminated except in UNION ALL.

• Column names from the first query appear in the result.

• The output is sorted in ascending order by default except in UNION ALL.

• Parentheses can be used to alter the sequence of execution.

sql: part 3

Documents