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84136 - Week 7Group by Concept  The Group by clause is used to rearrange a table into categories  These categories are then analysed usually using a statistical function  example Wh

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84136 - Week 7

Week 7 - SQL more complex problems

unit: 84136

by

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84136 - Week 7

Overview

 group by concept

(alternative to week 6 work)

 multiple group by queries

 useful product queries

 simple query nesting

 relational calculus and SQL

 more examples

(reading 3: p.25-p.28)

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84136 - Week 7

Group by Concept

 The Group by clause is used to rearrange

a table into categories

 These categories are then analysed usually using a statistical function

 example What is the total assessment for each student?

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84136 - Week 7

Group by Process

Steps to guide group by queries

Step 1: the partition the table into separate row categories

(group by ) Step 2: the statistical calculation operations performed on one or more columns Step 3: the output

one row per category (select )

Steps to guide group by queries

Step 1: the partition

the table into separate row categories

(group by )

Step 2: the statistical calculation

operations performed on one or more columns

Step 3: the output

one row per category (select )

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84136 - Week 7 ITEM ID SUBMITTED MARK

-

1 871 908 80

1 862 907 60

1 854 908 70

1 872 910 55

1 868 906 90

1 869 909 70

2 871 1021 70

2 869 1022 80

2 872 1021 65

2 862 1022 70

2 868 1021 75

3 869 ? 95

3 872 ? 45

3 862 ? 40

3 868 ? 50

example 1 GROUP BY Item the Results table,

P 92 Edmond

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-

1 871 908 80

1 862 907 60

1 854 908 70

1 872 910 55

1 868 906 90

1 869 909 70

2 871 1021 70

2 869 1022 80

2 872 1021 65

2 862 1022 70

2 868 1021 75

3 869 ? 95

3 872 ? 45

3 862 ? 40

3 868 ? 50

3 871 ? 60

ITEM ID SUBMITTED MARK -

1 871 908 80

1 862 907 60

1 854 908 70

1 872 910 55

1 868 906 90

1 869 909 70

2 871 1021 70

2 869 1022 80

2 872 1021 65

2 862 1022 70

2 868 1021 75

3 869 ? 95

3 872 ? 45

3 862 ? 40

3 868 ? 50

3 871 ? 60

method the table is partitioned into separate row categories for each item ITEM ID SUBMITTED MARK -

1 871 908 80

1 862 907 60

1 854 908 70

1 872 910 55

1 868 906 90

1 869 909 70

2 871 1021 70

2 869 1022 80

2 872 1021 65

2 862 1022 70

2 868 1021 75

3 869 ? 95

3 872 ? 45

3 862 ? 40

3 868 ? 50

3 871 ? 60

ITEM ID SUBMITTED MARK -

1 871 908 80

1 862 907 60

1 854 908 70

1 872 910 55

1 868 906 90

1 869 909 70

2 871 1021 70

2 869 1022 80

2 872 1021 65

2 862 1022 70

2 868 1021 75

3 869 ? 95

3 872 ? 45

3 862 ? 40

3 868 ? 50

3 871 ? 60

example 1 GROUP BY Item

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84136 - Week 7

ITEM ID SUBMITTED MARK

-

1 871 908 80

1 862 907 60

1 854 908 70

1 872 910 55

1 868 906 90

1 869 909 70

2 871 1021 70

2 869 1022 80

2 872 1021 65

2 862 1022 70

2 868 1021 75

3 869 ? 95

3 872 ? 45

3 862 ? 40

3 868 ? 50

locate Min(Mark)

for each group

example 1

GROUP BY Item

Statistical calculation

Circle the answers

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-

1 871 908 80

1 862 907 60

1 854 908 70

1 872 910 55

1 868 906 90

1 869 909 70

2 871 1021 70

2 869 1022 80

2 872 1021 65

2 862 1022 70

2 868 1021 75

3 869 ? 95

3 872 ? 45

3 862 ? 40

3 868 ? 50

3 871 ? 60

Step 2:

locate Min(Mark) for

each group

Min(Mark) = 55 when Item = 1 (that is assignment 1)

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-

1 871 908 80

1 862 907 60

1 854 908 70

1 872 910 55

1 868 906 90

1 869 909 70

2 871 1021 70

2 869 1022 80

2 872 1021 65

2 862 1022 70

2 868 1021 75

3 869 ? 95

3 872 ? 45

3 862 ? 40

3 868 ? 50

Min(Mark) = 65

when Item = 2

(that is assignment 2)

Step 2:

each group

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-

1 871 908 80

1 862 907 60

1 854 908 70

1 872 910 55

1 868 906 90

1 869 909 70

2 871 1021 70

2 869 1022 80

2 872 1021 65

2 862 1022 70

2 868 1021 75

3 869 ? 95

3 872 ? 45

3 862 ? 40

3 868 ? 50

3 871 ? 60

Min(Mark) = 40

when Item = 3 (that is assignment 3)

Step 2:

each group

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84136 - Week 7

SELECT Item,Min(Mark)

FROM Results

GROUP BY Item

Step 3:

the output

-

1 871 908 80

1 862 907 60

1 854 908 70

1 872 910 55

1 868 906 90

1 869 909 70

2 871 1021 70

2 869 1022 80

2 872 1021 65

2 862 1022 70

2 868 1021 75

3 869 ? 95

3 872 ? 45

3 862 ? 40

3 868 ? 50

Result??

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84136 - Week 7

Result

-

1 871 908 80

1 862 907 60

1 854 908 70

1 872 910 55

1 868 906 90

1 869 909 70

2 871 1021 70

2 869 1022 80

2 872 1021 65

2 862 1022 70

2 868 1021 75

3 869 ? 95

3 872 ? 45

3 862 ? 40

3 868 ? 50

3 871 ? 60

Step 3:

the output

FROM Results

GROUP BY Item

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84136 - Week 7

FROM Results

GROUP BY Item

Step 3:

the output

ITEM MIN(MARK)

-

1 55

2 65

3 40

* END OF RESULT ****

* 3 ROWS DISPLAYED *

Output

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84136 - Week 7

FROM Results

GROUP BY Item ITEM MIN(MARK)

-

1 55

2 65

3 40

* END OF RESULT **** * 3 ROWS DISPLAYED * What is the actual query?

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84136 - Week 7

FROM Results

GROUP BY Item ITEM MIN(MARK)

-

1 55

2 65

3 40

* END OF RESULT ****

* 3 ROWS DISPLAYED *

What is the actual query?

What is the lowest mark for each assignment?

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84136 - Week 7

A similar problem

What is the highest mark for each assignment?

What key words indicate that this is a

group by problem?

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84136 - Week 7

A similar problem

GROUP BY… item

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It is usual to reference the group in the select statement

SELECT Item, MAX(Mark) 



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84136 - Week 7

SELECT Item, MAX(Mark)

FROM Results GROUP BY Item

SELECT Item, MAX(Mark)

FROM Results GROUP BY Item

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84136 - Week 7

List the total mark for each student?

Another Query

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GROUP BY

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84136 - Week 7

List the total mark for each student?

Another Query

SELECT FROM

GROUP BY Id

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GROUP BY Id

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GROUP BY Id

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Id is used for more useful output

statistical function

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SELECT ID, SUM(Mark)

FROM Results

GROUP BY ID

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84136 - Week 7

ID SUM(MARK) - -

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84136 - Week 7

 used when more than one table needs to

be joined to find all information

 Example :

Find the average mark of each

assignment and provide a description of each assignment

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1 Which words indicate a group by situation?

Average mark - statistical function Each assignment - each group

avg(Mark)



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84136 - Week 7

Average mark each assignment assignment names

2: Match these key words

Find the average mark of each assignment and name each assignment.

Column Tables

… … …

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84136 - Week 7

2: Match these key words

Find the average mark of each assignment and name each assignment.

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84136 - Week 7

Step 4:

Each assignment is used in the group by clause

Find the average mark of

name each assignment.

Column Tables Mark Results Item Results, Assess Description Assess

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84136 - Week 7

Step 4:

Column Tables Mark Results Item Results, Assess Description Assess

each assignment Item Results, Assess GROUP BY Item (from results table

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84136 - Week 7

Step 4:

BUT: Choose only one table Group by r.Item or

Group by a.Item (r = Results table, a = Assess table)

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84136 - Week 7

Find the

Step 5:

Statistical function

1 Average Mark = AVG(Mark)

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84136 - Week 7

Find the

Step 5:

Statistical function

1 Average Mark = AVG(Mark)

2 Writing the description = MAX(Description) (necessary only for group by problems)

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FROM Results r, Assess A

WHERE r.Item = a.Item

GROUP BY r.Item

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WHERE r.Item = a.Item

GROUP BY r.Item

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SELECT a.Id, a.Mark, b.Mark

FROM Results a, Results b

WHERE a.Id = b.Id

AND a.Item = 1

AND b.Item = 2

ORDER BY 1

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84136 - Week 7

Meaning of each line of Code

assignment 1 for first table assignment 2 for second table

all rows are ordered by column 1

ie the Id’s

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84136 - Week 7

Output of Similar Code

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84136 - Week 7

Process: Query Nesting

 Inner query finds the first result structure

SELECT

FROM

WHERE = (SELECT MIN(Mark) INNER QUERY

FROM Results WHERE item = 1)

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84136 - Week 7

Process: Query Nesting

 Outer Query uses this result structure

FROM Results

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From Results Where Item = 2))

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From Results Where Item = 2)) Step 1.

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From Results Where Item = 2)) Step 2.

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84136 - Week 7

What is the name of the student who got the highest mark in the second assignment?

Select First,Last From Students Where Id = (Select Id

Where item = 2 And mark = (Select Max(Mark) From Results

Where Item = 2))

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where item = 2 and mark = (select Max(Mark) From Results

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84136 - Week 7

What is the name of the student who got the highest mark in the second assignment?

Select First,Last From Students where Id = (Select Id from Results where item = 2 and mark = (select Max(Mark) From Results

FIRST LAST - - Rip Orff

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language: SQL

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r.mark > = 50 r.id}

implementation:

Select id from results where item = 1 and mark >= 50

An Example

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Informal text List the student id and mark for assignment 1 Formal text

{r:results| r.item = 1 r.id, r.mark}

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84136 - Week 7

Statistical Query

SQL

Select count(*) from assess

informal text How many assessment items are there? Formal text

count {a:assess }

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84136 - Week 7

Join Query

SQL Select first, last, mark from students s, results r where s.id = r.id

informat text

List the students and their marks for assignment 1.

format text

{s:students, r:results| s.id = r.id and r.item = 1

s.first, s.last , r.mark}

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84136 - Week 7

Product Query

SQL Select a.id from results a, results b where a.id = b.id

and a.item = 1 and b.item = 2 and a.mark > b.mark

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Informal text Which student attained

the best mark in assignment1

Formal text

{r:results| r.item = 1 and

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84136 - Week 7

Applications

The remaining screens will present

 SQL code from Appendix 2, Reading 3,

84136 Resource book.

 the output from the code, based on subject database on P 92

subject database on P 92 of Edmond text

 the associated question

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84136 - Week 7

What is the actual question?

SELECT ID, MAX(MARK)

FROM RESULTS

GROUP BY ID

ORDER BY ID

ID MAX(MARK) - -

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84136 - Week 7

What is the actual question?

SELECT ID, MAX(MARK)

FROM RESULTS

GROUP BY ID

ORDER BY ID

ID MAX(MARK) - -

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* END OF RESULT **

*** 3 ROWS

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List the output in the table, using p 92 Edmond

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Group3.dat

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For each assignment, state the minimum, maximum and average value List in assignment order.

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84136 - Week 7

Group4.dat

SELECT ID, SUM(MARK*WEIGHT/100)

FROM RESULTS R, ASSESS A

WHERE R.ITEM = A.ITEM

GROUP BY ID

ORDER BY ID

complete the table

ID SUM(MARK*WEIGHT/100)

-

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84136 - Week 7

Group4.dat

GROUP BY ID

ORDER BY ID

ID SUM(MARK*WEIGHT/100) -

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84136 - Week 7

Group4.dat

GROUP BY ID

ORDER BY ID

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84136 - Week 7

Group4.dat

GROUP BY ID

ORDER BY ID

For each student,

list their overall mark

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84136 - Week 7

SELECT R.ID, MAX(FIRST),MAX(LAST), SUM(MARK*WEIGHT/100)

FROM STUDENTS S, RESULTS R, ASSESS A WHERE S.ID = R.ID

AND R.ITEM = A.ITEM GROUP BY R.ID

ORDER BY R.ID complete the table

R.ID MAX(FIRST) MAX(LAST) SUM(MARK*WEIGHT/100)

- -

-

Group5.dat

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