Relational Algebra and My SQL

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Relational Algebra and My SQL

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SALES/PRODUCTS: the ids of the suppliers supplying ALL products. 9/21/09 ... all products such that there exists at least one supplier not supplying it. ... – PowerPoint PPT presentation

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Title: Relational Algebra and My SQL

1
Relational Algebra and My SQL
Lecture 5 CS157B

Prof. Sin Min Lee
Deparment of Computer Science
San Jose State University

2
Functional Dependency
A, B ? R(.) , we say FD A?B if

For all t1, t2 element of R if t1A t2A gt
t1B t2B

Definition A? R(.) is a key if FD A
?R
Definition A? R(.) is a candidate key if FD
A?R and not proper subset of A is a key.
3
4. Find all the candidate keys for the following
table ( Mid1 Study Guide)

R(A B C D)
1 2 3 4
2 2 3 5
3 2 5 1
1 2 5 6
S (A B C D)
1 2 3 4
2 2 3 5
3 2 5 1
1 2 5 6

4
Queries

Q1
A?R ()
A is not a key
Q2
B?R ()
B is not a key
Q3
C?R ()
C is not a key
Q4
D?R ()
Yes-D is a key
Q5
AB?R ()
AB together cannot form the key
Q6
AC?R ()
Yes- AC together cannot form the key
Q7

5
Relational Algebra

Basic operations
Selection ( ) Selects a subset of rows
from relation.
Projection ( ) Deletes unwanted columns
from relation.
Cross-product ( ) Allows us to combine two
relations.
Set-difference ( ) Tuples in reln. 1, but
not in reln. 2.
Union ( ) Tuples in reln. 1 and in reln. 2.
Additional operations
Intersection, join, division, renaming Not
essential, but (very!) useful.
Since each operation returns a relation,
operations can be composed! (Algebra is closed.)

6
Projection

Deletes attributes that are not in projection
list.
Schema of result contains exactly the fields in
the projection list, with the same names that
they had in the (only) input relation.
Projection operator has to eliminate duplicates!
(Why??, what are the consequences?)
Note real systems typically dont do duplicate
elimination unless the user explicitly asks for
it.

7
Selection

Selects rows that satisfy selection condition.
Schema of result identical to schema of (only)
input relation.
Result relation can be the input for another
relational algebra operation! (Operator
composition.)

8
Union, Intersection, Set-Difference

All of these operations take two input relations,
which must be union-compatible
Same number of fields.
Corresponding fields have the same type.
What is the schema of result?

9
Cross-Product

Each row of S1 is paired with each row of R1.
Result schema has one field per field of S1 and
R1, with field names inherited if possible.
Conflict Both S1 and R1 have a field called sid.

Renaming operator

10
Joins

Condition Join
Result schema same as that of cross-product.
Fewer tuples than cross-product. Filters tuples
not satisfying the join condition.
Sometimes called a theta-join.

11
Joins

Equi-Join A special case of condition join
where the condition c contains only equalities.
Result schema similar to cross-product, but only
one copy of fields for which equality is
specified.
Natural Join Equijoin on all common fields.

12
Division

Not supported as a primitive operator, but useful
for expressing queries like

Find sailors who
have reserved all boats.
Precondition in A/B, the attributes in B must be
included in the schema for A. Also, the result
has attributes A-B.
SALES(supId, prodId)
PRODUCTS(prodId)
Relations SALES and PRODUCTS must be built using
projections.
SALES/PRODUCTS the ids of the suppliers
supplying ALL products.

13
Examples of Division A/B
B1
B2
B3
A/B1
A/B2
A/B3
A
14
Expressing A/B Using Basic Operators

Division is not essential op just a useful
shorthand.
(Also true of joins, but joins are so common that
systems implement joins specially. Division is
NOT implemented in SQL).
Idea For SALES/PRODUCTS, compute all products
such that there exists at least one supplier not
supplying it.
x value is disqualified if by attaching y value
from B, we obtain an xy tuple that is not in A.

The answer is ?sid(Sales) - A
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17
EQUALITY JOIN, NATURAL JOIN, JOIN, SEMI-JOIN

Equality join connects tuples from two relations
that match on certain attributes. The specified
joining columns are kept in the resulting
relation.
?name(?dnametoy(Emp Dept)))
Natural join connects tuples from two relations
that match on the specified common attributes
?name(?dnametoy(Emp Dept)))
How is an equality join between Emp and Dept
using dno different than a natural join between
Emp and Dept using dno?
Equality join SS, name, age, salary, Emp.dno,
Dept.dno,
Natural join SS, name, age, salary, dno,
dname,
Join is similar to equality join using different
comparison operators
A S op , ?, , , lt, gt
att op att

(dno)
(dno)
18
EXAMPLE JOIN

Equality Join, (Emp Dept)))

Dept
EMP
(dno)
19
EXAMPLE JOIN

Natural Join, (Emp Dept)))

Dept
EMP
(dno)
20
EXAMPLE JOIN

Join, (Emp ?x(Emp))))

Dept
EMP
Salary gt 5 salary
21
EQUALITY JOIN, NATURAL JOIN, JOIN, SEMI-JOIN
(Cont)

Example retrieve the name of employees who earn
more than Joe
?name(Emp (salgtx.sal)?nameJoe(? x(Emp)))
Semi-Join selects the columns of one relation
that joins with another. It is equivalent to a
join followed by a projection
Emp (dno)Dept ?SS, name, age, salary,
dno(Emp Dept)

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24
JOIN OPERATORS

Condition Joins
- Defined as a cross-product followed by a
selection
R ?c S sc(R ? S)
(? is called the bow-tie)
where c is the condition.
- Example
Given the sample relational instances S1 and R1

The condition join S ?S1.sidltR1.sid R1 yields
25
JOIN OPERATORS

Condition Joins
- Defined as a cross-product followed by a
selection
R ?c S sc(R ? S)
(? is called the bow-tie)
where c is the condition.
- Example
Given the sample relational instances S1 and R1

The condition join S ?S1.sidltR1.sid R1 yields
26
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29

Equijoin
Special case of the condition join where the join
condition consists solely of equalities between
two fields in R and S connected by the logical
AND operator (?).
Example Given the two sample relational
instances S1 and R1

The operator S1 R.sidSsid R1 yields
30
SQL

SQL (Structured Query Language) is the standard
language for commercial DBMSs
SEQUEL (Structured English QUEry Language)was
originally defined by IBM for System R
standardization of SQL began in the 80s
current standard is SQL-99
SQL is more than a query language it includes a
DDL, DML and administration commands
SQL is an example of a transform-oriented
language.
A language designed to use relations to transform
inputs into required outputs.

31
Basic structure of an SQL query
2
32
2
The SituationStudent Particulars
field type width contents id numeric
4 student id number name character
10 name dob date 8 date of
birth sex character 1 sex M /
F class character 2 class hcode character
1 house code R, Y, B, G dcode character
3 district code remission logical 1 fee
remission mtest numeric 2 Math test score
33
General Structure
I
SELECT ...... FROM ...... WHERE ......
SELECT ALL / DISTINCT expr1 AS col1, expr2
AS col2 FROM tablename WHERE condition
34
General Structure
I
SELECT ALL / DISTINCT expr1 AS col1, expr2
AS col2 FROM tablename WHERE condition

The query will select rows from the source
tablename and output the result in table form.

Expressions expr1, expr2 can be
(1) a column, or
(2) an expression of functions and fields.

And col1, col2 are their corresponding column
names in the output table.

3.2 SQL
SELECT a1, ..., an
FROM R1, R2, , Rm
WHERE Con1, ,Conk
This means
p a1, ..., an( s Con1( (s Conk ( R1 R2
Rm))))

Example
Find the SSN and tax for each person.
SELECT SSN, Tax
FROM Taxrecord, Taxtable
WHERE wages interest capital_gain
income

AS keyword used to rename relations
Two SQL expressions can be combined by
INTERSECT
UNION
MINUS set difference

Example
Find the names of the streets that intersect.
SELECT S.NAME, T.NAME
FROM Streets AS S, Streets AS T
WHERE S.X T.X and
S.Y T.Y

Example Assume we have the relations
Broadcast ( Radio, X , Y )
Town ( Name, X, Y )
Find the parts of Lincoln, NE that can
be reached by at least one Radio station.
(SELECT X, Y
FROM Town
WHERE Name Lincoln)
INTERSECT
(SELECT X, Y
FROM Broadcast)

Example
Find the SSN and tax for each person.
pSSN,Tax swagesinterestcapital_gain
income Taxrecord Taxtable
Example
Find the area of Lincoln reached by a radio
station.
( pX,Y ( sNameLincoln Town ) ) ? (
pX,Y Broadcast )

Another way of connecting SQL expressions
is using the IN keyword.
SELECT ..
FROM ..
WHERE a IN ( SELECT b
FROM ..
WHERE .. )

SQL with aggregation
SELECT aggregate_function
FROM .
WHERE
aggregate_function
Max (c1a1 .. cnan)
where ai are attributes
Min (c1a1 .. cnan)
and ci are constants
Sum(a) where a is an
attribute that is
Avg(a) constant in
each constraint tuple
Count(a)

Example
Package(Serial_No, From, Destination, Weight)
Postage (Weight , Fee)
Find the total postage of all packages sent
from Omaha.
SELECT Sum(Fee)
FROM Package, Postage
WHERE Package.Weight Postage.Weight AND
Package.From Omaha

GROUP BY
SELECT a1, , an, aggregate_function
FROM ..
WHERE
GROUP BY a1, ..., ak
Evaluates basic SQL query
Groups the tuples according to different values
of a1,..,ak
Applies the aggregate function to each group
separately
a1, , ak ? a1, , an

Example
Find the total postage sent out from each
city.
SELECT Package.From, Sum(Postage.Fee)
FROM Package, Postage
WHERE Package.Weight Postage.Weight
GROUP BY Package.From

46
General Structure
I
SELECT ALL / DISTINCT expr1 AS col1, expr2
AS col2 FROM tablename WHERE condition

DISTINCT will eliminate duplication in the output
while ALL will keep all duplicated rows.

condition can be
(1) an inequality, or
(2) a string comparison
using logical operators AND, OR, NOT.

47
General Structure
I

Before using SQL, open the student file
USE student

eg. 1 List all the student records.
SELECT FROM student
48
General Structure
I
eg. 2 List the names and house code of 1A
students.
SELECT name, hcode, class FROM student
WHERE class"1A"
49
General Structure
I
eg. 2 List the names and house code of 1A
students.
50
General Structure
I
eg. 3 List the residential district of the Red
House members.
SELECT DISTINCT dcode FROM student WHERE
hcode"R"
51
Data Manipulation

Select query data in the database
Insert insert data into a table
Update updates data in a table
Delete delete data from a table

Source Database Systems Connolly/Begg
52
Retrieve all columns and all rows

SELECT firstColumn,,lastColumn
FROM tableName
SELECT
FROM tableName

53
Use of Distinct

SELECT DISTINCT columnName
FROM tableName

54
Calculated fields

SELECT columnName/2
FROM tableName

55
Comparison Search Condition

equals
lt gt is not equal to (ISO standard)
! (allowed in some dialects)
lt is less than
gt is greater than
lt is less than or equal to
gt is greater than or equal to

Source Database Systems Connolly/Begg
56
Comparison Search Condition

An expression is evaluated left to right.
Subexpressions in brackets are evaluated first.
NOTs are evaluated before ANDs and ORs.
ANDs are evaluated before ORs.

Source Database Systems Connolly/Begg
57
Range Search Condition

SELECT columnName
FROM tableName
WHERE columnName BETWEEN 20 AND 30
SELECT columnName
FROM tableName
WHERE columnName gt 20
AND columnName lt 30

58
Set membership search condition

SELECT columnName
FROM tableName
WHERE columnName
IN (name1, name2)
SELECT columnName
FROM tableName
WHERE columnName name1
OR columnName name2

59
Pattern matching symbols

represents any sequence of zero
or more characters (wildcard).
_ represents any single character

Source Database Systems Connolly/Begg
60
Pattern match search condition

h begins with the character h .
h_ _ _ four character string beginning with
the
character h.
e any sequence of characters, of length
at
least 1, ending with the character e.
CS157B any sequence of characters of any
length containing CS157B

Source Database Systems Connolly/Begg
61
Pattern match search condition

LIKE h
begins with the character h .
NOT LIKE h
does not begin with the character h.

Source Database Systems Connolly/Begg
62
Pattern match search condition

To search a string that includes a
pattern-matching character
15
Use an escape character to represent
the pattern-matching character.
LIKE 15 ESCAPE

Source Database Systems Connolly/Begg
63
NULL search condition

DOES NOT WORK
comment
comment !
DOES WORK
comment IS NULL
comment IS NOT NULL

64
Sorting

The ORDER BY clause
consists of list of column identifiers that the
result is to be sorted on, separated by commas.
Allows the retrieved rows to be ordered by
ascending (ASC) or descending (DESC) order

Source Database Systems Connolly/Begg
65
Sorting

Column identifier may be
A column name
A column number (deprecated)

Source Database Systems Connolly/Begg
66
Sorting

SELECT type, rent
FROM tableName
ORDER BY type, rent ASC

Source Database Systems Connolly/Begg
67
Aggregate Functions

COUNT returns the number
SUM returns the sum
AVG returns the average
MIN returns the smallest
MAX returns the largest
value in a specified column.

Source Database Systems Connolly/Begg
68
Use of COUNT( )

How many students in CS157B?
SELECT COUNT( ) AS my count
FROM CS157B

69
GROUP BY clause

When GROUP BY is used, each item in the SELECT
list must be single-valued per group.
The SELECT clause may contain only
Column names
Aggregate functions
Constants
An expression involving combinations of the above

Source Database Systems Connolly/Begg
70
Grouping

SELECT dept, COUNT(staffNo) AS my count
SUM(salary)
FROM tableName
GROUP BY dept
ORDER BY dept

71
Restricting Grouping

HAVING clause
is with the GROUP BY clause.
filters groups into resulting table.
includes at least one aggregate function.
WHERE clause
filters individual rows into resulting table.
Aggregate functions cannot be used.

Source Database Systems Connolly/Begg
72

SELECT dept, COUNT(staffNo) AS my count,
SUM(salary) AS my sum
FROM Staff
GROUP BY dept
HAVING COUNT(staffNo) gt 1
ORDER BY dept

Source Database Systems Connolly/Begg
73
Subqueries

SELECT columnNameA
FROM tableName1
WHERE columnNameB (SELECT columnNameB
FROM tableName2
WHERE condition)

result from inner SELECT applied as a condition
for the outer SELECT
Source Database Systems Connolly/Begg
74
Subquery with Aggregate Function
List all staff whose salary is greater than the
average salary, show by how much their salary is
greater than the average.