Lecture SQL 1

8/4/2019 Lecture SQL 1

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SQL

SELECT Statement


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Introduction

SQL is the Structured Query Language

It is used to interact with the DBMS SQL can

Create Schemas in the DBMS

Alter Schemas

Add data

Remove data

Change data

Access Data


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DSL

SQL is a Data Sub Language DSL

This is a combination of two languages

DDL Data Definition Language

DML Data Manipulation Language

The main way of accessing data is using the DMLcommand SELECT.

The abilities of the SELECT command forms themajority of this material on SQL


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Database Models

A data model comprises

a data structure

a set of integrity constraints

operations associated with the data structure

Examples of data models include: hierarchic

network

relational


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Relational Databases

The relational data model comprises:

relational data structure

relational integrity constraints

relational algebra or equivalent (SQL) SQL is an ISO language based on relational

algebra

relational algebra is a mathematical formulation


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Relational Data Structure

A relational data structure is a collection of

tables or relations.

A relation is a collection of rows or tuples

A tuple is a collection of columns or attributes

A domain is a pool of values from which theactual attribute values are taken.


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Relational Structure cont

Description Price

Tuple

Attributes

Domain

MENU Relation or

Table


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Domain and Integrity Constraints

Domain Constraints limit the range of domain values of an attribute

specify uniqueness and nullness of an attribute specify a default value for an attribute when no value is

provided.

Entity Integrity every tuple is uniquely identified by a unique non-null

attribute, the primary key.

Referential Integrity rows in different tables are correctly related by valid key

values (foreign keys refer to primary keys).


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Example Database

In order to better understand SQL, all the examplequeries make use of a simple database.

The database is formed from 2 tables, CAR andDRIVER.

Each car may be owned by a DRIVER.

A DRIVER may own multiple CARs.

DRIVER CAR


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DRIVER

NAME DOB

Jim Smith 11 Jan 1980

Bob Smith 23 Mar 1981

Bob Jones 3 Dec 1986


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CAR

REGNO MAKE COLOUR PRICE OWNER

F611 AAA FORD RED 12000 Jim Smith

J111 BBB SKODA BLUE 11000 Jim Smith

A155 BDE MERCEDE

S

BLUE 22000 Bob Smith

K555 GHT FIAT GREEN 6000 Bob Jones

SC04 BFE SMART BLUE 13000


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Each column holds data of a particular type Integer, string, decimal, blobs

The range of values can be further constrained

If a column in a row contains no data, it is NULL.

It can indicate no possible value or unavailable data.

All rows must differ from each other in some way Sometimes a row is called a tuple

Cardinality is the number of rows of a table

Arity is the number of columns of a table


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Primary Keys

In the design section the idea of a Primary Key isdefined.

A Primary Key is a group of 1 or more columnswhich, when taken together, is unique in the table

No part of a primary key can be NULL.

In our example,

DRIVER: the primary key is NAME CAR: the primary key is REGNO

In our example this means that no two drivers canhave the same name. In the real world this would bea problem, but this is just an example.


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Referential Integrity

Note that there is a link between CAR and DRIVERvia OWNER.

If there is a value in OWNER, then this value mustalso appear somewhere in DRIVER.

If you change a drivers name in DRIVER, you must

make sure the same change is made in OWNER of

CAR.

The DBMS enforces the rules.

If you try to break the rules the DBMS reports theproblem as a REFERENTIAL INTEGRITY error.


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SQL Basics

Basic SQL statements include

CREATE a data structure

SELECT read one or more rows from a table

INSERT one of more rows into a table

DELETE one or more rows from a table

UPDATE change the column values in a row

DROP a data structure

In this lecture the focus is on SELECT.


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Simple SELECT SELECT column FROM tablename;

SELECT column1,column2,column3 FROM tablename;

SELECT * from tablename;

SELECT * from CAR;




A155 BDE MERCEDES BLUE 22000 Bob SmithK555 GHT FIAT GREEN 6000 Bob Jones



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SELECT regno from CAR;

REGNOF611 AAA

J111 BBB

A155 BDE

K555 GHTSC04 BFE


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SELECT colour,owner from CAR;

COLOUR OWNER

RED Jim Smith

BLUE Jim Smith

BLUE Bob Smith

GREEN Bob Jones

BLUE


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Formatting

SPACES do not matter

NEWLINES do not matter

Good practice to put ; at the end of the query.

CASE (except between single quotes) does notmatter.

These are all valid:

SELECT REGNO FROM CAR;

SElecT regno

From Car

;


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Comments

To give you the ability to make notes in queries youare allowed to have comments.

Comments are not executed A comment starts with -- and ends with a newline

They are only permitted within a query.

SELECT regno -- The registration number

FROM car -- The car storage table

;


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SELECT filters

You can have rules in your queries

These rules are tested for each row your queryproduces

If the rule is true, the row is displayed

If the rule is false, the row is not displayed

The rule starts with WHERE

SELECT columns

FROM table

WHERE rule


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Simple Rule

A simple rule might be tolook for a car with a colour ofRED.

The rule would be colour ='RED'

SELECT regno FROM CAR

SELECT regno from CAR

WHERE colour = 'RED'

REGNO

F611 AAAJ111 BBB

A155 BDE

K555 GHT

SC04 BFE

REGNO

F611 AAA


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Note

Things between quotes is CASE SENSITIVE.

RED is not the same as Red or red

Rules which mention fields they can beused if they appear on the SELECT line or

not.


WHERE colour = 'RED'

REGNO COLOUR

F611 AAA RED


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Comparisons

Valid comparisons include =,!=,,=

Colour = RED The colour must be red Colour != RED The colour is not red

Colour Red Same as !=

Price > 10000 More than 10000

Price >= 10000 More than or equal to 10000 Price < 10000 Cheaper than 10000

Price


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DATE

Date comparisons can be tricky

You can use all the normal comparators with dates.

SELECT name,dob SELECT name,dob from driver

from driver; where DOB = 3 Dec 1986;

NAME DOB

Jim Smith 11 Jan 1980



NAME DOB



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The tricky part with dates is remembering that datesget bigger as you move into the future.

DATE1>DATE2 indicates DATE1 is in the futureafter DATE2.

SELECT name,dob from driver

WHERE DOB >= 1 Jan 1981;

NAME DOB




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DATE Syntax

It must be in quotes

Each DBMS handles dates in a slightly different way

Dates like 1 Jan 2003 work quite well.

Oracle permits dates like 1-Jan-2003

Oracle also permits dates like 1-Jan-03

Be careful if you type this it will assume 2003. If you mean 1984 type 1984 not04.

You must always specify a day and a month. If youdo not the DBMS will report an error.


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BETWEEN

When dealing with dates sometimes you want to testto see if a field value falls between two dates.

The easiest way to do this is with BETWEEN

Find all drivers born between 1995 and 1999

SELECT name,dob from driver

WHERE DOB between 1 Jan 1985 and 31 Dec1999

Between works for other things, not just dates


where price between 5000 and 10000;


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NULL

NULL indicates that something has no value

It is not a value, and you cannot use normal comparison

operators. For instance, looking for cars without owners

Wrong: SELECT regno from car where owner = NULL;

Wrong: SELECT regno from car where owner = NULL;

Instead there are two special operators, IS NULL, andIS NOT NULL


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SELECT regno from car

WHERE OWNER is null

SELECT regno from carWHERE OWNER is not null

REGNO

SC04 BFE

REGNO

F611 AAA

J111 BBB

A155 BDE

K555 GHT

SC04 BFE


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LIKE

Sometimes you want to have a rule involvingpartial strings, substrings, or wildcards

LIKE does this, and is a slot-in replacementfor =

If the string contains % or _, LIKE uses

them to support wildcards. % - Matches 0 or more characters in the string

_ - Matches exactly 1 character in the string


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Examples

Name LIKE Jim Smith e.g. Jim Smith

Name LIKE _im Smith e.g. Tim Smith

Name LIKE ___ Smith e.g. Bob Smith Name LIKE % Smith e.g. Frank Smith

Name LIKE % S% e.g. Brian Smart

Name LIKE Bob % e.g. Bob Martin

Name LIKE % i.e. match anyone

LIKE is more expensive than =

If you are not using wildcards, always use = rather than LIKE.


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SQL

Logical Operators andaggregation


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Logical Operators

Combining rules in a single WHERE clause would beuseful

AND and OR allow us to do this

NOT also allows us to modify rule behaviour

When these are combined together, problems in ruleordering can occur.

This is solved using parentheses.


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AND

AND combines rules together so that they ALL must be true.

Lets revisit the CAR table:




A155 BDE MERCEDES BLUE 22000 Bob Smith

K555 GHT FIAT GREEN 6000 Bob Jones



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SELECT regno from car SELECT regno from car

where colour = BLUE; WHERE regno LIKE %5;

REGNO

J111 BBB

A155 BDE

SC04 BFE

REGNO

A155 BDE

K555 GHT


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SELECT regno from car

WHERE colour = BLUE and regno LIKE %5%;

REGNO

A155 BDE


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Multiple AND rules

You can have as many rules as you like ANDedtogether.

For example:

SELECT regno

FROM carWHERE colour = BLUE

AND regno like %5%

AND owner like Bob %

;


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OR

OR is like either. So long as one of the rules is true then thefilter is true.

Looks for cars which are EITHER red or blue

SELECT regno,colour from CAR

WHERE colour = RED OR colour = BLUE;

REGNO COLOUR

F611 AAA RED

J111 BBB BLUE

A155 BDE BLUE

SC04 BFE BLUE


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NOT

NOT inverts the rule it is put in front of:

WHERE colour = RED

This could be inverted as: WHERE colour != RED

WHERE NOT colour = RED

NOT is not really useful in this example, butcomes into its own in more complex rulesets.


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Precedence

Precedence is the order in which the rules are evaluated andcombined together.

It is NOT in the order they are written. Rules are combined together firstly at AND, then OR, and

finally at NOT.

Consider : Car has a 5 in reg and is either red or blue.

SELECT regno,colour from carWHERE colour = RED -- Line 1

OR colour = BLUE -- Line 2

AND regno LIKE %5% -- Line 3


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Brackets

Rewrite as:SELECT regno,colour from car

WHERE (colour = REDOR colour = BLUE )AND regno LIKE %5%;

Might be clearer as:SELECT regno,colour from carWHERE ( colour = RED OR colour = BLUE )AND regno LIKE %5%;


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DISTINCT

Find all the colours used in cars.

SELECT colour from car; COLOUR

RED

BLUE

BLUEGREEN

BLUE


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DISTINCT

SELECT DISTINCT colour from car;COLOUR

RED

BLUE

GREEN


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ORDER BY

It would be nice to be able to order the output usinga sort.

SELECT make from car; MAKE

FORD

SKODA

MERCEDESFIAT

SMART


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ASCending order

Sort by alphabetical or numeric order: ASC

ORDER BY ASC is the default.

SELECT make from car

ORDER BY make;

MAKE

FORD

FIAT

MERCEDESSKODA

SMART


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DESCending order

Sort by reverse alphabetical or numeric order: DESC

ORDER BY DESC must be selected.

SELECT make from car

ORDER BY make DESC;MAKE

SMART

SKODA

MERCEDES

FIAT

FORD


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Multi Column Sort

ORDER BY can take multiple columns.

SELECT make,colour FROM carORDER BY colour,make;

MAKE COLOUR

SKODA BLUE

SMART BLUE

MERCEDES BLUE

FIAT GREEN

FORD RED


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IN

When you have a list of OR, all on the sameattribute, then IN could be a simpler way:

Rather Than:

SELECT regno,make FROM car

WHERE make = SKODA or make = SMART

Have

SELECT regno,make FROM car

WHERE make in (SKODA,SMART);


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Single Value Functions

A single value is one number,

Such asa literal number,544.343

A variable in PL/SQL


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Cont..

Value1 + value 2

Value1 * value 2

Value1 - value 2

Value1 + value 2

ABS(value)

COS(value) LOG(value)

NVL(value,subsitute)

Power(value,exponent)

SIGN(value)


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NULL

Any arithmetic operation that includes a nullvalue has null as result.

The calculated columns(whose values arethe result of a calculation)plus,subr,times,and divided are empty.

Null is not zero,it is unknown.


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NVL (null value substitution)

Select client,weightfrom shipping;

Client weight

----------- ------------

John 59

Dagg 27

Nony

Select client,NVL(weight,43) from

shipping; Client weight

----------- ------------

John 59

Dagg 27

Nony 43


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NVL(value,subsitute)

If values is null, this function is equal tosubsitute.

If value is not null ,this function is equal tovalue.

Subsitute can be a number,another columnor computation.


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Aggregate Functions

Aggregate functions allow you to write queries toproduce statistics on the data in the database.

These functions are sometimes also called SETfunctions.

These include:

AVG (calculate the average) SUM

MAX

MIN

COUNT


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Null in aggregate function

Group functions ignore NULL values andcalculate a result in spite of them.

Count is special case.

It will count all the non-NULL rows of acolumn,

Or it will count all the rows.


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AVERAGE

SELECT price FROM car;

SELECT avg(price) FROM car;

PRICE

12000

11000

22000

6000

13000

AVG(PRICE)

12800


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SUM

Add up all the values in a column

SELECT sum(price) FROM car;

SUM(PRICE)

64000


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MAX

What is the maximum value in a column

SELECT max(price) FROM car;

max(PRICE)

22000


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MIN

What is the minimum value in a column

SELECT min(price) FROM car;

MIN(PRICE)

6000


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COUNT

How many rows make up a column

SELECT count(price) FROM car;COUNT(PRICE)

5

Count(*) is similar, but also counts when price isNULL.

SELECT count(*) FROM car;


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COUNT DISTINCT

Sometimes you do not want to count how many rowsare in a column, but how many different values could

be found in that column. There is a special variant of count which does this:

SELECT count(colour) from car;

SELECT count(DISTINCT colour) from car;

COUNT(colour)

5

COUNT(colour)

3


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Combining Group-value and single-

value functions

Select city,noon-midnight from comfort wherecity=KEENE;

City noon-midnight

.. ----------------------

KEENE 41.1

KEENE 18.4 KEENE 17.2

KEENE -6


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example

Select avg(noon-midnight) from comfort

Where city =KEENE;

Avg(Noon-midnight)

------------------------

17.675

select avg(abs(noon-midnight) from comfort

where city=KEENE; Avg(abs(Noon-

midnight))

------------------------

20.675


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GROUP BY

Aggregation functions so far have only been shown in querieswith only the single aggregation function on the select line.

You can combine functions and non-functions on the selectline.

To do this you need GROUP BY.

Question: What is the most expensive car for each colour.

Intuitively the following seems right, but will not execute!

SELECT colour,max(price)

FROM car;


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SELECT colour,priceFROM car;

SELECT colour,max(price)FROM carGROUP BY colour;

COLOUR PRICE

RED 12000

BLUE 11000

BLUE 22000

GREEN 6000

BLUE 13000

COLOUR PRICE

RED 12000

BLUE 22000

GREEN 6000


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HAVING

WHILE allows rules for each row.

HAVING allows rules for each group of a GROUP

BY.

Consider the problem Who has more than 1 car.

We would like to say:SELECT owner from car where count(owner) > 1

Aggregate functions are not allowed in WHERE.

They are allowed in HAVING.


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SELECT owner,count(regno)

FROM car

GROUP BY ownerHAVING count(regno) > 1

OR

SELECT owner

FROM carGROUP BY owner

HAVING count(regno) > 1

Lecture SQL 1

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