NCSS: Databases and SQLncss.edu.au/ckeditor_assets/attachments/9/slides.pdf · Motivation SQLite...

NCSS: Databases and SQL

Tim Dawborn

Lecture 1, January, 2016

Motivation SQLite SELECT WHERE JOIN Tips 2

Outline

1 Motivation

2 SQLite

3 Searching for Data

4 Filtering Results

5 Joining multiple tables together

6 Summing it up

Tim Dawborn Databases & SQL Lecture 1, January, 2016

Databases are everywhere

• Who has ever purchased something on the internet?

• Who has a Facebook page?

• Who uses Wikipedia?

The Big Picture

• There are two ways to build a website:

1 A static website is a fixed set of html files2 A dynamic website can display information based on user-input

• Static websites cannot change with user interaction.• and require change to the static files for updates

A dynamic website...

• ... keeps user- and content-specific data in a database

• ... uses several template files to define the look and feel

• ... runs code on the webserver that personalises the templates(using the database) and displays the resulting page to users

ClientWeb-Server

Database

HTML & template files

SQLite

• There are many different database implementations

• Some of the major players: MySQL, PostgreSQL, Oracle

• SQLite is another implementation

SQLite is an embedded database system:

It is a software library that provides an application with aself-contained SQL database engine.

Databases

• A SQLite database consists of a number of tables (relations)

• The data in a table is defined by the columns (fields)

• The data entries in a table are called rows (records)

id012345678

fnameBarryPrueAndrewMathewMaraScottAlecKarenGrant

lnameSchultzRobinsonVarvelNemesBarberHerdmanNewtonBarberOvzinsky

genderMFMMFMMFM

age161716131716161417

Column (field)

Row (record)

Table (relation)

Data Types

• SQLite is a lightweight database system

• Only four different data types are supported• TEXT for textual data• INTEGER for whole integer values• REAL for floating point values• BLOB for “blob-like” (binary) data

• NULL can be used in place of any of these data types• Like the special None value in Python

Implementations

• We will be interacting with SQLite in two different ways

• There is an interpreter where you can type commands directly

1 sqlite>

• We can also access a SQLite database from within Python

1 import sqlite3

SQL Syntax

• SQL (Structured Query Language) is the standard languagefor interacting with databases

• Syntax basics• All statements end in a semicolon• Strings are enclosed in single quotes• Comments are between /* and */ or from -- to EOL• Whitespace independent

1 -- this is our example query

2 SELECT fruit, price

3 FROM /* comments can go in here too */ inventory

4 WHERE fruit != 'apple' AND price > 1.23;

Case Study: Sports Carnival

•• Consider the database system used for a sports carnival whichis run at your high school

• This database needs to keep track of all of the results for allof the events which occurred during the day

• Simplified example database:

people

idfnamelnamegenderage

results

eventpersonresult

events

idnameagegenderat

Searching for Data

• The most frequent operation performed on a database is toask for some data contained within it

• The SQL SELECT keyword allows us to query the database fordata which meets some criterion

• The basic syntax for the SELECT clause is

1 SELECT <field_names>

2 FROM <table_name>

• <field_names> is either a comma-separated list of fields, orthe special * character

Examples of SELECT

• How to we list everything from the people table?

1 SELECT *

2 FROM people;

• The wildcard character means “every field” in this table

1 SELECT id, fname, lname, gender, age

2 FROM people;

• What if we wanted to only list people’s first and last names?

1 SELECT fname, lname

2 FROM people;

Examples of SELECT

1 SELECT *

2 FROM people;

Examples of SELECT

1 SELECT *

2 FROM people;

Examples of SELECT

1 SELECT *

2 FROM people;

Examples of SELECT

1 SELECT *

2 FROM people;

Functions and Aggregation

• The values in the columns in a SELECT clause can betransformed via functions

• Database implementations have a number of builtin functions• SQLite: http://www.sqlite.org/lang corefunc.html

• Example: Return all of the surnames in uppercase

1 SELECT fname, UPPER(lname)

2 FROM people;

• Aggregate functions reduce a whole table or column to justone value, e.g. for counting:

1 SELECT COUNT(*)

2 FROM people;

1 SELECT COUNT(*)

2 FROM people;

1 SELECT COUNT(*)

2 FROM people;

Columns as Expressions

• Column names can also be any valid expression

• What would we expect this query to produce?

1 SELECT fname,

2 2016 - age AS birthyear,

3 age / 10 AS decade

4 FROM people;

Distinct Values

• Sometimes you want to list only the unique values in a column

• How would we list all surnames?

1 SELECT lname

2 FROM people;

• How about all unique surnames?

• The SQL DISTINCT keyword is used in this situation

1 SELECT DISTINCT lname

2 FROM people;

Distinct Values

1 SELECT lname

2 FROM people;

Distinct Values

1 SELECT lname

2 FROM people;

Distinct Values

1 SELECT lname

2 FROM people;

Ordering the Results

• Often we would like the result ordered in some specific way

• In SQL, this is done with the ORDER BY clause

• For example:List the people in the database alphabetically by last name

1 SELECT *

2 FROM people

3 ORDER BY lname;

• How about in descending order?

1 SELECT *

2 FROM people

3 ORDER BY lname DESC;

1 SELECT *

2 FROM people

3 ORDER BY lname;

1 SELECT *

2 FROM people

1 SELECT *

2 FROM people

3 ORDER BY lname;

1 SELECT *

2 FROM people

1 SELECT *

2 FROM people

3 ORDER BY lname;

1 SELECT *

2 FROM people

Multiple Ordering Criteria

• Order all people by capitalised last name in ascending order,and break ties using their first name in descending order

1 SELECT *

2 FROM people

3 ORDER BY UPPER(lname), fname DESC;

• Note that the ordering values can be expressions

• Note also that the ordering modifier keyword applies only tothe previous ordering criterion

1 SELECT *

2 FROM people

1 SELECT *

2 FROM people

Limiting no. Results

• We can place an upper bound on the number of rows returnedusing the LIMIT SQL keyword

1 SELECT *

2 FROM people

3 LIMIT 3;

• Normally used in conjunction with ORDER BY

• Example: Find the 3 people whose surnames are lastalphabetically

1 SELECT *

2 FROM people

3 ORDER BY lname DESC

4 LIMIT 3;

Limiting no. Results

• We can place an upper bound on the number of rows returnedusing the LIMIT SQL keyword

1 SELECT *

2 FROM people

3 LIMIT 3;

• Normally used in conjunction with ORDER BY

• Example: Find the 3 people whose surnames are lastalphabetically

1 SELECT *

2 FROM people

3 ORDER BY lname DESC

4 LIMIT 3;

Filtering Results using WHERE

• Always returning every row from a table is not alwaysdesirable

• imagine how big Facebook’s database is...

• You often want to return only rows which meet some condition

• The SELECT clause can contain a WHERE clause

• Syntax is WHERE <condition>

• Six standard comparison operators for constructingconditional expressions: =, !=, <, <=, >, and >=

Filtering Results

• We can list only the people in the database who are male bycreating a condition expression on the gender column

1 SELECT *

2 FROM people

3 WHERE gender = 'M';

• What (and why) will the following query return?

1 SELECT *

2 FROM people

3 WHERE 1 = 0;

Filtering Results

• We can list only the people in the database who are male bycreating a condition expression on the gender column

1 SELECT *

2 FROM people

3 WHERE gender = 'M';

• What (and why) will the following query return?

1 SELECT *

2 FROM people

3 WHERE 1 = 0;

Comparing Data Types

• Comparison operators with different data types

1 sqlite> SELECT 'one' = 'one';2 1

3 sqlite> SELECT 'one' = 'One';4 0

5 sqlite> SELECT 'a' > 'b';6 0

7 sqlite> SELECT 'a' < 'b';8 1

9 sqlite> SELECT 1 > -1;

11 sqlite> SELECT 1 > 1;

13 sqlite> SELECT 1.0 = 1;

• How about when we compare NULL values?

1 sqlite> SELECT NULL = NULL;

2 NULL

3 sqlite> SELECT NULL != NULL;

4 NULL

• SQL provides the special IS NULL syntax for NULL checking

1 sqlite> SELECT NULL IS NULL;

3 sqlite> SELECT NULL IS NOT NULL;

• How about when we compare NULL values?

1 sqlite> SELECT NULL = NULL;

2 NULL

3 sqlite> SELECT NULL != NULL;

4 NULL

• SQL provides the special IS NULL syntax for NULL checking

1 sqlite> SELECT NULL IS NULL;

3 sqlite> SELECT NULL IS NOT NULL;

Filtering Results

• List all results where the result field is NULL

1 SELECT *

2 FROM results

3 WHERE result IS NULL;

• List only the fastest result from event with ID ‘0’

1 SELECT DISTINCT result

2 FROM results

3 WHERE event = 0

4 AND result IS NOT NULL

5 ORDER BY result

6 LIMIT 1;

Filtering Results

1 SELECT *

2 FROM results

3 WHERE event = 0

5 ORDER BY result

6 LIMIT 1;

Filtering Results

1 SELECT *

2 FROM results

3 WHERE event = 0

5 ORDER BY result

6 LIMIT 1;

Filtering Results

1 SELECT *

2 FROM results

3 WHERE event = 0

5 ORDER BY result

6 LIMIT 1;

Joining Tables

• Sometimes, we need to combine data from multiple tables...

• E.g. “List all personal details about competitors who achieveda result of 20 seconds or less in some event”

• The results and people tables are linked by the foreign id

values

Joining Two Tables

• Who achieved the different times of each event?

1 SELECT fname, lname, event, result

2 FROM results, people

3 WHERE person = id;

• Note: Whenever you list more than one table under FROM, youprobably want an explicit join condition in your WHERE clause

• What happens in the above query if we didn’t have the WHERE

condition in place?

Joining Two Tables

condition in place?

Joining Two Tables

condition in place?

Inner Join

• There is a more efficient notation via the JOIN operator

1 SELECT *

2 FROM results

3 JOIN people ON results.person = people.id;

• Table aliases just like column aliases

1 SELECT *

2 FROM results r

3 JOIN people p ON r.person = p.id;

Some Notes on Schema Creation and Updates

• CREATE TABLE• creates a new table in a database (cf. .schema command)

• INSERT a new row to a table:

1 INSERT INTO results VALUES (3, 2, '00:22');

• UPDATE an existing entry:

1 UPDATE results

2 SET result = '00:21'3 WHERE event = 3 AND person = 2;

• DELETE (one or more – careful!) rows:

1 DELETE FROM results

2 WHERE event = 3 AND person = 2;

More Complex SQL

• List all of the females ordered by last name

1 SELECT *

2 FROM people

3 WHERE gender = 'F'4 ORDER BY lname;

• List all females, plus all males who are over 18

1 SELECT *

2 FROM people

3 WHERE gender = 'F'4 OR (gender = 'M' AND age > 18);

More Complex SQL

1 SELECT *

2 FROM people

1 SELECT *

2 FROM people

More Complex SQL

1 SELECT *

2 FROM people

1 SELECT *

2 FROM people

More Complex SQL

1 SELECT *

2 FROM people

1 SELECT *

2 FROM people

More Complex SQL

• List the name and completion result for each competitor inthe male 16 years 100m event, ordering the results increasingin completion result.

1 SELECT p.fname, p.lname, r.result

2 FROM results r

3 JOIN people p ON r.person = p.id

4 JOIN events e ON r.event = e.id

5 WHERE e.name = '100m'6 AND e.gender = 'M'7 AND e.age = 16

8 AND r.result IS NOT NULL

9 ORDER BY r.result ASC;

More Complex SQL

• List the name and completion result for each competitor inthe male 16 years 100m event, ordering the results increasingin completion result.

1 SELECT p.fname, p.lname, r.result

2 FROM results r

3 JOIN people p ON r.person = p.id

4 JOIN events e ON r.event = e.id

5 WHERE e.name = '100m'6 AND e.gender = 'M'7 AND e.age = 16

8 AND r.result IS NOT NULL

9 ORDER BY r.result ASC;

Summing up

• Databases are used to efficiently store data

• We can query them using the SQL language

• SQLite is a database implementation that we can use fromPython

• Next time: SQL from Python, and good database design

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