Download sql101-webdev-nov-2012

SQL 101
for Web Developers
14 November 2012
• What is a database and why have one?
• Tables, relationships, normalization
• SQL
– What SQL is and isn’t
– CRUD: four basic operations
– Select queries step by step
– Using JOINs in SELECT queries
• Next steps and resources
What is a database and why?
• A database can be as simple as a single, excellike table
• For your website, a database brings you the
power of dynamic data
• Different technologies are used (e.g., MySQL,
SQL Server, Access, Oracle, Sybase, DB2,
SQLite) but most website-connected dbs are
relational databases
Student records – one table
name
phone
course
term
grade
Tom Rees
111-2222
ENG 101 Spring 2009
3.5
Beth Hardy
333-4444
EEB 102
Fall 2009
3.0
Tom Rees
111-2222
EEB 102
Fall 2010
3.5
A database is a whole spreadsheet file.
A table is a tab/sheet in the spreadsheet, with
each one being given a name.
A column is a column in both.
A row is a row in both.
Student records – one table
name
phone
course
term
grade
Tom Rees
111-2222
ENG 101 Spring 2009
3.5
Beth Hardy
333-4444
EEB 102
Fall 2009
3.0
Tom Rees
111-2222
EEB 102
Fall 2010
3.5
fname
lname
phone
course_
prefix
course_
num
term
year
grade
Tom
Rees
111-2222
ENG
101
Spring
2009
3.5
Beth
Hardy
333-4444
EEB
102
Fall
2009
3.0
Tom
Rees
111-2222
EEB
102
Fall
2010
3.5
Normalized Data
Normalized Data
Table relationships
• Primary keys – unique identifier for each
record in one table
• Primary keys – also used to build relationships
among tables
– one-to-one (studentID to email address)
– one-to-many (one student – many enrollment
records)
– many-to-many (many students to many sections)
Table relationships
Many to many: books and authors
Author table: authorID name
Book table: bookID, title, ISBN
Book_Author table:
Book_AuthorID
bookID
authorID
1
3
6
2
3
9
3
4
7
4
5
9
SQL statements
• SQL is a standard language for accessing
databases.
• It is not a programming language or a scripting
language.
• Four basic types of queries: “CRUD” stands for
– create
– read
– update
– delete
SQL statements
• Four basic types: “CRUD” stands for
– (create)
CREATE (table) INSERT INTO (table) VALUES
– (read) SELECT column_names FROM table_name
– (update) UPDATE
– (delete) DELETE rows, DROP tables
fname
lname
phone
course_
prefix
course_
num
term
year
grade
Tom
Rees
111-2222
ENG
101
Spring
2009
3.5
Beth
Hardy
333-4444
EEB
102
Fall
2009
3.0
Tom
Rees
111-2222
EEB
102
Fall
2010
3.5
SELECT * FROM student_records; (will get entire
contents of table)
SELECT fname FROM student_records;
+---------+
| fname
|
+---------+
| Tom
|
| Beth
|
| Tom
|
+---------+
fname
lname
phone
course_
prefix
course_
num
term
year
grade
Tom
Rees
111-2222
ENG
101
Spring
2009
3.5
Beth
Hardy
333-4444
EEB
102
Fall
2009
3.0
Tom
Rees
111-2222
EEB
102
Fall
2010
3.5
SELECT grade FROM student_records WHERE
lname = 'Rees';
+---------+
| grade
|
+---------+
| 3.5
|
| 3.5
|
+---------+
• The WHERE statement lets you filter records
• Lots of operators in addition to =
• SELECT * FROM student_records
WHERE lname = 'Rees' AND term = 'Fall';
• SELECT * FROM student_records
WHERE lname = 'Rees' AND
(term = 'Fall' OR term = 'Spring');
• SELECT * FROM student_records
WHERE lname LIKE 'R%';
• SELECT * FROM student_records WHERE lname
IN ('Rees', 'Hardy');
SELECT fname, lname FROM student_records
ORDER BY lname;
+---------+---------+
| fname
| lname
|
+---------+---------+
| Tom
| Rees
|
| Tom
| Rees
|
| Beth
| Hardy
|
+---------+---------+
SELECT DISTINCT lname FROM
student_records;
+---------+
| lname
|
+---------+
| Rees
|
| Hardy
|
+---------+
Why you must be familiar with your data:
When you get results back from a query, how do
you know you got the right results?
Joins
Joins
• JOIN (Inner Join): Return rows when there is at least one
match in both tables
• LEFT (Outer) JOIN: Return all rows from the left table,
even if there are no matches in the right table
• RIGHT (Outer) JOIN: Return all rows from the right
table, even if there are no matches in the left table
• FULL (Outer) JOIN: Return rows when there is a match
in one of the tables
Joins
• The syntax used determines which table will
be fully represented.
• A row of NULL values is substituted when a
matching row is not present.
table “Author” contains names of authors, with the primary key
“Author_ID”
table “Book_Author” contains ISBN numbers, keyed with the
primary key “Book_Author_ID” and linked to the Author table
by the foreign key “Author_ID.”
SELECT First_Name, Last_Name, ISBN FROM Author INNER JOIN
Book_Author ON Author.Author_ID = Book_Author.Author_ID;
+------------+-----------+------------+
| First_Name | Last_Name | ISBN
|
+------------+-----------+------------+
| Chad
| Russell
| 1590593324 |
| Jon
| Stephens | 1590593324 |
+------------+-----------+------------+
This INNER JOIN only gets rows where there is a match. If an
author doesn’t have a book, and a book doesn’t have an
author, neither type of record shows up.
When we need at least one row in the result set for every row in
a given table, regardless of matching rows, we use an OUTER
JOIN query.
SELECT First_Name, Last_Name, ISBN FROM Author LEFT OUTER
JOIN Book_Author ON Author.Author_ID =
Book_Author.Author_ID;
+------------+-----------+------------+
| First_Name | Last_Name | ISBN
|
+------------+-----------+------------+
| Chad
| Russell
| 1590593324 |
| Jon
| Stephens | 1590593324 |
| Mike
| Hillyer
| NULL
|
+------------+-----------+------------+
This query will NOT return rows for books that don’t have an
author – just authors that don’t have a book.
SELECT s.first_name,
s.last_name, c.course_prefix,
c.course_number
FROM students s,
courses c, enrollment e,
sections sc
WHERE s.student_id
= e.student_id
AND e.section_id
= sc.section_id
AND c.course_id
= sc.course_id;
Although not explicitly named
as such, this is an INNER JOIN.
How do you grab the data output from a query?
Depends on your database type and your
programming or scripting language
(for example, you can use PHP functions to read
the result set one row at a time and put the
results into an array.)
What didn’t we cover?
•
•
•
•
•
•
•
•
Database optimization and performance
Security issues like sql injection
Data integrity and constraints
Data types
Views
Triggers
Transactions
PDOs
What else didn’t we cover?
•
•
•
•
•
•
Logic
Math expressions
String manipulation
Parameterized queries
Database design
Different database technologies
Why do I need to know any SQL when my CMS
does it all for me?
• It helps to know your data
• Sometimes the fastest way to fix a problem is with a straight
database query
• You can check up on the queries written by your CMS
• You can’t always use a CMS – sometimes you need to write a
custom web app.
• You might need to migrate data from your department’s
Access database into another db.
• What is a database and why have one?
• Tables, relationships, normalization
• SQL
– What SQL is and isn’t
– CRUD: four basic operations
– Select queries step by step
– Using JOINs in SELECT queries
• Next steps and resources: depends on the
technologies you are using. There are a lot of
good online tutorials, and of course books.
I like “Learn SQL the hard way”
http://sql.learncodethehardway.org/book/
(coaches you through creating a SQLite db on
your local computer, entering data into it, and
then running queries on it)
Why is it “the hard way”? Enforces precision, attention to detail,
and persistence by requiring you to type each exercise (no
copy-paste!) and make it run.