Web Programming Step by Step

Chapter 11
Relational Databases and SQL

References: SQL syntax reference, w3schools tutorial

Except where otherwise noted, the contents of this presentation are Copyright 2009 Marty Stepp and Jessica Miller.

Relational databases

• relational database: A method of structuring data as tables associated to each other by shared attributes.
• a table row corresponds to a unit of data called a record; a column corresponds to an attribute of that record
• relational databases typically use Structured Query Language (SQL) to define, manage, and search data

Why use a database? (11.1.1)

• powerful: can search it, filter data, combine data from multiple sources
• fast: can search/filter a database very quickly compared to a file
• big: scale well up to very large data sizes
• safe: built-in mechanisms for failure recovery (e.g. transactions)
• multi-user: concurrency features let many users view/edit data at same time
• abstract: provides layer of abstraction between stored data and app(s)
  • many database programs understand the same SQL commands

Database software

• Oracle
• Microsoft SQL Server (powerful) and Microsoft Access (simple)
• PostgreSQL (powerful/complex free open-source database system)
• SQLite (transportable, lightweight free open-source database system)
• MySQL (simple free open-source database system)
  • many servers run "LAMP" (Linux, Apache, MySQL, and PHP)
  • Wikipedia is run on PHP and MySQL
  • we will use MySQL in this course

Example world database (11.1.2)

SQL basics

SELECT name FROM Cities WHERE id = 17;

INSERT INTO Countries VALUES ('SLD', 'ENG', 'T', 100.0);

• Structured Query Language (SQL): a language for searching and updating a database
• a standard syntax that is used by all database software (with minor incompatiblities)
• a declarative language: describes what data you are seeking, not exactly how to find it

Issuing SQL commands directly in MySQL (11.2.1 - 11.2.2)

SHOW DATABASES;
USE database;
SHOW TABLES;

• SSH to a web server, then type:

$ mysql -u yourusername -p
Password: 
Welcome to the MySQL monitor.  Commands end with ; or \g.

mysql> USE world;
Database changed

mysql> SHOW TABLES;
+--------------------+
| Cities             | 
| Countries          | 
| CountriesLanguages | 
+--------------------+
3 rows in set (0.00 sec)

The SQL SELECT statement (11.2.3)

SELECT column(s) FROM table;

SELECT name, code FROM Countries;

• the SELECT statement searches a database and returns a set of results
  • the column name(s) written after SELECT filter which parts of the rows are returned
  • table and column names are case-sensitive
  • SELECT * FROM table; keeps all columns

The DISTINCT modifier

SELECT DISTINCT column(s) FROM table;

SELECT language
FROM CountriesLanguages;

SELECT DISTINCT language
FROM CountriesLanguages;

• eliminates duplicates from the result set

The WHERE clause (11.2.4)

SELECT column(s) FROM table WHERE condition(s);

SELECT name, population FROM Cities WHERE country_code = "FSM";

• WHERE clause filters out rows based on their columns' data values
• in large databases, it's critical to use a WHERE clause to reduce the result set size
• suggestion: when trying to write a query, think of the FROM part first, then the WHERE part, and lastly the SELECT part

More about the WHERE clause

WHERE column operator value(s)

SELECT name, gnp FROM Countries WHERE gnp > 2000000;

• the WHERE portion of a SELECT statement can use the following operators:
  • =, >, >=, <, <=
  • <> : not equal
  • BETWEEN min AND max
  • LIKE pattern
  • IN (value, value, ..., value)

Multiple WHERE clauses: AND, OR

SELECT * FROM Cities WHERE code = 'USA' AND population >= 2000000;

• multiple WHERE conditions can be combined using AND and OR

Approximate matches: LIKE

WHERE column LIKE pattern

SELECT code, name, population FROM Countries WHERE name LIKE 'United%';

• LIKE 'text%' searches for text that starts with a given prefix
• LIKE '%text' searches for text that ends with a given suffix
• LIKE '%text%' searches for text that contains a given substring

Sorting by a column: ORDER BY (11.2.5)

ORDER BY column(s)

SELECT code, name, population FROM Countries
WHERE name LIKE 'United%' ORDER BY population;

• can write ASC or DESC to sort in ascending (default) or descending order:

  SELECT * FROM Countries ORDER BY population DESC;
  

• can specify multiple orderings in decreasing order of significance:

  SELECT * FROM Countries ORDER BY population DESC, gnp;
  

The SQL INSERT statement (11.2.6)

INSERT INTO table
VALUES (value, value, ..., value);

INSERT INTO student 
VALUES (789, "Nelson", "muntz@fox.com");

• adds a new row to the given table

The SQL UPDATE and DELETE statements

UPDATE table
SET column = value,
    ...,
    column = value
WHERE condition;

DELETE FROM table
WHERE condition;

UPDATE student
SET email = "lisasimpson@gmail.com"
WHERE SID = 888;

DELETE FROM student WHERE SID < 800;

• modifies or deletes an existing row(s) in a table

PHP MySQL functions

Complete PHP MySQL example

# connect to world database on local computer
$db = mysql_connect("localhost", "traveler", "packmybags");
mysql_select_db("world");

# execute a SQL query on the database
$results = mysql_query("SELECT * FROM Countries WHERE population > 100000000;");

# loop through each country
while ($row = mysql_fetch_array($results)) {
	?>

	<li> <?= $row["name"] ?>, ruled by <?= $row["head_of_state"] ?> </li>

	<?php
}
?>

Connecting to MySQL: mysql_connect (11.3.1)

mysql_connect("host", "username", "password");
mysql_select_db("database name");

# connect to world database on local computer
mysql_connect("localhost", "traveler", "packmybags");
mysql_select_db("world");

• mysql_connect opens connection to database on its server
  • any/all of the 3 parameters can be omitted (default: localhost, anonymous)
• mysql_select_db sets which database to examine

Performing queries: mysql_query (11.3.2)

mysql_connect("host", "username", "password");
mysql_select_db("database name");

$results = mysql_query("SQL query");
...

$results = mysql_query("SELECT * FROM Cities WHERE code = 'USA'
                        AND population >= 2000000;");

• mysql_query sends a SQL query to the database
• returns a special result-set object that you don't interact with directly, but instead pass to later functions
• SQL queries are in " ", end with ;, and nested quotes can be ' or \"

Result rows: mysql_fetch_array

mysql_connect("host", "username", "password");
mysql_select_db("database name");
$results = mysql_query("SQL query");

while ($row = mysql_fetch_array($results)) {
	do something with $row;
}

• mysql_fetch_array returns one result row as an associative array
  • the column names are its keys, and each column's values are its values
  • example: $row["population"] gives the population from that row of the results

Error-checking: mysql_error (11.3.3)

if (!mysql_connect("localhost", "traveler", "packmybags")) {
	die("SQL error occurred on connect: " . mysql_error());
}
if (!mysql_select_db("world")) {
	die("SQL error occurred selecting DB: " . mysql_error());
}
$query = "SELECT * FROM Countries WHERE population > 100000000;";
$results = mysql_query($query);
if (!$results) {
	die("SQL query failed:\n$query\n" . mysql_error());
}

• SQL commands can fail: database down, bad password, bad query, ...
• for debugging, always test the results of PHP's mysql functions
  • if they fail, stop script with die function, and print mysql_error result to see what failed
  • give a descriptive error message and also print the query, if any

Complete example w/ error checking

# connect to world database on local computer
check(mysql_connect("localhost", "traveler", "packmybags"), "connect");
check(mysql_select_db("world"), "selecting db");

# execute a SQL query on the database
$query = "SELECT * FROM Countries WHERE population > 100000000;";
$results = mysql_query($query);
check($results, "query of $query");

# loop through each country
while ($row = mysql_fetch_array($results)) {
	?>
	<li> <?= $row["name"] ?>, ruled by <?= $row["head_of_state"] ?> </li>
	<?php
}

# makes sure result is not false/null; else prints error
function check($result, $message) {
	if (!$result) {
		die("SQL error during $message: " . mysql_error());
	}
}
?>

Other MySQL PHP functions

Example simpsons database

Querying multi-table databases

When we have larger datasets spread across multiple tables, we need queries that can answer high-level questions such as:

• What courses has Bart taken and gotten a B- or better?
• What courses have been taken by both Bart and Lisa?
• Who are all the teachers Bart has had?
• How many total students has Ms. Krabappel taught, and what are their names?

To do this, we'll have to join data from several tables in our SQL queries.

Cross product with JOIN (11.4.1)

SELECT column(s) FROM table1 JOIN table2;

SELECT * FROM students JOIN grades;

• cross product or Cartesian product: combines each row of first table with each row of second
  • produces M * N rows, where table 1 has M rows and table 2 has N
  • problem: produces too much irrelevant/meaningless data

Joining with ON clauses (11.4.2)

SELECT column(s)
FROM table1
     JOIN table2 ON condition(s)
     ...
     JOIN tableN ON condition(s);

SELECT *
FROM students
     JOIN grades ON id = student_id;

• join: a relational database operation that combines records from two or more tables if they satisfy certain conditions
• the ON clause specifies which records from each table are matched
• often the rows are linked by their key columns

Join example

SELECT *
FROM students
     JOIN grades ON id = student_id;

• table.column can be used to disambiguate column names:

  SELECT *
  FROM students
       JOIN grades ON students.id = grades.student_id;
  

Filtering columns in a join

SELECT name, course_id, grade
FROM students
     JOIN grades ON students.id = student_id;

• if a column exists in multiple tables, it may be written as table.column

Giving names to tables

SELECT name, g.*
FROM students s
     JOIN grades g ON s.id = g.student_id;

• can give names to tables, like a variable name in Java
• to specify all columns from a table, write table.*

Filtered join (JOIN with WHERE) (11.4.3)

SELECT name, course_id, grade
FROM students s
     JOIN grades g ON s.id = g.student_id
WHERE s.id = 123;

• FROM / JOIN glue the proper tables together, and WHERE filters the results
• what goes in the ON clause, and what goes in WHERE?
  • ON directly links columns of the joined tables
  • WHERE sets additional constraints such as particular values (123, 'Bart')

Multi-way join

SELECT c.name
FROM courses c
     JOIN grades g ON g.course_id = c.id
     JOIN students bart ON g.student_id = bart.id
WHERE bart.name = 'Bart' AND g.grade <= 'B-';

• grade column sorts alphabetically, so grades better than B- are ones <= it

A suboptimal query

• What courses have been taken by both Bart and Lisa?

  SELECT bart.course_id
  FROM grades bart
       JOIN grades lisa ON lisa.course_id = bart.course_id
  WHERE bart.student_id = 123
        AND lisa.student_id = 888;
  

• problem: requires us to know Bart/Lisa's Student IDs, and only spits back course IDs, not names.
• Write a version of this query that gets us the course names, and only requires us to know Bart/Lisa's names, not their IDs.

Improved query

• What courses have been taken by both Bart and Lisa?

  SELECT DISTINCT c.name
  FROM courses c
       JOIN grades g1 ON g1.course_id = c.id
       JOIN students bart ON g1.student_id = bart.id
       JOIN grades g2 ON g2.course_id = c.id
       JOIN students lisa ON g2.student_id = lisa.id
  WHERE bart.name = 'Bart'
        AND lisa.name = 'Lisa';
  

Practice queries

• What are the names of all teachers Bart has had?

  SELECT DISTINCT t.name
  FROM teachers t
       JOIN courses c ON c.teacher_id = t.id
       JOIN grades g ON g.course_id = c.id 
       JOIN students s ON s.id = g.student_id
  WHERE s.name = 'Bart';
  

• How many total students has Ms. Krabappel taught, and what are their names?

  SELECT DISTINCT s.name
  FROM students s
       JOIN grades g ON s.id = g.student_id
       JOIN courses c ON g.course_id = c.id
       JOIN teachers t ON t.id = c.teacher_id
  WHERE t.name = 'Krabappel';
  

Example imdb database (11.1.2)

• also available, imdb_small with fewer records (for testing queries)
• other tables:
  • directors (id, first_name, last_name)
  • movies_directors (director_id, movie_id)
  • movies_genres (movie_id, genre)

IMDb query example

[stepp@webster ~]$ mysql -u myusername -p
Enter password: 
Welcome to the MySQL monitor.  Commands end with ; or \g.

mysql> use imdb_small;
Database changed

mysql> select * from actors where first_name like '%mick%';
+--------+------------+-----------+--------+
| id     | first_name | last_name | gender |
+--------+------------+-----------+--------+
|  71699 | Mickey     | Cantwell  | M      | 
| 115652 | Mickey     | Dee       | M      | 
| 470693 | Mick       | Theo      | M      | 
| 716748 | Mickie     | McGowan   | F      | 
+--------+------------+-----------+--------+
4 rows in set (0.01 sec)

IMDb table relationships / ids (11.4.3)

Designing a query (11.4.4)

• Figure out the proper SQL queries in the following way:
  • Which table(s) contain the critical data? (FROM)
  • Which columns do I need in the result set? (SELECT)
  • How are tables connected (JOIN) and values filtered (WHERE)?
• Test on a small data set (imdb_small).
• Confirm on the real data set (imdb).
• Try out the queries first in the MySQL console.
• Write the PHP code to run those same queries.
  • Make sure to check for SQL errors at every step!!