Web Programming Step by Step, 2nd Edition

Lecture 11: Relational Databases and SQL

Reading: 13.1–13.2, 13.4–13.4.2

Except where otherwise noted, the contents of this document are Copyright 2012 Marty Stepp, Jessica Miller, and Victoria Kirst. All rights reserved. Any redistribution, reproduction, transmission, or storage of part or all of the contents in any form is prohibited without the author's expressed written permission.

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?

• 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 simpsons database

• to test queries on this database, use username homer, password d0ughnut

Example world database

• to test queries on this database, use username traveler, password packmybags

Example imdb database

• also available, imdb_small with fewer records (for testing queries)
• to test queries on this database, use the username/password that we will email to you soon

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)
  • generally case-insensitive
• a declarative language: describes what data you are seeking, not exactly how to find it

The MySQL console

SHOW DATABASES;
USE database;
SHOW TABLES;

• connect to Webster in an SSH program like PuTTY, 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 | 
| languages | 
+-----------+
3 rows in set (0.00 sec)

SQL console 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)

The SQL SELECT statement

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;

• eliminates duplicates from the result set

SELECT language
FROM languages;

SELECT DISTINCT language
FROM languages;

The WHERE clause

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

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;
  

• see also: GROUP BY

Limiting rows: LIMIT

LIMIT number

SELECT name FROM cities WHERE name LIKE 'K%' LIMIT 5;

• can be used to get the top-N of a given category (ORDER BY and LIMIT)
• also useful as a sanity check to make sure your query doesn't return 10⁷ rows

Querying a Database in PHP with PDO

$name = new PDO("dbprogram:dbname=database;host=server", username, password);
$name->query("SQL query");

// connect to world database on local server
$db = new PDO("mysql:dbname=world;host=localhost", "traveler", "packmybags");
$db->query("SELECT * FROM countries WHERE population > 100000000;");

• PDO database library allows you to connect to many different database programs
  • replaces older, less versatile functions like mysql_connect
• PDO object's query function returns rows that match a query

Result rows: query

$db = new PDO("dbprogram:dbname=database;host=server", username, password);
$stmt = $db->query("SQL query");
foreach ($stmt as $row) {
	do something with $row;
}

• query returns a result set (“statement object”)
  • can iterate over $stmt using foreach to get each row in the results
  • each row is an associative array of [column name -> value]
  • example: $row["population"] gives the value of the population column

A complete example

$db = new PDO("mysql:dbname=imdb_small", "jessica", "guinness");
$stmt = $db->query("SELECT * FROM actors WHERE last_name LIKE 'Del%'");
foreach ($stmt as $row) {
	?>
	<li> First name: <?= $row["first_name"] ?>,
	     Last name:  <?= $row["last_name"]  ?> </li>
	<?php
}

PDO object methods

PDOStatement methods

The $stmt variable returned by PDO's query method is technically not an array but an object of type PDOStatement. It can be foreach-ed over like an array, but it also has the following methods:

if ($stmt->rowCount() > 0) {
	$first_row = $stmt->fetch();
}
$remaining_rows = $stmt->fetchAll();