CSE 190M Web Programming

Lecture 12: More SQL, JOINS

Reading: 13.2 - 13.5

Except where otherwise noted, the contents of this document are Copyright 2012 Marty Stepp, Jessica Miller, Victoria Kirst and Roy McElmurry IV. 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.

Related tables and keys

• primary key: a column guaranteed to be unique for each record (e.g. Lisa Simpson's ID 888)
• foreign key: a column in table A storing a primary key value from table B
  • (e.g. records in grades with student_id of 888 are Lisa's grades)
• normalizing: splitting tables to improve structure / redundancy (linked by unique IDs)

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

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

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: combines records from two or more tables if they satisfy certain conditions
• the ON clause specifies which records from each table are matched
• the rows are often linked by their key columns (id)

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 id = student_id;

Filtered join (JOIN with WHERE)

SELECT name, course_id, grade
FROM students
JOIN grades ON id = student_id
WHERE name = 'Bart';

• 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')

What's wrong with this?

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

• The above query produces the same rows as the previous one, but it is poor style. Why?
• The JOIN ON clause is poorly chosen. It doesn't really say what connects a grades record to a students record.
  • They are related when they are for a student with the same id.
  • Filtering out by a specific ID or name should be done in the WHERE clause, not JOIN ON.

Giving names to tables

SELECT s.name, g.*
FROM students s
JOIN grades g ON s.id = g.student_id
WHERE g.grade <= 'C';

• can give names to tables, like a variable name in Java
• to specify all columns from a table, write table.*
• (grade column sorts alphabetically, so grades C or better are ones <= it)

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-';

• More than 2 tables can be joined, as shown above
• What does the above query represent?
• The names of all courses in which Bart has gotten a B- or better.

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';
  

Designing a query

• 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!!

Example imdb database

• also available, imdb_small with fewer records (for testing queries)

IMDb table relationships / ids

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)

Learning about databases and tables

SHOW DATABASES;
SHOW TABLES;
DESCRIBE table;

SHOW TABLES;
+-----------+
| students  |
| courses   |
| grades    |
| teachers  |
+-----------+    4 rows in set

The SQL INSERT statement

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

INSERT INTO students
VALUES (789, "Nelson", "muntz@fox.com", "haha!");

• adds a new row to the given table
• columns' values should be listed in the same order as in the table
• How would we record that Nelson took CSE 190M and got a D+ in it?

More about INSERT

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

INSERT INTO students (name, email)
VALUES ("Lewis", "lewis@fox.com");

• some columns have default or auto-assigned values (such as IDs)
• omitting them from the INSERT statement uses the defaults

The SQL REPLACE statement

REPLACE INTO table (columnName, columnName, ..., columnName)
VALUES (value, value, ..., value);

REPLACE INTO students
VALUES (789, "Martin", "prince@fox.com");

• just like INSERT, but if an existing row exists for that key (ID), it will be replaced
• can pass optional list of column names, like with INSERT

The SQL UPDATE statement

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

UPDATE students
SET email = "lisasimpson@gmail.com"
WHERE id = 888;

• modifies an existing row(s) in a table
• BE CAREFUL! If you omit the WHERE, it modifies ALL rows

The SQL DELETE statement

DELETE FROM table
WHERE condition;

DELETE FROM students
WHERE id = 888;

• removes existing row(s) in a table
• can be used with other syntax like LIMIT, LIKE, ORDER BY, etc.
• BE CAREFUL! If you omit the WHERE, it deletes ALL rows

Creating and deleting an entire database

CREATE DATABASE name;
DROP DATABASE name;

CREATE DATABASE warcraft;

• adds/deletes an entire database from the server

Creating and deleting a table

CREATE TABLE name (
	columnName type constraints,
	...
	columnName type constraints
);
DROP TABLE name;

CREATE TABLE students (
	id INTEGER,
	name VARCHAR(20),
	email VARCHAR(32),
	password VARCHAR(16)
);

• adds/deletes a table from this database
• all columns' names and types must be listed (see next slide)

SQL data types

• BOOLEAN: either TRUE or FALSE
• INTEGER
• DOUBLE
• VARCHAR(length) : a string
• ENUM(value, ..., value): a fixed set of values
• DATE, TIME, DATETIME
• BLOB : binary data
• quick reference

Column constraints

CREATE TABLE students (
	id INTEGER UNSIGNED NOT NULL PRIMARY KEY,
	name VARCHAR(20) NOT NULL,
	email VARCHAR(32),
	password VARCHAR(16) NOT NULL DEFAULT "12345"
);

• NOT NULL: not allowed to insert a null/empty value in any row for that column
• PRIMARY KEY / UNIQUE: no two rows can have the same value
• DEFAULT value: if no value is provided, use the given default
• AUTO_INCREMENT: default value is the last row's value plus 1 (useful for IDs)
• UNSIGNED: don't allow negative numbers (INTEGER only)

Rename a table

ALTER TABLE name RENAME TO newName;

ALTER TABLE students RENAME TO children;

• changes the name of an existing table

Add/remove/modify a column in a table

ALTER TABLE name
	ADD COLUMN columnName type constraints;

ALTER TABLE name DROP COLUMN columnName;

ALTER TABLE name
	CHANGE COLUMN oldColumnName newColumnName type constraints;

• adds/deletes/respecifies a column in an existing table
• if a column is added, all existing rows are given a default value for that column

Database design principles

• database design : the act of deciding the schema for a database
• database schema: a description of what tables a database should have, what columns each table should contain, which columns' values must be unique, etc.
• some database design principles:
  • keep it simple, stupid (KISS)
  • provide an identifier by which any row can be uniquely fetched
  • eliminate redundancy, especially of lengthy data (strings)
    • integers are smaller than strings and better to repeat
  • favor integer data for comparisons and repeated values
    • integers are smaller than strings and better to repeat
    • integers can be compared/searched more quickly than strings, real numbers

First database design

• what's good and bad about this design?
  • good: simple (one table), can see all data in one place
  • bad: redundancy (name, email, course repeated frequently)
  • bad: most searches (e.g. find a student's courses) will have to rely on string comparisons
  • bad: there is no single column whose value will be unique in each row

Improved database design

• normalizing: splitting tables to improve structure / redundancy (linked by unique IDs)
• primary key: a column guaranteed to be unique for each record (e.g. Lisa Simpson's ID 888)
• foreign key: a column in table A storing a primary key value from table B
  • (e.g. records in grades with student_id of 888 are Lisa's grades)

Database design exercise

Suppose we want to write a web store like Amazon.com. The store sells products that can be purchased by customers online. The customer can add items to their shopping cart and then order them. The customer can also check the order's status, whether it has shipped, etc.

• What are some database tables and columns we could use?
• Is your design normalized? Does it have any redundancy?

Connecting to a server in a terminal

• You can talk to a database directly using a secure shell (SSH) terminal program such as PuTTY.
• Open your SSH program and connect to webster.cs.washington.edu.
  • in a Mac/Linux Terminal window, type: ssh yourUWnetID@webster.cs.washington.edu
• Log in with your normal UW NetID and password.

Using the MySQL console

Once connected via SSH, type the following command at the prompt:

mysql -u username -p

Type in your MySQL password (should have been emailed to you). The screen will not show anything as you type the password.

Now at the mysql> prompt, you can type SQL commands. End each with a semicolon ;.