Web Programming Step by Step, 2nd Edition

Lecture 16: Multi-table SQL Queries (Joins)

Reading: 13.3 - 13.5

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.

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

The SELECT statement

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
  • =, >, >=, <, <=
  • <> : not equal
  • BETWEEN min AND max
  • LIKE pattern
  • IN (value, value, ..., value)

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

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.

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.

A suboptimal query

• Exercise: 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';
  

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 practice queries

• What are the names of all movies released in 1995?
• How many people played a part in the movie "Lost in Translation"?
• What are the names of all the people who played a part in the movie "Lost in Translation"?
• Who directed the movie "Fight Club"?
• How many movies has Clint Eastwood directed?
• What are the names of all movies Clint Eastwood has directed?
• What are the names of all directors who have directed at least one horror film?
• What are the names of every actor who has appeared in a movie directed by Christopher Nolan?