Home Accessibility Courses Twitter The Mouth Facebook Resources Site Map About Us Contact
 
Python and Tcl - public course schedule [here]
Private courses on your site - see [here]
Please ask about maintenance training for Perl, PHP, Lua, etc
 
Ruby / SQLite3 example program, showing JOIN v LEFT JOIN

Ruby includes support for the SQLite3 database ... and SQLite is an excellent tool for developing and testing ("Spike solutions") your database structure and concepts before implementing a larger system. For a smaller and straightforward requirement, SQLite may be all that you require.

Towards the end of the Ruby course that ran last week, I demonstrated how simple it is to connect to a SQLite database, insert data and select data ... and I have it all rolled into a single example - full source code [here].

Here are the basics ...
Connect to the database:
  require 'sqlite3'
  db = SQLite3::Database.new( "data.db" )
Insert a record:
  db.execute 'insert into aisles values (2,"Sweeties")'
Select and iterate through a result set:
  db.execute( "select * from aisles" ) do |row|
    p row
  end
Close the connection:
  db.close

I've also coded a slighly more complex example - [here] - in which I created two tables, and used joins to connect them together. In my example, I have a shop with a number of aisles, each of which has a number of products on it. If you visit the example link, you'll find sample output too showing this in action

• If I select from both tables, I get all possible joined records out - so with 3 aisles and four products in my example, this gave me 12 rows. Clearly not a sensible thing to do!
select * from aisles,products

• By using a JOIN, I got all combined records with a match between the tables in specified columns. That works very well, but it does not report any "orphan" data - i.e. aisles that have no products in them, or products which are not in any know aisle. That's 3 records in my example
select * from aisles join products on aisles.aid = products.aid

• By using a LEFT JOIN, I got all the combined records of the example above PLUS orphan records in the left hand table - in my example, that's aisles with no products.
select * from aisles left join products on aisles.aid = products.aid

• And finally, by adding where pid is NULL - pid being the primary key in the second table - I'm able to list out only the orphan records. In my example, there's just one of those.
select * from aisles left join products on aisles.aid = products.aid where PID is NULL
(written 2013-02-16)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
R117 - Ruby GUIs, XML, SQL Database Connectivity
  [4679] Reading in XML in Ruby with xmlsimple - (2016-05-18)
  [2714] A simple example - XML from a Ruby program - (2010-04-10)
  [1890] MySQL database from Ruby - an example - (2008-11-16)

S200 - MySQL - SQLite
  [4024] SQL databases from Python - an SQLite example - (2013-03-02)
  [4007] Which database should I use? MySQL v SQLite - (2013-02-16)
  [3269] Files or Databases? MySQL, SQLite, or Oracle? - (2011-04-28)
  [3139] Steering our Python courses towards wxPython, SQLite and Django - (2011-01-19)
  [3099] Perl - database access - DBD, DBI and DBIx modules - (2010-12-22)
  [2786] Factory methods and SqLite in use in a Python teaching example - (2010-05-29)
  [2746] Model - View - Controller demo, Sqlite - Python 3 - Qt4 - (2010-04-29)
  [2745] Connecting Python to sqlite and MySQL databases - (2010-04-28)
  [2744] PyQt (Python and Qt) and wxPython - GUI comparison - (2010-04-28)
  [2561] The future of MySQL - (2010-01-03)


Back to
New guest ... becoming returning guest
Previous and next
or
Horse's mouth home
Forward to
Which database should I use? MySQL v SQLite
Some other Articles
Really Simple Rails
Clear, concise examples - Ruby classes and objects.
Reading and checking user inputs - first lessons - Ruby
Ruby / SQLite3 example program, showing JOIN v LEFT JOIN
New guest ... becoming returning guest
Annual Accounts - a big job for a small business
Web and console - same principle, same code - Ruby example
Images of Melksham you havent seen before
Helping search engines with appropriate 400 error codes
4759 posts, page by page
Link to page ... 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96 at 50 posts per page


This is a page archived from The Horse's Mouth at http://www.wellho.net/horse/ - the diary and writings of Graham Ellis. Every attempt was made to provide current information at the time the page was written, but things do move forward in our business - new software releases, price changes, new techniques. Please check back via our main site for current courses, prices, versions, etc - any mention of a price in "The Horse's Mouth" cannot be taken as an offer to supply at that price.

Link to Ezine home page (for reading).
Link to Blogging home page (to add comments).

You can Add a comment or ranking to this page

© WELL HOUSE CONSULTANTS LTD., 2019: 404 The Spa • Melksham, Wiltshire • United Kingdom • SN12 6QL
PH: 01225 708225 • EMAIL: info@wellho.net • WEB: http://www.wellho.net • SKYPE: wellho

PAGE: http://www.wellho.net/mouth/4006_Rub ... -JOIN.html • PAGE BUILT: Sat May 27 16:49:10 2017 • BUILD SYSTEM: WomanWithCat