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For 2021 - online Python 3 training - see ((here)).

Our plans were to retire in summer 2020 and see the world, but Coronavirus has lead us into a lot of lockdown programming in Python 3 and PHP 7.
We can now offer tailored online training - small groups, real tutors - works really well for groups of 4 to 14 delegates. Anywhere in the world; course language English.

Please ask about private 'maintenance' training for Python 2, Tcl, 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
S200 - MySQL - SQLite
  [2561] The future of MySQL - (2010-01-03)
  [2744] PyQt (Python and Qt) and wxPython - GUI comparison - (2010-04-28)
  [2745] Connecting Python to sqlite and MySQL databases - (2010-04-28)
  [2746] Model - View - Controller demo, Sqlite - Python 3 - Qt4 - (2010-04-29)
  [2786] Factory methods and SqLite in use in a Python teaching example - (2010-05-29)
  [3099] Perl - database access - DBD, DBI and DBIx modules - (2010-12-22)
  [3139] Steering our Python courses towards wxPython, SQLite and Django - (2011-01-19)
  [3269] Files or Databases? MySQL, SQLite, or Oracle? - (2011-04-28)
  [4007] Which database should I use? MySQL v SQLite - (2013-02-16)
  [4024] SQL databases from Python - an SQLite example - (2013-03-02)

R117 - Ruby GUIs, XML, SQL Database Connectivity
  [1890] MySQL database from Ruby - an example - (2008-11-16)
  [2714] A simple example - XML from a Ruby program - (2010-04-10)
  [4679] Reading in XML in Ruby with xmlsimple - (2016-05-18)


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Which database should I use? MySQL v SQLite
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Really Simple Rails
Clear, concise examples - Ruby classes and objects.
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Ruby / SQLite3 example program, showing JOIN v LEFT JOIN
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Web and console - same principle, same code - Ruby example
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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.

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