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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:
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|
Close the connection:
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
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 articlesR117 - Ruby GUIs, XML, SQL Database Connectivity 
Reading in XML in Ruby with xmlsimple - (2016-05-18) 
A simple example - XML from a Ruby program - (2010-04-10) 
MySQL database from Ruby - an example - (2008-11-16)S200 - MySQL - SQLite 
SQL databases from Python - an SQLite example - (2013-03-02) 
Which database should I use? MySQL v SQLite - (2013-02-16) 
Files or Databases? MySQL, SQLite, or Oracle? - (2011-04-28) 
Steering our Python courses towards wxPython, SQLite and Django - (2011-01-19) 
Perl - database access - DBD, DBI and DBIx modules - (2010-12-22) 
Factory methods and SqLite in use in a Python teaching example - (2010-05-29) 
Model - View - Controller demo, Sqlite - Python 3 - Qt4 - (2010-04-29) 
Connecting Python to sqlite and MySQL databases - (2010-04-28) 
PyQt (Python and Qt) and wxPython - GUI comparison - (2010-04-28) 
The future of MySQL - (2010-01-03)
Some other Articles
Really Simple RailsClear, concise examples - Ruby classes and objects.Reading and checking user inputs - first lessons - RubyRuby / SQLite3 example program, showing JOIN v LEFT JOINNew guest ... becoming returning guestAnnual Accounts - a big job for a small businessWeb and console - same principle, same code - Ruby exampleImages of Melksham you havent seen beforeHelping search engines with appropriate 400 error codes
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