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Special Forums UNIX and Linux Applications ruby/SQLite database interface Post 302663291 by LMHmedchem on Thursday 28th of June 2012 12:58:28 AM
Old 06-28-2012
ruby/SQLite database interface

Hello,

I'm not sure this is quite the right place, but there do seem to be allot of posts with folks using ruby to play nicely with databases so I thought I would give it a go.

I am starting a long process of developing a database application bases on SQLite and ruby. This will run on various linux flavors as well as windows/cygwin. I am using ruby because the interface will eventually be browser based and ruby on rails or Sinatra seems like a good way to get that working. I have looked at some tutorials, but they are either too simple or too complicated. My first step is to do a simple import of a tab delimited text file to create a SQLlite database. There would be one database and several tables, so I need to know how to map each column in the file to table/column. It could be just one table for now if that is simpler.

Can some point me in the right direction to get me going? I have worked through allot of this tutorial,
SQLite Ruby tutorial

This code,
Code:
#!/usr/bin/ruby

require 'sqlite3'

begin
    
    db = SQLite3::Database.open "test.db"
    db.execute "CREATE TABLE IF NOT EXISTS Cars(Id INTEGER PRIMARY KEY, 
        Name TEXT, Price INT)"
    db.execute "INSERT INTO Cars VALUES(1,'Audi',52642)"
    db.execute "INSERT INTO Cars VALUES(2,'Mercedes',57127)"
    db.execute "INSERT INTO Cars VALUES(3,'Skoda',9000)"
    db.execute "INSERT INTO Cars VALUES(4,'Volvo',29000)"
    db.execute "INSERT INTO Cars VALUES(5,'Bentley',350000)"
    db.execute "INSERT INTO Cars VALUES(6,'Citroen',21000)"
    db.execute "INSERT INTO Cars VALUES(7,'Hummer',41400)"
    db.execute "INSERT INTO Cars VALUES(8,'Volkswagen',21600)"
    
rescue SQLite3::Exception => e 
    
    puts "Exception occured"
    puts e
    
ensure
    db.close if db
end

inserts some data to a table, but a hard coded comma separated list is a long way from a delimited text file. I think it's fine for the mapping to be hard coded for now, but this is something that would eventually be entered from a browser interface. At the end of the day, one function would be to select a file from a web interface and load it into a SQLite database. There would be mapping for each column in the file to a table/column in the database, and also a hash to determine if the record already exists. Where a record already exists, data could be added, where it doesn't exist, it will be created.

This is relatively straightforward stuff, but it always seems to take a while to get going.

I can post some sample files if that would help.

Thanks for the advice,

LMHmedchem
 

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VALUES(7)						  PostgreSQL 9.2.7 Documentation						 VALUES(7)

NAME
VALUES - compute a set of rows SYNOPSIS
VALUES ( expression [, ...] ) [, ...] [ ORDER BY sort_expression [ ASC | DESC | USING operator ] [, ...] ] [ LIMIT { count | ALL } ] [ OFFSET start [ ROW | ROWS ] ] [ FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY ] DESCRIPTION
VALUES computes a row value or set of row values specified by value expressions. It is most commonly used to generate a "constant table" within a larger command, but it can be used on its own. When more than one row is specified, all the rows must have the same number of elements. The data types of the resulting table's columns are determined by combining the explicit or inferred types of the expressions appearing in that column, using the same rules as for UNION (see Section 10.5, "UNION, CASE, and Related Constructs", in the documentation). Within larger commands, VALUES is syntactically allowed anywhere that SELECT is. Because it is treated like a SELECT by the grammar, it is possible to use the ORDER BY, LIMIT (or equivalently FETCH FIRST), and OFFSET clauses with a VALUES command. PARAMETERS
expression A constant or expression to compute and insert at the indicated place in the resulting table (set of rows). In a VALUES list appearing at the top level of an INSERT, an expression can be replaced by DEFAULT to indicate that the destination column's default value should be inserted. DEFAULT cannot be used when VALUES appears in other contexts. sort_expression An expression or integer constant indicating how to sort the result rows. This expression can refer to the columns of the VALUES result as column1, column2, etc. For more details see ORDER BY Clause. operator A sorting operator. For details see ORDER BY Clause. count The maximum number of rows to return. For details see LIMIT Clause. start The number of rows to skip before starting to return rows. For details see LIMIT Clause. NOTES
VALUES lists with very large numbers of rows should be avoided, as you might encounter out-of-memory failures or poor performance. VALUES appearing within INSERT is a special case (because the desired column types are known from the INSERT's target table, and need not be inferred by scanning the VALUES list), so it can handle larger lists than are practical in other contexts. EXAMPLES
A bare VALUES command: VALUES (1, 'one'), (2, 'two'), (3, 'three'); This will return a table of two columns and three rows. It's effectively equivalent to: SELECT 1 AS column1, 'one' AS column2 UNION ALL SELECT 2, 'two' UNION ALL SELECT 3, 'three'; More usually, VALUES is used within a larger SQL command. The most common use is in INSERT: INSERT INTO films (code, title, did, date_prod, kind) VALUES ('T_601', 'Yojimbo', 106, '1961-06-16', 'Drama'); In the context of INSERT, entries of a VALUES list can be DEFAULT to indicate that the column default should be used here instead of specifying a value: INSERT INTO films VALUES ('UA502', 'Bananas', 105, DEFAULT, 'Comedy', '82 minutes'), ('T_601', 'Yojimbo', 106, DEFAULT, 'Drama', DEFAULT); VALUES can also be used where a sub-SELECT might be written, for example in a FROM clause: SELECT f.* FROM films f, (VALUES('MGM', 'Horror'), ('UA', 'Sci-Fi')) AS t (studio, kind) WHERE f.studio = t.studio AND f.kind = t.kind; UPDATE employees SET salary = salary * v.increase FROM (VALUES(1, 200000, 1.2), (2, 400000, 1.4)) AS v (depno, target, increase) WHERE employees.depno = v.depno AND employees.sales >= v.target; Note that an AS clause is required when VALUES is used in a FROM clause, just as is true for SELECT. It is not required that the AS clause specify names for all the columns, but it's good practice to do so. (The default column names for VALUES are column1, column2, etc in PostgreSQL, but these names might be different in other database systems.) When VALUES is used in INSERT, the values are all automatically coerced to the data type of the corresponding destination column. When it's used in other contexts, it might be necessary to specify the correct data type. If the entries are all quoted literal constants, coercing the first is sufficient to determine the assumed type for all: SELECT * FROM machines WHERE ip_address IN (VALUES('192.168.0.1'::inet), ('192.168.0.10'), ('192.168.1.43')); Tip For simple IN tests, it's better to rely on the list-of-scalars form of IN than to write a VALUES query as shown above. The list of scalars method requires less writing and is often more efficient. COMPATIBILITY
VALUES conforms to the SQL standard. LIMIT and OFFSET are PostgreSQL extensions; see also under SELECT(7). SEE ALSO
INSERT(7), SELECT(7) PostgreSQL 9.2.7 2014-02-17 VALUES(7)
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