Question

For a simple android app I’m creating as a teaching tool for myself (for using relational dbs/SQL among other things – pardon the simplicity of the question if you will). I’m pre-creating a sqlite db to ship with the application. I’m doing this based on the following SO question.

I’ve got two tables with a many to many relationship and a junction table to define those relationships as follows:

CREATE TABLE Names (_id INTEGER PRIMARY KEY,
                    name TEXT
                   );

CREATE TABLE Categories (_id INTEGER PRIMARY KEY,
                         category TEXT
                        );

CREATE TABLE Name_Category (name_id INTEGER,
                            category_id INTEGER,
                            PRIMARY KEY (name_id, category_id),
                            foreign key (name_id) references Names(_id),
                            foreign key (category_id) references Categories(_id) 
                           );

I’ve got sets of insert statements to fill the Names and Categories tables. I’m now faced with the task of filling the junction table. I’m sure that I could create the insert statements by hand by looking up the ids of the names and categories that I want to match, but that seems a bit silly.

In order to automatically create the insert statements for the junction table, I imagine that I could create a script based on a set of name and category pairs that will search for the appropriate ids and dump an insert statement. (I came up with this as I was asking the question and will research it. Don’t you love it when that happens?)

Does anybody have any suggestions for ways to do this?

EDIT I added the foreign keys because, as pointed out below, they’ll help maintain integrity between the tables.

EDIT #2 To solve this, I created a simple Perl script that would take a text file with name – category pairs and dump them out into another file with the appropriate SQL statements.

The name – category text file has a format as follows:

'Name' 'Category'

The Perl script looks like this:

use strict;
use warnings;

open (my $name_category_pair_file, "<", "name_category.txt") or die "Can't open name_category.txt: $!";
open (my $output_sql_file, ">", "load_name_category_junction_table.sqlite") or die "Can't open load_name_category_junction_table.sqlite: $!";

while (<$name_category_pair_file>) {
    if (/('[a-zA-Z ]*') ('[a-zA-Z ]*')/) {
        my $sql_statement = "INSERT INTO Name_Category VALUES (
                             (SELECT _id FROM Names WHERE name = $1),
                             (SELECT _id FROM Categories WHERE category = $2))\;\n\n";

        print $output_sql_file $sql_statement;
    }
}

close $name_category_pair_file or die "$name_category_pair_file: $!";
close $output_sql_file or die "$output_sql_file: $!";

Answer 1

You can use this insert in your script or code (replacing the strings or using ?):

insert into Name_Category values(
   (select _id from Categories where category='CAT1'),
   (select _id from Names where name='NAME1'));

Also, you can alter the Name_Category table to constraint on the values that can be inserted and/or deleted:

CREATE TABLE Name_Category ( name_id INTEGER NOT NULL,
   category_id INTEGER NOT NULL,
   PRIMARY KEY (name_id, category_id),
   foreign key (name_id) references Names(_id),
   foreign key (category_id) references Categories(_id));