stag-pgslurp

- NAME
- SYNOPSIS
- DESCRIPTION
- MAKING DATABASE FROM XML FILES

- Step 1: Prepare input file
- Step 2: Generating CREATE TABLE statements
- Step 3: Store the data in the db

# NAME

stag-pgslurp.pl

# SYNOPSIS

      stag-pgslurp.pl -d "dbi:Pg:dbname=mydb;host=localhost" myfile.xml

# DESCRIPTION

This script is for storing data (specified in a nested file format such as XML or S-Expressions) in a database. It assumes a database schema corresponding to the tags in the input data already exists.

# MAKING DATABASE FROM XML FILES

It is possible to automatically generate a database schema and populate it directly from XML files (or from Stag objects or other Stag compatible files). Of course, this is no substitute for proper relational design, but often it can be necessary to quickly generate databases from heterogeneous XML data sources, for the purposes of data mining.

There are 3 steps involved:

1. Prepare the input XML (for instance, modifying db reserved words). 2. Autogenerate the CREATE TABLE statements, and make a db from these. 3. Store the XML data in the database.

## Step 1: Prepare input file

You may need to make modifications to your XML before it can be used to make a schema. If your XML elements contain any words that are reserved by your DB you should change these.

Any XML processing tool (eg XSLT) can be used. Alternatively you can use the script 'stag-mogrify'

e.g. to get rid of '-' characters (this is how Stag treates attributes) and to change the element with postgresql reserved word 'date', do this:

      stag-mogrify.pl -xml -r 's/^date$/moddate/' -r 's/\-//g' data.xml > data.mog.xml

You may also need to explicitly make elements where you will need linking tables. For instance, if the relationship between 'movie' and 'star' is many-to-many, and your input data looks like this:

      (movie
       (name "star wars")
       (star
        (name "mark hamill")))

You will need to *interpose* an element between these two, like this:

      (movie
       (name "star wars")
       (movie2star
        (star
         (name "mark hamill"))))

you can do this with the -i switch:

      stag-mogrify.pl -xml -i movie,star,movie2star data.xml > data.mog.xml

or if you simply do:

      stag-mogrify.pl -xml -i star data.xml > data.mog.xml

the mogrifier will simply interpose an element above every time it sees 'star'; the naming rule is to use the two elements with an underscore between (in this case, 'movie_star').

## Step 2: Generating CREATE TABLE statements

Use the stag-autoddl.pl script;

      stag-autoddl.pl data.mog.xml > table.sql

The default rule is to create foreign keys from the nested element to the outer element; you will want linking tables tobe treated differently (a linking table will point to parent and child elements).

      stag-autoddl.pl -l movie2star -l star2character data.mog.xml > table.sql

Once you have done this, load the statements into your db; eg for postgresql (for other databases, use the SQL::Translator manpage)

      psql -a mydb < table.sql

If something goes wrong, go back to step 1 and sort it out!

Note that certain rules are followed: ever table generated gets a surrogate primary key of type 'serial'; this is used to generate foreign key relationships. The rule used is primary and foreign key names are the name of the table with the '_id' suffix.

Feel free to modify the autogenerated schema at this stage (eg add uniqueness constraints)

## Step 3: Store the data in the db

      stag-pgslurp.pl -u movie -d 'dbi:Pg:mydb' data.mog.xml

You generally dont need extra metadata here; everything can be infered by introspecting the database.

The -u|unit switch controls when transactions are committed

If this works, you should now be able to retreive XML from the database, eg

      selectall_xml.pl -d 'dbi:Pg:mydb' 'SELECT * FROM x NATURAL JOIN y'