Question

I need to create a configuration file from a data file that looks as follows:

MAN1_TIME '01-JAN-2010 00:00:00.0000 UTC'
MAN1_RX 123.45
MAN1_RY 123.45
MAN1_RZ 123.45
MAN1_NEXT 'MAN2'
MAN2_TIME '01-MAR-2010 00:00:00.0000 UTC'
MAN2_RX 123.45
[...]
MAN2_NEXT 'MANX'
[...]
MANX_TIME [...]

This file describes different “legs” of a trajectory. In this case, MAN1 is chained to MAN2, and MAN2 to MANX. In the original file, the chains are not as obvious (i.e., they are non-sequential).

I’ve managed to read the file and store in an Sqlite3 database (I’m using the Python interface). The table is stored with three columns: Id, Par, and Val; for instance, Id='MAN1', Par='RX', and Val='123.45'.

I’m interested in querying such database for obtaining the information related to ‘n’ legs. In English, that would be:

"Select RX,RY,RZ for the next five legs starting on MAN1"

So the query would go to MAN1, retrieve RX, RY, RZ, then read the parameter NEXT and go to that Id, retrieve RX, RY, RZ; read the parameter NEXT; go to that one … like this five times.

How can I pass such query with “dynamic parameters”?

Thank you.

Answer 1

I found the answer to my own question in the SQLAlchemy website. From the documentation:

The adjacency list pattern is a common
relational pattern whereby a table
contains a foreign key reference to
itself. This is the most common and
simple way to represent hierarchical
data in flat tables. The other way is
the “nested sets” model, sometimes
called “modified preorder”. Despite
what many online articles say about
modified preorder, the adjacency list
model is probably the most appropriate
pattern for the large majority of
hierarchical storage needs, for
reasons of concurrency, reduced
complexity, and that modified preorder
has little advantage over an
application which can fully load
subtrees into the application space.

SQLAlchemy commonly refers to an
adjacency list relationship as a
self-referential mapper. In this
example, we’ll work with a single
table called treenodes to represent a
tree structure:

A graph such as the following:

root --+---> child1
       +---> child2 --+--> subchild1
       |              +--> subchild2
       +---> child3

Would be represented with data such as:

id       parent_id     data
---      -------       ----
1        NULL          root
2        1             child1
3        1             child2
4        3             subchild1
5        3             subchild2
6        1             child3