Selective PostgreSQL database querying - database

Question

Is it possible to have selective queries in PostgreSQL which select different tables/columns based on values of rows already selected?
Basically, I've got a table in which each row contains a sequence of two to five characters (tbl_roots), optionally with a length field which specifies how many characters the sequence is supposed to contain (it's meant to be made redundant once I figure out a better way, i.e. by counting the length of the sequences).
There are four tables containing patterns (tbl_patterns_biliteral, tbl_patterns_triliteral, ...etc), each of which corresponds to a root_length, and a fifth table (tbl_patterns) which is used to synchronise the pattern tables by providing an identifier for each row—so row #2 in tbl_patterns_biliteral corresponds to the same row in tbl_patterns_triliteral. The six pattern tables are restricted such that no row in tbl_patterns_(bi|tri|quadri|quinqui)literal can have a pattern_id that doesn't exist in tbl_patterns.
Each pattern table has nine other columns which corresponds to an identifier (root_form).
The last table in the database (tbl_words), contains a column for each of the major tables (word_id, root_id, pattern_id, root_form, word). Each word is defined as being a root of a particular length and form, spliced into a particular pattern. The splicing is relatively simple: translate(pattern, '12345', array_to_string(root, '')) as word_combined does the job.
Now, what I want to do is select the appropriate pattern table based on the length of the sequence in tbl_roots, and select the appropriate column in the pattern table based on the value of root_form.
How could this be done? Can it be combined into a simple query, or will I need to make multiple passes? Once I've built up this query, I'll then be able to code it into a PHP script which can search my database.
EDIT
Here's some sample data (it's actually the data I'm using at the moment) and some more explanations as to how the system works: https://gist.github.com/823609
It's conceptually simpler than it appears at first, especially if you think of it as a coordinate system.

Answer 1

I think you're going to have to change the structure of your tables to have any hope. Here's a first draft for you to think about. I'm not sure what the significance of the "i", "ii", and "iii" are in your column names. In my ignorance, I'm assuming they're meaningful to you, so I've preserved them in the table below. (I preserved their information as integers. Easy to change that to lowercase roman numerals if it matters.)
create table patterns_bilateral (
pattern_id integer not null,
root_num integer not null,
pattern varchar(15) not null,
primary key (pattern_id, root_num)
);
insert into patterns_bilateral values
(1,1, 'ya1u2a'),
(1,2, 'ya1u22a'),
(1,3, 'ya12u2a'),
(1,4, 'me11u2a'),
(1,5, 'te1u22a'),
(1,6, 'ina12u2a'),
(1,7, 'i1u22a'),
(1,8, 'ya1u22a'),
(1,9, 'e1u2a');
I'm pretty sure a structure like this will be much easier to query, but you know your field better than I do. (On the other hand, database design is my field . . . )
Expanding on my earlier answer and our comments, take a look at this query. (The test table isn't even in 3NF, but the table's not important right now.)
create table test (
root_id integer,
root_substitution varchar[],
length integer,
form integer,
pattern varchar(15),
primary key (root_id, length, form, pattern));
insert into test values
(4,'{s,ş,m}', 3, 1, '1o2i3');
This is the important part.
select root_id
, root_substitution
, length
, form
, pattern
, translate(pattern, '12345', array_to_string(root_substitution, ''))
from test;
That query returns, among other things, the translation soşim.
Are we heading in the right direction?

Answer 2

Well, that's certainly a bizarre set of requirements! Here's my best guess, but obviously I haven't tried it. I used UNION ALL to combine the patterns of different sizes and then filtered them based on length. You might need to move the length condition inside each of the subqueries for speed reasons, I don't know. Then I chose the column using the CASE expression.
select word,
translate(
case root_form
when 1 then patinfo.pattern1
when 2 then patinfo.pattern2
... up to pattern9
end,
'12345',
array_to_string(root.root, '')) as word_combined
from tbl_words word
join tbl_root root
on word.root_id = root.root_id
join tbl_patterns pat
on word.pattern_id = pat.pattern_id
join (
select 2 as pattern_length, pattern_id, pattern1, ..., pattern9
from tbl_patterns_biliteral bi
union all
select 3, pattern_id, pattern1, pattern2, ..., pattern9
from tbl_patterns_biliteral tri
union all
...same for quad and quin...
) patinfo
on
patinfo.pattern_id = pat.pattern_id
and length(root.root) = patinfo.pattern_length
Consider combining all the different patterns into one pattern_details table with a root_length field to filter on. I think that would be easier than combining them all together with UNION ALL. It might be even easier if you had multiple rows in the pattern_details table and filtered based on root_form. Maybe the best would be to lay out pattern_details with fields for pattern_id, root_length, root_form, and pattern. Then you just join from the word table through the pattern table to the pattern detail that matches all the right criteria.
Of course, maybe I've completely misunderstood what you're looking for. If so, it would be clearer if you posted some example data and an example result.